layout/generic/nsFlexContainerFrame.cpp
author Daniel Holbert <dholbert@cs.stanford.edu>
Thu, 27 Oct 2016 18:58:25 -0700
changeset 346578 86a391e3e1638407465610b0dc5b5fc4694a0a08
parent 346576 ae632144724fc5efae24a32acab2d7d938384123
child 346579 7aa8199183fca08ca7d9b309fa01580affb45da7
permissions -rw-r--r--
Bug 1269045 part 1: Adjust flex item "order"-sorting code to treat placeholder frames as <= anything they're compared against, including each other. r=mats This patch makes the following specific changes: (1) Adds an early-return to both versions of the IsOrderLEQ function, to treat placeholder children as LEQ everything (including each other). This will tend to sort them to the beginning of the child list, which is unimportant but fine. More importantly, though: this means our "order"-sorting code won't reorder placeholders *with respect to each other* (since our sort algorithm is stable). So their painting order won't be affected by the "order" property, which is required by the spec. (2) Drops some nsPlaceholderFrame::GetRealFrameFor() calls -- they're unnecessary, since any placeholder frames will have prompted us to return earlier. One caveat to (2): this patch does leave a few "nsPlaceholderFrame::GetRealFrameFor()" calls in place, *for the moment*. These remaining calls are for handling placeholders that are wrapped, i.e. inside of anonymous flex items. These calls are still needed to avoid assertion-failures (i.e. to get a consistent ordering) at this point, but they'll be removed in a later patch in this same bug, when we stop wrapping placeholders in anonymous flex items. MozReview-Commit-ID: 1R6NW30Kxgv

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=2 et sw=2 tw=80: */

/* This Source Code is subject to the terms of the Mozilla Public License
 * version 2.0 (the "License"). You can obtain a copy of the License at
 * http://mozilla.org/MPL/2.0/. */

/* rendering object for CSS "display: flex" */

#include "mozilla/UniquePtr.h"
#include "nsFlexContainerFrame.h"
#include "nsContentUtils.h"
#include "nsCSSAnonBoxes.h"
#include "nsDisplayList.h"
#include "nsIFrameInlines.h"
#include "nsLayoutUtils.h"
#include "nsPlaceholderFrame.h"
#include "nsPresContext.h"
#include "nsRenderingContext.h"
#include "nsStyleContext.h"
#include "mozilla/Logging.h"
#include <algorithm>
#include "mozilla/LinkedList.h"
#include "mozilla/FloatingPoint.h"
#include "WritingModes.h"

using namespace mozilla;
using namespace mozilla::layout;

// Convenience typedefs for helper classes that we forward-declare in .h file
// (so that nsFlexContainerFrame methods can use them as parameters):
typedef nsFlexContainerFrame::FlexItem FlexItem;
typedef nsFlexContainerFrame::FlexLine FlexLine;
typedef nsFlexContainerFrame::FlexboxAxisTracker FlexboxAxisTracker;
typedef nsFlexContainerFrame::StrutInfo StrutInfo;

static mozilla::LazyLogModule gFlexContainerLog("nsFlexContainerFrame");

// XXXdholbert Some of this helper-stuff should be separated out into a general
// "main/cross-axis utils" header, shared by grid & flexbox?
// (Particularly when grid gets support for align-*/justify-* properties.)

// Helper enums
// ============

// Represents a physical orientation for an axis.
// The directional suffix indicates the direction in which the axis *grows*.
// So e.g. eAxis_LR means a horizontal left-to-right axis, whereas eAxis_BT
// means a vertical bottom-to-top axis.
// NOTE: The order here is important -- these values are used as indices into
// the static array 'kAxisOrientationToSidesMap', defined below.
enum AxisOrientationType {
  eAxis_LR,
  eAxis_RL,
  eAxis_TB,
  eAxis_BT,
  eNumAxisOrientationTypes // For sizing arrays that use these values as indices
};

// Represents one or the other extreme of an axis (e.g. for the main axis, the
// main-start vs. main-end edge.
// NOTE: The order here is important -- these values are used as indices into
// the sub-arrays in 'kAxisOrientationToSidesMap', defined below.
enum AxisEdgeType {
  eAxisEdge_Start,
  eAxisEdge_End,
  eNumAxisEdges // For sizing arrays that use these values as indices
};

// This array maps each axis orientation to a pair of corresponding
// [start, end] physical mozilla::Side values.
static const mozilla::Side
kAxisOrientationToSidesMap[eNumAxisOrientationTypes][eNumAxisEdges] = {
  { eSideLeft,   eSideRight  },  // eAxis_LR
  { eSideRight,  eSideLeft   },  // eAxis_RL
  { eSideTop,    eSideBottom },  // eAxis_TB
  { eSideBottom, eSideTop }      // eAxis_BT
};

// Helper structs / classes / methods
// ==================================
// Returns true iff the given nsStyleDisplay has display:-webkit-{inline-}-box.
static inline bool
IsDisplayValueLegacyBox(const nsStyleDisplay* aStyleDisp)
{
  return aStyleDisp->mDisplay == mozilla::StyleDisplay::WebkitBox ||
    aStyleDisp->mDisplay == mozilla::StyleDisplay::WebkitInlineBox;
}

/* static */ bool
nsFlexContainerFrame::IsLegacyBox(const nsIFrame* aFrame)
{
  nsStyleContext* styleContext = aFrame->StyleContext();
  const nsStyleDisplay* styleDisp = styleContext->StyleDisplay();

  // Trivial case: just check "display" directly.
  bool isLegacyBox = IsDisplayValueLegacyBox(styleDisp);

  // If this frame is for a scrollable element, then it will actually have
  // "display:block", and its *parent* will have the real flex-flavored display
  // value. So in that case, check the parent to find out if we're legacy.
  if (!isLegacyBox && styleDisp->mDisplay == mozilla::StyleDisplay::Block) {
    nsStyleContext* parentStyleContext = styleContext->GetParent();
    NS_ASSERTION(parentStyleContext &&
                 (styleContext->GetPseudo() == nsCSSAnonBoxes::buttonContent ||
                  styleContext->GetPseudo() == nsCSSAnonBoxes::scrolledContent),
                 "The only way a nsFlexContainerFrame can have 'display:block' "
                 "should be if it's the inner part of a scrollable or button "
                 "element");
    isLegacyBox = IsDisplayValueLegacyBox(parentStyleContext->StyleDisplay());
  }

  NS_ASSERTION(!isLegacyBox ||
               aFrame->GetType() == nsGkAtoms::flexContainerFrame,
               "legacy box with unexpected frame type");
  return isLegacyBox;
}

// Returns the "align-items" value that's equivalent to the legacy "box-align"
// value in the given style struct.
static uint8_t
ConvertLegacyStyleToAlignItems(const nsStyleXUL* aStyleXUL)
{
  // -[moz|webkit]-box-align corresponds to modern "align-items"
  switch (aStyleXUL->mBoxAlign) {
    case StyleBoxAlign::Stretch:
      return NS_STYLE_ALIGN_STRETCH;
    case StyleBoxAlign::Start:
      return NS_STYLE_ALIGN_FLEX_START;
    case StyleBoxAlign::Center:
      return NS_STYLE_ALIGN_CENTER;
    case StyleBoxAlign::Baseline:
      return NS_STYLE_ALIGN_BASELINE;
    case StyleBoxAlign::End:
      return NS_STYLE_ALIGN_FLEX_END;
  }

  MOZ_ASSERT_UNREACHABLE("Unrecognized mBoxAlign enum value");
  // Fall back to default value of "align-items" property:
  return NS_STYLE_ALIGN_STRETCH;
}

// Returns the "justify-content" value that's equivalent to the legacy
// "box-pack" value in the given style struct.
static uint8_t
ConvertLegacyStyleToJustifyContent(const nsStyleXUL* aStyleXUL)
{
  // -[moz|webkit]-box-pack corresponds to modern "justify-content"
  switch (aStyleXUL->mBoxPack) {
    case StyleBoxPack::Start:
      return NS_STYLE_ALIGN_FLEX_START;
    case StyleBoxPack::Center:
      return NS_STYLE_ALIGN_CENTER;
    case StyleBoxPack::End:
      return NS_STYLE_ALIGN_FLEX_END;
    case StyleBoxPack::Justify:
      return NS_STYLE_ALIGN_SPACE_BETWEEN;
  }

  MOZ_ASSERT_UNREACHABLE("Unrecognized mBoxPack enum value");
  // Fall back to default value of "justify-content" property:
  return NS_STYLE_ALIGN_FLEX_START;
}

// Indicates whether advancing along the given axis is equivalent to
// increasing our X or Y position (as opposed to decreasing it).
static inline bool
AxisGrowsInPositiveDirection(AxisOrientationType aAxis)
{
  return eAxis_LR == aAxis || eAxis_TB == aAxis;
}

// Given an AxisOrientationType, returns the "reverse" AxisOrientationType
// (in the same dimension, but the opposite direction)
static inline AxisOrientationType
GetReverseAxis(AxisOrientationType aAxis)
{
  AxisOrientationType reversedAxis;

  if (aAxis % 2 == 0) {
    // even enum value. Add 1 to reverse.
    reversedAxis = AxisOrientationType(aAxis + 1);
  } else {
    // odd enum value. Subtract 1 to reverse.
    reversedAxis = AxisOrientationType(aAxis - 1);
  }

  // Check that we're still in the enum's valid range
  MOZ_ASSERT(reversedAxis >= eAxis_LR &&
             reversedAxis <= eAxis_BT);

  return reversedAxis;
}

/**
 * Converts a "flex-relative" coordinate in a single axis (a main- or cross-axis
 * coordinate) into a coordinate in the corresponding physical (x or y) axis. If
 * the flex-relative axis in question already maps *directly* to a physical
 * axis (i.e. if it's LTR or TTB), then the physical coordinate has the same
 * numeric value as the provided flex-relative coordinate. Otherwise, we have to
 * subtract the flex-relative coordinate from the flex container's size in that
 * axis, to flip the polarity. (So e.g. a main-axis position of 2px in a RTL
 * 20px-wide container would correspond to a physical coordinate (x-value) of
 * 18px.)
 */
static nscoord
PhysicalCoordFromFlexRelativeCoord(nscoord aFlexRelativeCoord,
                                   nscoord aContainerSize,
                                   AxisOrientationType aAxis) {
  if (AxisGrowsInPositiveDirection(aAxis)) {
    return aFlexRelativeCoord;
  }
  return aContainerSize - aFlexRelativeCoord;
}

// Helper-macro to let us pick one of two expressions to evaluate
// (a width expression vs. a height expression), to get a main-axis or
// cross-axis component.
// For code that has e.g. a nsSize object, FlexboxAxisTracker::GetMainComponent
// and GetCrossComponent are cleaner; but in cases where we simply have
// two separate expressions for width and height (which may be expensive to
// evaluate), these macros will ensure that only the expression for the correct
// axis gets evaluated.
#define GET_MAIN_COMPONENT(axisTracker_, width_, height_)  \
  (axisTracker_).IsMainAxisHorizontal() ? (width_) : (height_)

#define GET_CROSS_COMPONENT(axisTracker_, width_, height_)  \
  (axisTracker_).IsCrossAxisHorizontal() ? (width_) : (height_)

// Logical versions of helper-macros above:
#define GET_MAIN_COMPONENT_LOGICAL(axisTracker_, wm_, isize_, bsize_)  \
  wm_.IsOrthogonalTo(axisTracker_.GetWritingMode()) != \
    (axisTracker_).IsRowOriented() ? (isize_) : (bsize_)

#define GET_CROSS_COMPONENT_LOGICAL(axisTracker_, wm_, isize_, bsize_)  \
  wm_.IsOrthogonalTo(axisTracker_.GetWritingMode()) != \
    (axisTracker_).IsRowOriented() ? (bsize_) : (isize_)

// Encapsulates our flex container's main & cross axes.
class MOZ_STACK_CLASS nsFlexContainerFrame::FlexboxAxisTracker {
public:
  FlexboxAxisTracker(const nsFlexContainerFrame* aFlexContainer,
                     const WritingMode& aWM);

  // Accessors:
  // XXXdholbert [BEGIN DEPRECATED]
  AxisOrientationType GetMainAxis() const  { return mMainAxis;  }
  AxisOrientationType GetCrossAxis() const { return mCrossAxis; }

  bool IsMainAxisHorizontal() const {
    // If we're row-oriented, and our writing mode is NOT vertical,
    // or we're column-oriented and our writing mode IS vertical,
    // then our main axis is horizontal. This handles all cases:
    return mIsRowOriented != mWM.IsVertical();
  }
  bool IsCrossAxisHorizontal() const {
    return !IsMainAxisHorizontal();
  }
  // XXXdholbert [END DEPRECATED]

  // Returns the flex container's writing mode.
  WritingMode GetWritingMode() const { return mWM; }

  // Returns true if our main axis is in the reverse direction of our
  // writing mode's corresponding axis. (From 'flex-direction: *-reverse')
  bool IsMainAxisReversed() const {
    return mIsMainAxisReversed;
  }
  // Returns true if our cross axis is in the reverse direction of our
  // writing mode's corresponding axis. (From 'flex-wrap: *-reverse')
  bool IsCrossAxisReversed() const {
    return mIsCrossAxisReversed;
  }

  bool IsRowOriented() const { return mIsRowOriented; }
  bool IsColumnOriented() const { return !mIsRowOriented; }

  nscoord GetMainComponent(const nsSize& aSize) const {
    return GET_MAIN_COMPONENT(*this, aSize.width, aSize.height);
  }
  int32_t GetMainComponent(const LayoutDeviceIntSize& aIntSize) const {
    return GET_MAIN_COMPONENT(*this, aIntSize.width, aIntSize.height);
  }

  nscoord GetCrossComponent(const nsSize& aSize) const {
    return GET_CROSS_COMPONENT(*this, aSize.width, aSize.height);
  }
  int32_t GetCrossComponent(const LayoutDeviceIntSize& aIntSize) const {
    return GET_CROSS_COMPONENT(*this, aIntSize.width, aIntSize.height);
  }

  nscoord GetMarginSizeInMainAxis(const nsMargin& aMargin) const {
    return IsMainAxisHorizontal() ?
      aMargin.LeftRight() :
      aMargin.TopBottom();
  }
  nscoord GetMarginSizeInCrossAxis(const nsMargin& aMargin) const {
    return IsCrossAxisHorizontal() ?
      aMargin.LeftRight() :
      aMargin.TopBottom();
  }

  // Returns aFrame's computed value for 'height' or 'width' -- whichever is in
  // the cross-axis. (NOTE: This is cross-axis-specific for now. If we need a
  // main-axis version as well, we could generalize or clone this function.)
  const nsStyleCoord& ComputedCrossSize(const nsIFrame* aFrame) const {
    const nsStylePosition* stylePos = aFrame->StylePosition();

    return IsCrossAxisHorizontal() ?
      stylePos->mWidth :
      stylePos->mHeight;
  }

  /**
   * Converts a "flex-relative" point (a main-axis & cross-axis coordinate)
   * into a LogicalPoint, using the flex container's writing mode.
   *
   *  @arg aMainCoord  The main-axis coordinate -- i.e an offset from the
   *                   main-start edge of the flex container's content box.
   *  @arg aCrossCoord The cross-axis coordinate -- i.e an offset from the
   *                   cross-start edge of the flex container's content box.
   *  @arg aContainerMainSize  The main size of flex container's content box.
   *  @arg aContainerCrossSize The cross size of flex container's content box.
   *  @return A LogicalPoint, with the flex container's writing mode, that
   *          represents the same position. The logical coordinates are
   *          relative to the flex container's content box.
   */
  LogicalPoint
  LogicalPointFromFlexRelativePoint(nscoord aMainCoord,
                                    nscoord aCrossCoord,
                                    nscoord aContainerMainSize,
                                    nscoord aContainerCrossSize) const {
    nscoord logicalCoordInMainAxis = mIsMainAxisReversed ?
      aContainerMainSize - aMainCoord : aMainCoord;
    nscoord logicalCoordInCrossAxis = mIsCrossAxisReversed ?
      aContainerCrossSize - aCrossCoord : aCrossCoord;

    return mIsRowOriented ?
      LogicalPoint(mWM, logicalCoordInMainAxis, logicalCoordInCrossAxis) :
      LogicalPoint(mWM, logicalCoordInCrossAxis, logicalCoordInMainAxis);
  }

  /**
   * Converts a "flex-relative" size (a main-axis & cross-axis size)
   * into a LogicalSize, using the flex container's writing mode.
   *
   *  @arg aMainSize  The main-axis size.
   *  @arg aCrossSize The cross-axis size.
   *  @return A LogicalSize, with the flex container's writing mode, that
   *          represents the same size.
   */
  LogicalSize LogicalSizeFromFlexRelativeSizes(nscoord aMainSize,
                                               nscoord aCrossSize) const {
    return mIsRowOriented ?
      LogicalSize(mWM, aMainSize, aCrossSize) :
      LogicalSize(mWM, aCrossSize, aMainSize);
  }

  // Are my axes reversed with respect to what the author asked for?
  // (We may reverse the axes in the FlexboxAxisTracker constructor and set
  // this flag, to avoid reflowing our children in bottom-to-top order.)
  bool AreAxesInternallyReversed() const
  {
    return mAreAxesInternallyReversed;
  }

private:
  // Delete copy-constructor & reassignment operator, to prevent accidental
  // (unnecessary) copying.
  FlexboxAxisTracker(const FlexboxAxisTracker&) = delete;
  FlexboxAxisTracker& operator=(const FlexboxAxisTracker&) = delete;

  // Helpers for constructor which determine the orientation of our axes, based
  // on legacy box properties (-webkit-box-orient, -webkit-box-direction) or
  // modern flexbox properties (flex-direction, flex-wrap) depending on whether
  // the flex container is a "legacy box" (as determined by IsLegacyBox).
  void InitAxesFromLegacyProps(const nsFlexContainerFrame* aFlexContainer);
  void InitAxesFromModernProps(const nsFlexContainerFrame* aFlexContainer);

  // XXXdholbert [BEGIN DEPRECATED]
  AxisOrientationType mMainAxis;
  AxisOrientationType mCrossAxis;
  // XXXdholbert [END DEPRECATED]

  const WritingMode mWM; // The flex container's writing mode.

  bool mIsRowOriented; // Is our main axis the inline axis?
                       // (Are we 'flex-direction:row[-reverse]'?)

  bool mIsMainAxisReversed; // Is our main axis in the opposite direction
                            // as mWM's corresponding axis? (e.g. RTL vs LTR)
  bool mIsCrossAxisReversed; // Is our cross axis in the opposite direction
                             // as mWM's corresponding axis? (e.g. BTT vs TTB)

  // Implementation detail -- this indicates whether we've decided to
  // transparently reverse our axes & our child ordering, to avoid having
  // frames flow from bottom to top in either axis (& to make pagination saner).
  bool mAreAxesInternallyReversed;
};

/**
 * Represents a flex item.
 * Includes the various pieces of input that the Flexbox Layout Algorithm uses
 * to resolve a flexible width.
 */
class nsFlexContainerFrame::FlexItem : public LinkedListElement<FlexItem>
{
public:
  // Normal constructor:
  FlexItem(ReflowInput& aFlexItemReflowInput,
           float aFlexGrow, float aFlexShrink, nscoord aMainBaseSize,
           nscoord aMainMinSize, nscoord aMainMaxSize,
           nscoord aTentativeCrossSize,
           nscoord aCrossMinSize, nscoord aCrossMaxSize,
           const FlexboxAxisTracker& aAxisTracker);

  // Simplified constructor, to be used only for generating "struts":
  // (NOTE: This "strut" constructor uses the *container's* writing mode, which
  // we'll use on this FlexItem instead of the child frame's real writing mode.
  // This is fine - it doesn't matter what writing mode we use for a
  // strut, since it won't render any content and we already know its size.)
  FlexItem(nsIFrame* aChildFrame, nscoord aCrossSize, WritingMode aContainerWM);

  // Accessors
  nsIFrame* Frame() const          { return mFrame; }
  nscoord GetFlexBaseSize() const  { return mFlexBaseSize; }

  nscoord GetMainMinSize() const   {
    MOZ_ASSERT(!mNeedsMinSizeAutoResolution,
               "Someone's using an unresolved 'auto' main min-size");
    return mMainMinSize;
  }
  nscoord GetMainMaxSize() const   { return mMainMaxSize; }

  // Note: These return the main-axis position and size of our *content box*.
  nscoord GetMainSize() const      { return mMainSize; }
  nscoord GetMainPosition() const  { return mMainPosn; }

  nscoord GetCrossMinSize() const  { return mCrossMinSize; }
  nscoord GetCrossMaxSize() const  { return mCrossMaxSize; }

  // Note: These return the cross-axis position and size of our *content box*.
  nscoord GetCrossSize() const     { return mCrossSize;  }
  nscoord GetCrossPosition() const { return mCrossPosn; }

  nscoord ResolvedAscent() const {
    if (mAscent == ReflowOutput::ASK_FOR_BASELINE) {
      // XXXdholbert We should probably be using the *container's* writing-mode
      // here, instead of the item's -- though it doesn't much matter right
      // now, because all of the baseline-handling code here essentially
      // assumes that the container & items have the same writing-mode. This
      // will matter more (& can be expanded/tested) once we officially support
      // logical directions & vertical writing-modes in flexbox, in bug 1079155
      // or a dependency.
      // Use GetFirstLineBaseline(), or just GetBaseline() if that fails.
      if (!nsLayoutUtils::GetFirstLineBaseline(mWM, mFrame, &mAscent)) {
        mAscent = mFrame->GetLogicalBaseline(mWM);
      }
    }
    return mAscent;
  }

  // Convenience methods to compute the main & cross size of our *margin-box*.
  // The caller is responsible for telling us the right axis, so that we can
  // pull out the appropriate components of our margin/border/padding structs.
  nscoord GetOuterMainSize(AxisOrientationType aMainAxis) const
  {
    return mMainSize + GetMarginBorderPaddingSizeInAxis(aMainAxis);
  }

  nscoord GetOuterCrossSize(AxisOrientationType aCrossAxis) const
  {
    return mCrossSize + GetMarginBorderPaddingSizeInAxis(aCrossAxis);
  }

  // Returns the distance between this FlexItem's baseline and the cross-start
  // edge of its margin-box. Used in baseline alignment.
  // (This function needs to be told which edge we're measuring the baseline
  // from, so that it can look up the appropriate components from mMargin.)
  nscoord GetBaselineOffsetFromOuterCrossEdge(
    AxisEdgeType aEdge,
    const FlexboxAxisTracker& aAxisTracker) const;

  float GetShareOfWeightSoFar() const { return mShareOfWeightSoFar; }

  bool IsFrozen() const            { return mIsFrozen; }

  bool HadMinViolation() const     { return mHadMinViolation; }
  bool HadMaxViolation() const     { return mHadMaxViolation; }

  // Indicates whether this item received a preliminary "measuring" reflow
  // before its actual reflow.
  bool HadMeasuringReflow() const  { return mHadMeasuringReflow; }

  // Indicates whether this item's cross-size has been stretched (from having
  // "align-self: stretch" with an auto cross-size and no auto margins in the
  // cross axis).
  bool IsStretched() const         { return mIsStretched; }

  // Indicates whether we need to resolve an 'auto' value for the main-axis
  // min-[width|height] property.
  bool NeedsMinSizeAutoResolution() const
    { return mNeedsMinSizeAutoResolution; }

  // Indicates whether this item is a "strut" left behind by an element with
  // visibility:collapse.
  bool IsStrut() const             { return mIsStrut; }

  WritingMode GetWritingMode() const { return mWM; }
  uint8_t GetAlignSelf() const     { return mAlignSelf; }

  // Returns the flex factor (flex-grow or flex-shrink), depending on
  // 'aIsUsingFlexGrow'.
  //
  // Asserts fatally if called on a frozen item (since frozen items are not
  // flexible).
  float GetFlexFactor(bool aIsUsingFlexGrow)
  {
    MOZ_ASSERT(!IsFrozen(), "shouldn't need flex factor after item is frozen");

    return aIsUsingFlexGrow ? mFlexGrow : mFlexShrink;
  }

  // Returns the weight that we should use in the "resolving flexible lengths"
  // algorithm.  If we're using the flex grow factor, we just return that;
  // otherwise, we return the "scaled flex shrink factor" (scaled by our flex
  // base size, so that when both large and small items are shrinking, the large
  // items shrink more).
  //
  // I'm calling this a "weight" instead of a "[scaled] flex-[grow|shrink]
  // factor", to more clearly distinguish it from the actual flex-grow &
  // flex-shrink factors.
  //
  // Asserts fatally if called on a frozen item (since frozen items are not
  // flexible).
  float GetWeight(bool aIsUsingFlexGrow)
  {
    MOZ_ASSERT(!IsFrozen(), "shouldn't need weight after item is frozen");

    if (aIsUsingFlexGrow) {
      return mFlexGrow;
    }

    // We're using flex-shrink --> return mFlexShrink * mFlexBaseSize
    if (mFlexBaseSize == 0) {
      // Special-case for mFlexBaseSize == 0 -- we have no room to shrink, so
      // regardless of mFlexShrink, we should just return 0.
      // (This is really a special-case for when mFlexShrink is infinity, to
      // avoid performing mFlexShrink * mFlexBaseSize = inf * 0 = undefined.)
      return 0.0f;
    }
    return mFlexShrink * mFlexBaseSize;
  }

  const nsSize& IntrinsicRatio() const { return mIntrinsicRatio; }
  bool HasIntrinsicRatio() const { return mIntrinsicRatio != nsSize(); }

  // Getters for margin:
  // ===================
  const nsMargin& GetMargin() const { return mMargin; }

  // Returns the margin component for a given mozilla::Side
  nscoord GetMarginComponentForSide(mozilla::Side aSide) const
  { return mMargin.Side(aSide); }

  // Returns the total space occupied by this item's margins in the given axis
  nscoord GetMarginSizeInAxis(AxisOrientationType aAxis) const
  {
    mozilla::Side startSide = kAxisOrientationToSidesMap[aAxis][eAxisEdge_Start];
    mozilla::Side endSide = kAxisOrientationToSidesMap[aAxis][eAxisEdge_End];
    return GetMarginComponentForSide(startSide) +
      GetMarginComponentForSide(endSide);
  }

  // Getters for border/padding
  // ==========================
  const nsMargin& GetBorderPadding() const { return mBorderPadding; }

  // Returns the border+padding component for a given mozilla::Side
  nscoord GetBorderPaddingComponentForSide(mozilla::Side aSide) const
  { return mBorderPadding.Side(aSide); }

  // Returns the total space occupied by this item's borders and padding in
  // the given axis
  nscoord GetBorderPaddingSizeInAxis(AxisOrientationType aAxis) const
  {
    mozilla::Side startSide = kAxisOrientationToSidesMap[aAxis][eAxisEdge_Start];
    mozilla::Side endSide = kAxisOrientationToSidesMap[aAxis][eAxisEdge_End];
    return GetBorderPaddingComponentForSide(startSide) +
      GetBorderPaddingComponentForSide(endSide);
  }

  // Getter for combined margin/border/padding
  // =========================================
  // Returns the total space occupied by this item's margins, borders and
  // padding in the given axis
  nscoord GetMarginBorderPaddingSizeInAxis(AxisOrientationType aAxis) const
  {
    return GetMarginSizeInAxis(aAxis) + GetBorderPaddingSizeInAxis(aAxis);
  }

  // Setters
  // =======
  // Helper to set the resolved value of min-[width|height]:auto for the main
  // axis. (Should only be used if NeedsMinSizeAutoResolution() returns true.)
  void UpdateMainMinSize(nscoord aNewMinSize)
  {
    NS_ASSERTION(aNewMinSize >= 0,
                 "How did we end up with a negative min-size?");
    MOZ_ASSERT(mMainMaxSize >= aNewMinSize,
               "Should only use this function for resolving min-size:auto, "
               "and main max-size should be an upper-bound for resolved val");
    MOZ_ASSERT(mNeedsMinSizeAutoResolution &&
               (mMainMinSize == 0 || mFrame->IsThemed(mFrame->StyleDisplay())),
               "Should only use this function for resolving min-size:auto, "
               "so we shouldn't already have a nonzero min-size established "
               "(unless it's a themed-widget-imposed minimum size)");

    if (aNewMinSize > mMainMinSize) {
      mMainMinSize = aNewMinSize;
      // Also clamp main-size to be >= new min-size:
      mMainSize = std::max(mMainSize, aNewMinSize);
    }
    mNeedsMinSizeAutoResolution = false;
  }

  // This sets our flex base size, and then sets our main size to the
  // resulting "hypothetical main size" (the base size clamped to our
  // main-axis [min,max] sizing constraints).
  void SetFlexBaseSizeAndMainSize(nscoord aNewFlexBaseSize)
  {
    MOZ_ASSERT(!mIsFrozen || mFlexBaseSize == NS_INTRINSICSIZE,
               "flex base size shouldn't change after we're frozen "
               "(unless we're just resolving an intrinsic size)");
    mFlexBaseSize = aNewFlexBaseSize;

    // Before we've resolved flexible lengths, we keep mMainSize set to
    // the 'hypothetical main size', which is the flex base size, clamped
    // to the [min,max] range:
    mMainSize = NS_CSS_MINMAX(mFlexBaseSize, mMainMinSize, mMainMaxSize);
  }

  // Setters used while we're resolving flexible lengths
  // ---------------------------------------------------

  // Sets the main-size of our flex item's content-box.
  void SetMainSize(nscoord aNewMainSize)
  {
    MOZ_ASSERT(!mIsFrozen, "main size shouldn't change after we're frozen");
    mMainSize = aNewMainSize;
  }

  void SetShareOfWeightSoFar(float aNewShare)
  {
    MOZ_ASSERT(!mIsFrozen || aNewShare == 0.0f,
               "shouldn't be giving this item any share of the weight "
               "after it's frozen");
    mShareOfWeightSoFar = aNewShare;
  }

  void Freeze() { mIsFrozen = true; }

  void SetHadMinViolation()
  {
    MOZ_ASSERT(!mIsFrozen,
               "shouldn't be changing main size & having violations "
               "after we're frozen");
    mHadMinViolation = true;
  }
  void SetHadMaxViolation()
  {
    MOZ_ASSERT(!mIsFrozen,
               "shouldn't be changing main size & having violations "
               "after we're frozen");
    mHadMaxViolation = true;
  }
  void ClearViolationFlags()
  { mHadMinViolation = mHadMaxViolation = false; }

  // Setters for values that are determined after we've resolved our main size
  // -------------------------------------------------------------------------

  // Sets the main-axis position of our flex item's content-box.
  // (This is the distance between the main-start edge of the flex container
  // and the main-start edge of the flex item's content-box.)
  void SetMainPosition(nscoord aPosn) {
    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
    mMainPosn  = aPosn;
  }

  // Sets the cross-size of our flex item's content-box.
  void SetCrossSize(nscoord aCrossSize) {
    MOZ_ASSERT(!mIsStretched,
               "Cross size shouldn't be modified after it's been stretched");
    mCrossSize = aCrossSize;
  }

  // Sets the cross-axis position of our flex item's content-box.
  // (This is the distance between the cross-start edge of the flex container
  // and the cross-start edge of the flex item.)
  void SetCrossPosition(nscoord aPosn) {
    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
    mCrossPosn = aPosn;
  }

  // After a FlexItem has had a reflow, this method can be used to cache its
  // (possibly-unresolved) ascent, in case it's needed later for
  // baseline-alignment or to establish the container's baseline.
  // (NOTE: This can be marked 'const' even though it's modifying mAscent,
  // because mAscent is mutable. It's nice for this to be 'const', because it
  // means our final reflow can iterate over const FlexItem pointers, and we
  // can be sure it's not modifying those FlexItems, except via this method.)
  void SetAscent(nscoord aAscent) const {
    mAscent = aAscent; // NOTE: this may be ASK_FOR_BASELINE
  }

  void SetHadMeasuringReflow() {
    mHadMeasuringReflow = true;
  }

  void SetIsStretched() {
    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
    mIsStretched = true;
  }

  // Setter for margin components (for resolving "auto" margins)
  void SetMarginComponentForSide(mozilla::Side aSide, nscoord aLength)
  {
    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
    mMargin.Side(aSide) = aLength;
  }

  void ResolveStretchedCrossSize(nscoord aLineCrossSize,
                                 const FlexboxAxisTracker& aAxisTracker);

  uint32_t GetNumAutoMarginsInAxis(AxisOrientationType aAxis) const;

  // Once the main size has been resolved, should we bother doing layout to
  // establish the cross size?
  bool CanMainSizeInfluenceCrossSize(const FlexboxAxisTracker& aAxisTracker) const;

protected:
  // Helper called by the constructor, to set mNeedsMinSizeAutoResolution:
  void CheckForMinSizeAuto(const ReflowInput& aFlexItemReflowInput,
                           const FlexboxAxisTracker& aAxisTracker);

  // Our frame:
  nsIFrame* const mFrame;

  // Values that we already know in constructor: (and are hence mostly 'const')
  const float mFlexGrow;
  const float mFlexShrink;

  const nsSize mIntrinsicRatio;

  const nsMargin mBorderPadding;
  nsMargin mMargin; // non-const because we need to resolve auto margins

  // These are non-const so that we can lazily update them with the item's
  // intrinsic size (obtained via a "measuring" reflow), when necessary.
  // (e.g. for "flex-basis:auto;height:auto" & "min-height:auto")
  nscoord mFlexBaseSize;
  nscoord mMainMinSize;
  nscoord mMainMaxSize;

  const nscoord mCrossMinSize;
  const nscoord mCrossMaxSize;

  // Values that we compute after constructor:
  nscoord mMainSize;
  nscoord mMainPosn;
  nscoord mCrossSize;
  nscoord mCrossPosn;
  mutable nscoord mAscent; // Mutable b/c it's set & resolved lazily, sometimes
                           // via const pointer. See comment above SetAscent().

  // Temporary state, while we're resolving flexible widths (for our main size)
  // XXXdholbert To save space, we could use a union to make these variables
  // overlay the same memory as some other member vars that aren't touched
  // until after main-size has been resolved. In particular, these could share
  // memory with mMainPosn through mAscent, and mIsStretched.
  float mShareOfWeightSoFar;
  bool mIsFrozen;
  bool mHadMinViolation;
  bool mHadMaxViolation;

  // Misc:
  bool mHadMeasuringReflow; // Did this item get a preliminary reflow,
                            // to measure its desired height?
  bool mIsStretched; // See IsStretched() documentation
  bool mIsStrut;     // Is this item a "strut" left behind by an element
                     // with visibility:collapse?

  // Does this item need to resolve a min-[width|height]:auto (in main-axis).
  bool mNeedsMinSizeAutoResolution;

  const WritingMode mWM; // The flex item's writing mode.
  uint8_t mAlignSelf; // My "align-self" computed value (with "auto"
                      // swapped out for parent"s "align-items" value,
                      // in our constructor).
};

/**
 * Represents a single flex line in a flex container.
 * Manages a linked list of the FlexItems that are in the line.
 */
class nsFlexContainerFrame::FlexLine : public LinkedListElement<FlexLine>
{
public:
  FlexLine()
  : mNumItems(0),
    mNumFrozenItems(0),
    mTotalInnerHypotheticalMainSize(0),
    mTotalOuterHypotheticalMainSize(0),
    mLineCrossSize(0),
    mBaselineOffset(nscoord_MIN)
  {}

  // Returns the sum of our FlexItems' outer hypothetical main sizes.
  // ("outer" = margin-box, and "hypothetical" = before flexing)
  nscoord GetTotalOuterHypotheticalMainSize() const {
    return mTotalOuterHypotheticalMainSize;
  }

  // Accessors for our FlexItems & information about them:
  FlexItem* GetFirstItem()
  {
    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
               "mNumItems bookkeeping is off");
    return mItems.getFirst();
  }

  const FlexItem* GetFirstItem() const
  {
    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
               "mNumItems bookkeeping is off");
    return mItems.getFirst();
  }

  FlexItem* GetLastItem()
  {
    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
               "mNumItems bookkeeping is off");
    return mItems.getLast();
  }

  const FlexItem* GetLastItem() const
  {
    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
               "mNumItems bookkeeping is off");
    return mItems.getLast();
  }

  bool IsEmpty() const
  {
    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
               "mNumItems bookkeeping is off");
    return mItems.isEmpty();
  }

  uint32_t NumItems() const
  {
    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
               "mNumItems bookkeeping is off");
    return mNumItems;
  }

  // Adds the given FlexItem to our list of items (at the front or back
  // depending on aShouldInsertAtFront), and adds its hypothetical
  // outer & inner main sizes to our totals. Use this method instead of
  // directly modifying the item list, so that our bookkeeping remains correct.
  void AddItem(FlexItem* aItem,
               bool aShouldInsertAtFront,
               nscoord aItemInnerHypotheticalMainSize,
               nscoord aItemOuterHypotheticalMainSize)
  {
    if (aShouldInsertAtFront) {
      mItems.insertFront(aItem);
    } else {
      mItems.insertBack(aItem);
    }

    // Update our various bookkeeping member-vars:
    mNumItems++;
    if (aItem->IsFrozen()) {
      mNumFrozenItems++;
    }
    mTotalInnerHypotheticalMainSize += aItemInnerHypotheticalMainSize;
    mTotalOuterHypotheticalMainSize += aItemOuterHypotheticalMainSize;
  }

  // Computes the cross-size and baseline position of this FlexLine, based on
  // its FlexItems.
  void ComputeCrossSizeAndBaseline(const FlexboxAxisTracker& aAxisTracker);

  // Returns the cross-size of this line.
  nscoord GetLineCrossSize() const { return mLineCrossSize; }

  // Setter for line cross-size -- needed for cases where the flex container
  // imposes a cross-size on the line. (e.g. for single-line flexbox, or for
  // multi-line flexbox with 'align-content: stretch')
  void SetLineCrossSize(nscoord aLineCrossSize) {
    mLineCrossSize = aLineCrossSize;
  }

  /**
   * Returns the offset within this line where any baseline-aligned FlexItems
   * should place their baseline. Usually, this represents a distance from the
   * line's cross-start edge, but if we're internally reversing the axes (see
   * AreAxesInternallyReversed()), this instead represents the distance from
   * its cross-end edge.
   *
   * If there are no baseline-aligned FlexItems, returns nscoord_MIN.
   */
  nscoord GetBaselineOffset() const {
    return mBaselineOffset;
  }

  // Runs the "Resolving Flexible Lengths" algorithm from section 9.7 of the
  // CSS flexbox spec to distribute aFlexContainerMainSize among our flex items.
  void ResolveFlexibleLengths(nscoord aFlexContainerMainSize);

  void PositionItemsInMainAxis(uint8_t aJustifyContent,
                               nscoord aContentBoxMainSize,
                               const FlexboxAxisTracker& aAxisTracker);

  void PositionItemsInCrossAxis(nscoord aLineStartPosition,
                                const FlexboxAxisTracker& aAxisTracker);

  friend class AutoFlexLineListClearer; // (needs access to mItems)

private:
  // Helpers for ResolveFlexibleLengths():
  void FreezeItemsEarly(bool aIsUsingFlexGrow);

  void FreezeOrRestoreEachFlexibleSize(const nscoord aTotalViolation,
                                       bool aIsFinalIteration);

  LinkedList<FlexItem> mItems; // Linked list of this line's flex items.

  uint32_t mNumItems; // Number of FlexItems in this line (in |mItems|).
                      // (Shouldn't change after GenerateFlexLines finishes
                      // with this line -- at least, not until we add support
                      // for splitting lines across continuations. Then we can
                      // update this count carefully.)

  // Number of *frozen* FlexItems in this line, based on FlexItem::IsFrozen().
  // Mostly used for optimization purposes, e.g. to bail out early from loops
  // when we can tell they have nothing left to do.
  uint32_t mNumFrozenItems;

  nscoord mTotalInnerHypotheticalMainSize;
  nscoord mTotalOuterHypotheticalMainSize;
  nscoord mLineCrossSize;
  nscoord mBaselineOffset;
};

// Information about a strut left behind by a FlexItem that's been collapsed
// using "visibility:collapse".
struct nsFlexContainerFrame::StrutInfo {
  StrutInfo(uint32_t aItemIdx, nscoord aStrutCrossSize)
    : mItemIdx(aItemIdx),
      mStrutCrossSize(aStrutCrossSize)
  {
  }

  uint32_t mItemIdx;      // Index in the child list.
  nscoord mStrutCrossSize; // The cross-size of this strut.
};

static void
BuildStrutInfoFromCollapsedItems(const FlexLine* aFirstLine,
                                 nsTArray<StrutInfo>& aStruts)
{
  MOZ_ASSERT(aFirstLine, "null first line pointer");
  MOZ_ASSERT(aStruts.IsEmpty(),
             "We should only build up StrutInfo once per reflow, so "
             "aStruts should be empty when this is called");

  uint32_t itemIdxInContainer = 0;
  for (const FlexLine* line = aFirstLine; line; line = line->getNext()) {
    for (const FlexItem* item = line->GetFirstItem(); item;
         item = item->getNext()) {
      if (NS_STYLE_VISIBILITY_COLLAPSE ==
          item->Frame()->StyleVisibility()->mVisible) {
        // Note the cross size of the line as the item's strut size.
        aStruts.AppendElement(StrutInfo(itemIdxInContainer,
                                        line->GetLineCrossSize()));
      }
      itemIdxInContainer++;
    }
  }
}

// Convenience function to get either the "order" or the "box-ordinal-group"
// property-value for a flex item (depending on whether the container is a
// modern flex container or a legacy box).
static int32_t
GetOrderOrBoxOrdinalGroup(nsIFrame* aFlexItem, bool aIsLegacyBox)
{
  if (aIsLegacyBox) {
    // We'll be using mBoxOrdinal, which has type uint32_t. However, the modern
    // 'order' property (whose functionality we're co-opting) has type int32_t.
    // So: if we happen to have a uint32_t value that's greater than INT32_MAX,
    // we clamp it rather than letting it overflow. Chances are, this is just
    // an author using BIG_VALUE anyway, so the clamped value should be fine.
    // (particularly since sufficiently-huge values are busted in Chrome/WebKit
    // per https://bugs.chromium.org/p/chromium/issues/detail?id=599645 )
    uint32_t clampedBoxOrdinal = std::min(aFlexItem->StyleXUL()->mBoxOrdinal,
                                          static_cast<uint32_t>(INT32_MAX));
    return static_cast<int32_t>(clampedBoxOrdinal);
  }

  // Normal case: just use modern 'order' property.
  return aFlexItem->StylePosition()->mOrder;
}

// Helper-function to find the first non-anonymous-box descendent of aFrame.
static nsIFrame*
GetFirstNonAnonBoxDescendant(nsIFrame* aFrame)
{
  while (aFrame) {
    nsIAtom* pseudoTag = aFrame->StyleContext()->GetPseudo();

    // If aFrame isn't an anonymous container, then it'll do.
    if (!pseudoTag ||                                 // No pseudotag.
        !nsCSSAnonBoxes::IsAnonBox(pseudoTag) ||      // Pseudotag isn't anon.
        nsCSSAnonBoxes::IsNonElement(pseudoTag)) {    // Text, not a container.
      break;
    }

    // Otherwise, descend to its first child and repeat.

    // SPECIAL CASE: if we're dealing with an anonymous table, then it might
    // be wrapping something non-anonymous in its caption or col-group lists
    // (instead of its principal child list), so we have to look there.
    // (Note: For anonymous tables that have a non-anon cell *and* a non-anon
    // column, we'll always return the column. This is fine; we're really just
    // looking for a handle to *anything* with a meaningful content node inside
    // the table, for use in DOM comparisons to things outside of the table.)
    if (MOZ_UNLIKELY(aFrame->GetType() == nsGkAtoms::tableWrapperFrame)) {
      nsIFrame* captionDescendant =
        GetFirstNonAnonBoxDescendant(aFrame->GetChildList(kCaptionList).FirstChild());
      if (captionDescendant) {
        return captionDescendant;
      }
    } else if (MOZ_UNLIKELY(aFrame->GetType() == nsGkAtoms::tableFrame)) {
      nsIFrame* colgroupDescendant =
        GetFirstNonAnonBoxDescendant(aFrame->GetChildList(kColGroupList).FirstChild());
      if (colgroupDescendant) {
        return colgroupDescendant;
      }
    }

    // USUAL CASE: Descend to the first child in principal list.
    aFrame = aFrame->PrincipalChildList().FirstChild();
  }
  return aFrame;
}

/**
 * Sorting helper-function that compares two frames' "order" property-values,
 * and if they're equal, compares the DOM positions of their corresponding
 * content nodes. Returns true if aFrame1 is "less than or equal to" aFrame2
 * according to this comparison.
 *
 * Note: This can't be a static function, because we need to pass it as a
 * template argument. (Only functions with external linkage can be passed as
 * template arguments.)
 *
 * @return true if the computed "order" property of aFrame1 is less than that
 *         of aFrame2, or if the computed "order" values are equal and aFrame1's
 *         corresponding DOM node is earlier than aFrame2's in the DOM tree.
 *         Otherwise, returns false.
 */
bool
IsOrderLEQWithDOMFallback(nsIFrame* aFrame1,
                          nsIFrame* aFrame2)
{
  MOZ_ASSERT(aFrame1->IsFlexItem() && aFrame2->IsFlexItem(),
             "this method only intended for comparing flex items");
  MOZ_ASSERT(aFrame1->GetParent() == aFrame2->GetParent(),
             "this method only intended for comparing siblings");
  if (aFrame1 == aFrame2) {
    // Anything is trivially LEQ itself, so we return "true" here... but it's
    // probably bad if we end up actually needing this, so let's assert.
    NS_ERROR("Why are we checking if a frame is LEQ itself?");
    return true;
  }

  if (aFrame1->GetType() == nsGkAtoms::placeholderFrame ||
      aFrame2->GetType() == nsGkAtoms::placeholderFrame) {
    // Treat placeholders (for abspos/fixedpos frames) as LEQ everything.  This
    // ensures we don't reorder them w.r.t. one another, which is sufficient to
    // prevent them from noticeably participating in "order" reordering.
    return true;
  }

  bool isInLegacyBox = nsFlexContainerFrame::IsLegacyBox(aFrame1->GetParent());

  int32_t order1 = GetOrderOrBoxOrdinalGroup(aFrame1, isInLegacyBox);
  int32_t order2 = GetOrderOrBoxOrdinalGroup(aFrame2, isInLegacyBox);

  if (order1 != order2) {
    return order1 < order2;
  }

  // The "order" values are equal, so we need to fall back on DOM comparison.
  // For that, we need to dig through any anonymous box wrapper frames to find
  // the actual frame that corresponds to our child content.
  aFrame1 = GetFirstNonAnonBoxDescendant(aFrame1);
  aFrame2 = GetFirstNonAnonBoxDescendant(aFrame2);
  MOZ_ASSERT(aFrame1 && aFrame2,
             "why do we have an anonymous box without any "
             "non-anonymous descendants?");


  // Special case:
  // If either frame is for generated content from ::before or ::after, then
  // we can't use nsContentUtils::PositionIsBefore(), since that method won't
  // recognize generated content as being an actual sibling of other nodes.
  // We know where ::before and ::after nodes *effectively* insert in the DOM
  // tree, though (at the beginning & end), so we can just special-case them.
  nsIAtom* pseudo1 =
    nsPlaceholderFrame::GetRealFrameFor(aFrame1)->StyleContext()->GetPseudo();
  nsIAtom* pseudo2 =
    nsPlaceholderFrame::GetRealFrameFor(aFrame2)->StyleContext()->GetPseudo();

  if (pseudo1 == nsCSSPseudoElements::before ||
      pseudo2 == nsCSSPseudoElements::after) {
    // frame1 is ::before and/or frame2 is ::after => frame1 is LEQ frame2.
    return true;
  }
  if (pseudo1 == nsCSSPseudoElements::after ||
      pseudo2 == nsCSSPseudoElements::before) {
    // frame1 is ::after and/or frame2 is ::before => frame1 is not LEQ frame2.
    return false;
  }

  // Usual case: Compare DOM position.
  nsIContent* content1 = aFrame1->GetContent();
  nsIContent* content2 = aFrame2->GetContent();
  MOZ_ASSERT(content1 != content2,
             "Two different flex items are using the same nsIContent node for "
             "comparison, so we may be sorting them in an arbitrary order");

  return nsContentUtils::PositionIsBefore(content1, content2);
}

/**
 * Sorting helper-function that compares two frames' "order" property-values.
 * Returns true if aFrame1 is "less than or equal to" aFrame2 according to this
 * comparison.
 *
 * Note: This can't be a static function, because we need to pass it as a
 * template argument. (Only functions with external linkage can be passed as
 * template arguments.)
 *
 * @return true if the computed "order" property of aFrame1 is less than or
 *         equal to that of aFrame2.  Otherwise, returns false.
 */
bool
IsOrderLEQ(nsIFrame* aFrame1,
           nsIFrame* aFrame2)
{
  MOZ_ASSERT(aFrame1->IsFlexItem() && aFrame2->IsFlexItem(),
             "this method only intended for comparing flex items");
  MOZ_ASSERT(aFrame1->GetParent() == aFrame2->GetParent(),
             "this method only intended for comparing siblings");

  if (aFrame1->GetType() == nsGkAtoms::placeholderFrame ||
      aFrame2->GetType() == nsGkAtoms::placeholderFrame) {
    // Treat placeholders (for abspos/fixedpos frames) as LEQ everything.  This
    // ensures we don't reorder them w.r.t. one another, which is sufficient to
    // prevent them from noticeably participating in "order" reordering.
    return true;
  }

  bool isInLegacyBox = nsFlexContainerFrame::IsLegacyBox(aFrame1->GetParent());

  int32_t order1 = GetOrderOrBoxOrdinalGroup(aFrame1, isInLegacyBox);
  int32_t order2 = GetOrderOrBoxOrdinalGroup(aFrame2, isInLegacyBox);

  return order1 <= order2;
}

bool
nsFlexContainerFrame::IsHorizontal()
{
  const FlexboxAxisTracker axisTracker(this, GetWritingMode());
  return axisTracker.IsMainAxisHorizontal();
}

UniquePtr<FlexItem>
nsFlexContainerFrame::GenerateFlexItemForChild(
  nsPresContext* aPresContext,
  nsIFrame*      aChildFrame,
  const ReflowInput& aParentReflowInput,
  const FlexboxAxisTracker& aAxisTracker)
{
  // Create temporary reflow state just for sizing -- to get hypothetical
  // main-size and the computed values of min / max main-size property.
  // (This reflow state will _not_ be used for reflow.)
  ReflowInput
    childRI(aPresContext, aParentReflowInput, aChildFrame,
            aParentReflowInput.ComputedSize(aChildFrame->GetWritingMode()));

  // FLEX GROW & SHRINK WEIGHTS
  // --------------------------
  float flexGrow, flexShrink;
  if (IsLegacyBox(this)) {
    flexGrow = flexShrink = aChildFrame->StyleXUL()->mBoxFlex;
  } else {
    const nsStylePosition* stylePos = aChildFrame->StylePosition();
    flexGrow   = stylePos->mFlexGrow;
    flexShrink = stylePos->mFlexShrink;
  }

  WritingMode childWM = childRI.GetWritingMode();

  // MAIN SIZES (flex base size, min/max size)
  // -----------------------------------------
  nscoord flexBaseSize = GET_MAIN_COMPONENT_LOGICAL(aAxisTracker, childWM,
                                                    childRI.ComputedISize(),
                                                    childRI.ComputedBSize());
  nscoord mainMinSize = GET_MAIN_COMPONENT_LOGICAL(aAxisTracker, childWM,
                                                   childRI.ComputedMinISize(),
                                                   childRI.ComputedMinBSize());
  nscoord mainMaxSize = GET_MAIN_COMPONENT_LOGICAL(aAxisTracker, childWM,
                                                   childRI.ComputedMaxISize(),
                                                   childRI.ComputedMaxBSize());
  // This is enforced by the ReflowInput where these values come from:
  MOZ_ASSERT(mainMinSize <= mainMaxSize, "min size is larger than max size");

  // CROSS SIZES (tentative cross size, min/max cross size)
  // ------------------------------------------------------
  // Grab the cross size from the reflow state. This might be the right value,
  // or we might resolve it to something else in SizeItemInCrossAxis(); hence,
  // it's tentative. See comment under "Cross Size Determination" for more.
  nscoord tentativeCrossSize =
    GET_CROSS_COMPONENT_LOGICAL(aAxisTracker, childWM,
                                childRI.ComputedISize(),
                                childRI.ComputedBSize());
  nscoord crossMinSize =
    GET_CROSS_COMPONENT_LOGICAL(aAxisTracker, childWM,
                                childRI.ComputedMinISize(),
                                childRI.ComputedMinBSize());
  nscoord crossMaxSize =
    GET_CROSS_COMPONENT_LOGICAL(aAxisTracker, childWM,
                                childRI.ComputedMaxISize(),
                                childRI.ComputedMaxBSize());

  // SPECIAL-CASE FOR WIDGET-IMPOSED SIZES
  // Check if we're a themed widget, in which case we might have a minimum
  // main & cross size imposed by our widget (which we can't go below), or
  // (more severe) our widget might have only a single valid size.
  bool isFixedSizeWidget = false;
  const nsStyleDisplay* disp = aChildFrame->StyleDisplay();
  if (aChildFrame->IsThemed(disp)) {
    LayoutDeviceIntSize widgetMinSize;
    bool canOverride = true;
    aPresContext->GetTheme()->
      GetMinimumWidgetSize(aPresContext, aChildFrame,
                           disp->mAppearance,
                           &widgetMinSize, &canOverride);

    nscoord widgetMainMinSize =
      aPresContext->DevPixelsToAppUnits(
        aAxisTracker.GetMainComponent(widgetMinSize));
    nscoord widgetCrossMinSize =
      aPresContext->DevPixelsToAppUnits(
        aAxisTracker.GetCrossComponent(widgetMinSize));

    // GMWS() returns border-box. We need content-box, so subtract
    // borderPadding (but don't let that push our min sizes below 0).
    nsMargin& bp = childRI.ComputedPhysicalBorderPadding();
    widgetMainMinSize = std::max(widgetMainMinSize -
                                 aAxisTracker.GetMarginSizeInMainAxis(bp), 0);
    widgetCrossMinSize = std::max(widgetCrossMinSize -
                                  aAxisTracker.GetMarginSizeInCrossAxis(bp), 0);

    if (!canOverride) {
      // Fixed-size widget: freeze our main-size at the widget's mandated size.
      // (Set min and max main-sizes to that size, too, to keep us from
      // clamping to any other size later on.)
      flexBaseSize = mainMinSize = mainMaxSize = widgetMainMinSize;
      tentativeCrossSize = crossMinSize = crossMaxSize = widgetCrossMinSize;
      isFixedSizeWidget = true;
    } else {
      // Variable-size widget: ensure our min/max sizes are at least as large
      // as the widget's mandated minimum size, so we don't flex below that.
      mainMinSize = std::max(mainMinSize, widgetMainMinSize);
      mainMaxSize = std::max(mainMaxSize, widgetMainMinSize);

      if (tentativeCrossSize != NS_INTRINSICSIZE) {
        tentativeCrossSize = std::max(tentativeCrossSize, widgetCrossMinSize);
      }
      crossMinSize = std::max(crossMinSize, widgetCrossMinSize);
      crossMaxSize = std::max(crossMaxSize, widgetCrossMinSize);
    }
  }

  // Construct the flex item!
  auto item = MakeUnique<FlexItem>(childRI,
                                   flexGrow, flexShrink, flexBaseSize,
                                   mainMinSize, mainMaxSize,
                                   tentativeCrossSize,
                                   crossMinSize, crossMaxSize,
                                   aAxisTracker);

  // If we're inflexible, we can just freeze to our hypothetical main-size
  // up-front. Similarly, if we're a fixed-size widget, we only have one
  // valid size, so we freeze to keep ourselves from flexing.
  if (isFixedSizeWidget || (flexGrow == 0.0f && flexShrink == 0.0f)) {
    item->Freeze();
  }

  // Resolve "flex-basis:auto" and/or "min-[width|height]:auto" (which might
  // require us to reflow the item to measure content height)
  ResolveAutoFlexBasisAndMinSize(aPresContext, *item,
                                 childRI, aAxisTracker);
  return item;
}

// Static helper-functions for ResolveAutoFlexBasisAndMinSize():
// -------------------------------------------------------------
// Indicates whether the cross-size property is set to something definite.
// The logic here should be similar to the logic for isAutoWidth/isAutoHeight
// in nsLayoutUtils::ComputeSizeWithIntrinsicDimensions().
static bool
IsCrossSizeDefinite(const ReflowInput& aItemReflowInput,
                    const FlexboxAxisTracker& aAxisTracker)
{
  const nsStylePosition* pos = aItemReflowInput.mStylePosition;
  if (aAxisTracker.IsCrossAxisHorizontal()) {
    return pos->mWidth.GetUnit() != eStyleUnit_Auto;
  }
  // else, vertical. (We need to use IsAutoHeight() to catch e.g. %-height
  // applied to indefinite-height containing block, which counts as auto.)
  nscoord cbHeight = aItemReflowInput.mCBReflowInput->ComputedHeight();
  return !nsLayoutUtils::IsAutoHeight(pos->mHeight, cbHeight);
}

// If aFlexItem has a definite cross size, this function returns it, for usage
// (in combination with an intrinsic ratio) for resolving the item's main size
// or main min-size.
//
// The parameter "aMinSizeFallback" indicates whether we should fall back to
// returning the cross min-size, when the cross size is indefinite. (This param
// should be set IFF the caller intends to resolve the main min-size.) If this
// param is true, then this function is guaranteed to return a definite value
// (i.e. not NS_AUTOHEIGHT, excluding cases where huge sizes are involved).
//
// XXXdholbert the min-size behavior here is based on my understanding in
//   http://lists.w3.org/Archives/Public/www-style/2014Jul/0053.html
// If my understanding there ends up being wrong, we'll need to update this.
static nscoord
CrossSizeToUseWithRatio(const FlexItem& aFlexItem,
                        const ReflowInput& aItemReflowInput,
                        bool aMinSizeFallback,
                        const FlexboxAxisTracker& aAxisTracker)
{
  if (aFlexItem.IsStretched()) {
    // Definite cross-size, imposed via 'align-self:stretch' & flex container.
    return aFlexItem.GetCrossSize();
  }

  if (IsCrossSizeDefinite(aItemReflowInput, aAxisTracker)) {
    // Definite cross size.
    return GET_CROSS_COMPONENT_LOGICAL(aAxisTracker, aFlexItem.GetWritingMode(),
                                       aItemReflowInput.ComputedISize(),
                                       aItemReflowInput.ComputedBSize());
  }

  if (aMinSizeFallback) {
    // Indefinite cross-size, and we're resolving main min-size, so we'll fall
    // back to ussing the cross min-size (which should be definite).
    return GET_CROSS_COMPONENT_LOGICAL(aAxisTracker, aFlexItem.GetWritingMode(),
                                       aItemReflowInput.ComputedMinISize(),
                                       aItemReflowInput.ComputedMinBSize());
  }

  // Indefinite cross-size.
  return NS_AUTOHEIGHT;
}

// Convenience function; returns a main-size, given a cross-size and an
// intrinsic ratio. The intrinsic ratio must not have 0 in its cross-axis
// component (or else we'll divide by 0).
static nscoord
MainSizeFromAspectRatio(nscoord aCrossSize,
                        const nsSize& aIntrinsicRatio,
                        const FlexboxAxisTracker& aAxisTracker)
{
  MOZ_ASSERT(aAxisTracker.GetCrossComponent(aIntrinsicRatio) != 0,
             "Invalid ratio; will divide by 0! Caller should've checked...");

  if (aAxisTracker.IsCrossAxisHorizontal()) {
    // cross axis horiz --> aCrossSize is a width. Converting to height.
    return NSCoordMulDiv(aCrossSize, aIntrinsicRatio.height, aIntrinsicRatio.width);
  }
  // cross axis vert --> aCrossSize is a height. Converting to width.
  return NSCoordMulDiv(aCrossSize, aIntrinsicRatio.width, aIntrinsicRatio.height);
}

// Partially resolves "min-[width|height]:auto" and returns the resulting value.
// By "partially", I mean we don't consider the min-content size (but we do
// consider flex-basis, main max-size, and the intrinsic aspect ratio).
// The caller is responsible for computing & considering the min-content size
// in combination with the partially-resolved value that this function returns.
//
// Spec reference: http://dev.w3.org/csswg/css-flexbox/#min-size-auto
static nscoord
PartiallyResolveAutoMinSize(const FlexItem& aFlexItem,
                            const ReflowInput& aItemReflowInput,
                            const FlexboxAxisTracker& aAxisTracker)
{
  MOZ_ASSERT(aFlexItem.NeedsMinSizeAutoResolution(),
             "only call for FlexItems that need min-size auto resolution");

  nscoord minMainSize = nscoord_MAX; // Intentionally huge; we'll shrink it
                                     // from here, w/ std::min().

  // We need the smallest of:
  // * the used flex-basis, if the computed flex-basis was 'auto':
  // XXXdholbert ('auto' might be renamed to 'main-size'; see bug 1032922)
  if (eStyleUnit_Auto ==
      aItemReflowInput.mStylePosition->mFlexBasis.GetUnit() &&
      aFlexItem.GetFlexBaseSize() != NS_AUTOHEIGHT) {
    // NOTE: We skip this if the flex base size depends on content & isn't yet
    // resolved. This is OK, because the caller is responsible for computing
    // the min-content height and min()'ing it with the value we return, which
    // is equivalent to what would happen if we min()'d that at this point.
    minMainSize = std::min(minMainSize, aFlexItem.GetFlexBaseSize());
  }

  // * the computed max-width (max-height), if that value is definite:
  nscoord maxSize =
    GET_MAIN_COMPONENT_LOGICAL(aAxisTracker, aFlexItem.GetWritingMode(),
                               aItemReflowInput.ComputedMaxISize(),
                               aItemReflowInput.ComputedMaxBSize());
  if (maxSize != NS_UNCONSTRAINEDSIZE) {
    minMainSize = std::min(minMainSize, maxSize);
  }

  // * if the item has no intrinsic aspect ratio, its min-content size:
  //  --- SKIPPING THIS IN THIS FUNCTION --- caller's responsibility.

  // * if the item has an intrinsic aspect ratio, the width (height) calculated
  //   from the aspect ratio and any definite size constraints in the opposite
  //   dimension.
  if (aAxisTracker.GetCrossComponent(aFlexItem.IntrinsicRatio()) != 0) {
    // We have a usable aspect ratio. (not going to divide by 0)
    const bool useMinSizeIfCrossSizeIsIndefinite = true;
    nscoord crossSizeToUseWithRatio =
      CrossSizeToUseWithRatio(aFlexItem, aItemReflowInput,
                              useMinSizeIfCrossSizeIsIndefinite,
                              aAxisTracker);
    nscoord minMainSizeFromRatio =
      MainSizeFromAspectRatio(crossSizeToUseWithRatio,
                              aFlexItem.IntrinsicRatio(), aAxisTracker);
    minMainSize = std::min(minMainSize, minMainSizeFromRatio);
  }

  return minMainSize;
}

// Resolves flex-basis:auto, using the given intrinsic ratio and the flex
// item's cross size.  On success, updates the flex item with its resolved
// flex-basis and returns true. On failure (e.g. if the ratio is invalid or
// the cross-size is indefinite), returns false.
static bool
ResolveAutoFlexBasisFromRatio(FlexItem& aFlexItem,
                              const ReflowInput& aItemReflowInput,
                              const FlexboxAxisTracker& aAxisTracker)
{
  MOZ_ASSERT(NS_AUTOHEIGHT == aFlexItem.GetFlexBaseSize(),
             "Should only be called to resolve an 'auto' flex-basis");
  // If the flex item has ...
  //  - an intrinsic aspect ratio,
  //  - a [used] flex-basis of 'main-size' [auto?] [We have this, if we're here.]
  //  - a definite cross size
  // then the flex base size is calculated from its inner cross size and the
  // flex item’s intrinsic aspect ratio.
  if (aAxisTracker.GetCrossComponent(aFlexItem.IntrinsicRatio()) != 0) {
    // We have a usable aspect ratio. (not going to divide by 0)
    const bool useMinSizeIfCrossSizeIsIndefinite = false;
    nscoord crossSizeToUseWithRatio =
      CrossSizeToUseWithRatio(aFlexItem, aItemReflowInput,
                              useMinSizeIfCrossSizeIsIndefinite,
                              aAxisTracker);
    if (crossSizeToUseWithRatio != NS_AUTOHEIGHT) {
      // We have a definite cross-size
      nscoord mainSizeFromRatio =
        MainSizeFromAspectRatio(crossSizeToUseWithRatio,
                                aFlexItem.IntrinsicRatio(), aAxisTracker);
      aFlexItem.SetFlexBaseSizeAndMainSize(mainSizeFromRatio);
      return true;
    }
  }
  return false;
}

// Note: If & when we handle "min-height: min-content" for flex items,
// we may want to resolve that in this function, too.
void
nsFlexContainerFrame::
  ResolveAutoFlexBasisAndMinSize(nsPresContext* aPresContext,
                                 FlexItem& aFlexItem,
                                 const ReflowInput& aItemReflowInput,
                                 const FlexboxAxisTracker& aAxisTracker)
{
  // (Note: We should never have a used flex-basis of "auto" if our main axis
  // is horizontal; width values should always be resolvable without reflow.)
  const bool isMainSizeAuto = (!aAxisTracker.IsMainAxisHorizontal() &&
                               NS_AUTOHEIGHT == aFlexItem.GetFlexBaseSize());

  const bool isMainMinSizeAuto = aFlexItem.NeedsMinSizeAutoResolution();

  if (!isMainSizeAuto && !isMainMinSizeAuto) {
    // Nothing to do; this function is only needed for flex items
    // with a used flex-basis of "auto" or a min-main-size of "auto".
    return;
  }

  // We may be about to do computations based on our item's cross-size
  // (e.g. using it as a contstraint when measuring our content in the
  // main axis, or using it with the intrinsic ratio to obtain a main size).
  // BEFORE WE DO THAT, we need let the item "pre-stretch" its cross size (if
  // it's got 'align-self:stretch'), for a certain case where the spec says
  // the stretched cross size is considered "definite". That case is if we
  // have a single-line (nowrap) flex container which itself has a definite
  // cross-size.  Otherwise, we'll wait to do stretching, since (in other
  // cases) we don't know how much the item should stretch yet.
  const ReflowInput* flexContainerRI = aItemReflowInput.mParentReflowInput;
  MOZ_ASSERT(flexContainerRI,
             "flex item's reflow state should have ptr to container's state");
  if (NS_STYLE_FLEX_WRAP_NOWRAP == flexContainerRI->mStylePosition->mFlexWrap) {
    // XXXdholbert Maybe this should share logic with ComputeCrossSize()...
    // Alternately, maybe tentative container cross size should be passed down.
    nscoord containerCrossSize =
      GET_CROSS_COMPONENT_LOGICAL(aAxisTracker, aAxisTracker.GetWritingMode(),
                                  flexContainerRI->ComputedISize(),
                                  flexContainerRI->ComputedBSize());
    // Is container's cross size "definite"?
    // (Container's cross size is definite if cross-axis is horizontal, or if
    // cross-axis is vertical and the cross-size is not NS_AUTOHEIGHT.)
    if (aAxisTracker.IsCrossAxisHorizontal() ||
        containerCrossSize != NS_AUTOHEIGHT) {
      aFlexItem.ResolveStretchedCrossSize(containerCrossSize, aAxisTracker);
    }
  }

  nscoord resolvedMinSize; // (only set/used if isMainMinSizeAuto==true)
  bool minSizeNeedsToMeasureContent = false; // assume the best
  if (isMainMinSizeAuto) {
    // Resolve the min-size, except for considering the min-content size.
    // (We'll consider that later, if we need to.)
    resolvedMinSize = PartiallyResolveAutoMinSize(aFlexItem, aItemReflowInput,
                                                  aAxisTracker);
    if (resolvedMinSize > 0 &&
        aAxisTracker.GetCrossComponent(aFlexItem.IntrinsicRatio()) == 0) {
      // We don't have a usable aspect ratio, so we need to consider our
      // min-content size as another candidate min-size, which we'll have to
      // min() with the current resolvedMinSize.
      // (If resolvedMinSize were already at 0, we could skip this measurement
      // because it can't go any lower. But it's not 0, so we need it.)
      minSizeNeedsToMeasureContent = true;
    }
  }

  bool flexBasisNeedsToMeasureContent = false; // assume the best
  if (isMainSizeAuto) {
    if (!ResolveAutoFlexBasisFromRatio(aFlexItem, aItemReflowInput,
                                       aAxisTracker)) {
      flexBasisNeedsToMeasureContent = true;
    }
  }

  // Measure content, if needed (w/ intrinsic-width method or a reflow)
  if (minSizeNeedsToMeasureContent || flexBasisNeedsToMeasureContent) {
    if (aAxisTracker.IsMainAxisHorizontal()) {
      if (minSizeNeedsToMeasureContent) {
        nscoord frameMinISize =
          aFlexItem.Frame()->GetMinISize(aItemReflowInput.mRenderingContext);
        resolvedMinSize = std::min(resolvedMinSize, frameMinISize);
      }
      NS_ASSERTION(!flexBasisNeedsToMeasureContent,
                   "flex-basis:auto should have been resolved in the "
                   "reflow state, for horizontal flexbox. It shouldn't need "
                   "special handling here");
    } else {
      // If this item is flexible (vertically), or if we're measuring the
      // 'auto' min-height and our main-size is something else, then we assume
      // that the computed-height we're reflowing with now could be different
      // from the one we'll use for this flex item's "actual" reflow later on.
      // In that case, we need to be sure the flex item treats this as a
      // vertical resize, even though none of its ancestors are necessarily
      // being vertically resized.
      // (Note: We don't have to do this for width, because InitResizeFlags
      // will always turn on mHResize on when it sees that the computed width
      // is different from current width, and that's all we need.)
      bool forceVerticalResizeForMeasuringReflow =
        !aFlexItem.IsFrozen() ||         // Is the item flexible?
        !flexBasisNeedsToMeasureContent; // Are we *only* measuring it for
                                         // 'min-height:auto'?

      nscoord contentHeight =
        MeasureFlexItemContentHeight(aPresContext, aFlexItem,
                                     forceVerticalResizeForMeasuringReflow,
                                     *flexContainerRI);
      if (minSizeNeedsToMeasureContent) {
        resolvedMinSize = std::min(resolvedMinSize, contentHeight);
      }
      if (flexBasisNeedsToMeasureContent) {
        aFlexItem.SetFlexBaseSizeAndMainSize(contentHeight);
      }
    }
  }

  if (isMainMinSizeAuto) {
    aFlexItem.UpdateMainMinSize(resolvedMinSize);
  }
}

nscoord
nsFlexContainerFrame::
  MeasureFlexItemContentHeight(nsPresContext* aPresContext,
                               FlexItem& aFlexItem,
                               bool aForceVerticalResizeForMeasuringReflow,
                               const ReflowInput& aParentReflowInput)
{
  // Set up a reflow state for measuring the flex item's auto-height:
  WritingMode wm = aFlexItem.Frame()->GetWritingMode();
  LogicalSize availSize = aParentReflowInput.ComputedSize(wm);
  availSize.BSize(wm) = NS_UNCONSTRAINEDSIZE;
  ReflowInput
    childRIForMeasuringHeight(aPresContext, aParentReflowInput,
                              aFlexItem.Frame(), availSize,
                              nullptr, ReflowInput::CALLER_WILL_INIT);
  childRIForMeasuringHeight.mFlags.mIsFlexContainerMeasuringHeight = true;
  childRIForMeasuringHeight.Init(aPresContext);

  if (aFlexItem.IsStretched()) {
    childRIForMeasuringHeight.SetComputedWidth(aFlexItem.GetCrossSize());
    childRIForMeasuringHeight.SetHResize(true);
  }

  if (aForceVerticalResizeForMeasuringReflow) {
    childRIForMeasuringHeight.SetVResize(true);
  }

  ReflowOutput childDesiredSize(childRIForMeasuringHeight);
  nsReflowStatus childReflowStatus;
  const uint32_t flags = NS_FRAME_NO_MOVE_FRAME;
  ReflowChild(aFlexItem.Frame(), aPresContext,
              childDesiredSize, childRIForMeasuringHeight,
              0, 0, flags, childReflowStatus);

  MOZ_ASSERT(NS_FRAME_IS_COMPLETE(childReflowStatus),
             "We gave flex item unconstrained available height, so it "
             "should be complete");

  FinishReflowChild(aFlexItem.Frame(), aPresContext,
                    childDesiredSize, &childRIForMeasuringHeight,
                    0, 0, flags);

  aFlexItem.SetHadMeasuringReflow();
  aFlexItem.SetAscent(childDesiredSize.BlockStartAscent());

  // Subtract border/padding in vertical axis, to get _just_
  // the effective computed value of the "height" property.
  nscoord childDesiredHeight = childDesiredSize.Height() -
    childRIForMeasuringHeight.ComputedPhysicalBorderPadding().TopBottom();

  return std::max(0, childDesiredHeight);
}

FlexItem::FlexItem(ReflowInput& aFlexItemReflowInput,
                   float aFlexGrow, float aFlexShrink, nscoord aFlexBaseSize,
                   nscoord aMainMinSize,  nscoord aMainMaxSize,
                   nscoord aTentativeCrossSize,
                   nscoord aCrossMinSize, nscoord aCrossMaxSize,
                   const FlexboxAxisTracker& aAxisTracker)
  : mFrame(aFlexItemReflowInput.mFrame),
    mFlexGrow(aFlexGrow),
    mFlexShrink(aFlexShrink),
    mIntrinsicRatio(mFrame->GetIntrinsicRatio()),
    mBorderPadding(aFlexItemReflowInput.ComputedPhysicalBorderPadding()),
    mMargin(aFlexItemReflowInput.ComputedPhysicalMargin()),
    mMainMinSize(aMainMinSize),
    mMainMaxSize(aMainMaxSize),
    mCrossMinSize(aCrossMinSize),
    mCrossMaxSize(aCrossMaxSize),
    mMainPosn(0),
    mCrossSize(aTentativeCrossSize),
    mCrossPosn(0),
    mAscent(0),
    mShareOfWeightSoFar(0.0f),
    mIsFrozen(false),
    mHadMinViolation(false),
    mHadMaxViolation(false),
    mHadMeasuringReflow(false),
    mIsStretched(false),
    mIsStrut(false),
    // mNeedsMinSizeAutoResolution is initialized in CheckForMinSizeAuto()
    mWM(aFlexItemReflowInput.GetWritingMode())
    // mAlignSelf, see below
{
  MOZ_ASSERT(mFrame, "expecting a non-null child frame");
  MOZ_ASSERT(mFrame->GetType() != nsGkAtoms::placeholderFrame,
             "placeholder frames should not be treated as flex items");
  MOZ_ASSERT(!(mFrame->GetStateBits() & NS_FRAME_OUT_OF_FLOW),
             "out-of-flow frames should not be treated as flex items");

  const ReflowInput* containerRS = aFlexItemReflowInput.mParentReflowInput;
  if (IsLegacyBox(containerRS->mFrame)) {
    // For -webkit-box/-webkit-inline-box, we need to:
    // (1) Use "-webkit-box-align" instead of "align-items" to determine the
    //     container's cross-axis alignment behavior.
    // (2) Suppress the ability for flex items to override that with their own
    //     cross-axis alignment. (The legacy box model doesn't support this.)
    // So, each FlexItem simply copies the container's converted "align-items"
    // value and disregards their own "align-self" property.
    const nsStyleXUL* containerStyleXUL = containerRS->mFrame->StyleXUL();
    mAlignSelf = ConvertLegacyStyleToAlignItems(containerStyleXUL);
  } else {
    mAlignSelf = aFlexItemReflowInput.mStylePosition->UsedAlignSelf(
                   containerRS->mFrame->StyleContext());
    if (MOZ_LIKELY(mAlignSelf == NS_STYLE_ALIGN_NORMAL)) {
      mAlignSelf = NS_STYLE_ALIGN_STRETCH;
    }

    // XXX strip off the <overflow-position> bit until we implement that
    mAlignSelf &= ~NS_STYLE_ALIGN_FLAG_BITS;
  }

  SetFlexBaseSizeAndMainSize(aFlexBaseSize);
  CheckForMinSizeAuto(aFlexItemReflowInput, aAxisTracker);

  // Assert that any "auto" margin components are set to 0.
  // (We'll resolve them later; until then, we want to treat them as 0-sized.)
#ifdef DEBUG
  {
    const nsStyleSides& styleMargin =
      aFlexItemReflowInput.mStyleMargin->mMargin;
    NS_FOR_CSS_SIDES(side) {
      if (styleMargin.GetUnit(side) == eStyleUnit_Auto) {
        MOZ_ASSERT(GetMarginComponentForSide(side) == 0,
                   "Someone else tried to resolve our auto margin");
      }
    }
  }
#endif // DEBUG

  // If the flex item's inline axis is the same as the cross axis, then
  // 'align-self:baseline' is identical to 'flex-start'. If that's the case, we
  // just directly convert our align-self value here, so that we don't have to
  // handle this with special cases elsewhere.
  // Moreover: for the time being (until we support writing-modes),
  // all inline axes are horizontal -- so we can just check if the cross axis
  // is horizontal.
  // FIXME: Once we support writing-mode (vertical text), this
  // IsCrossAxisHorizontal check won't be sufficient anymore -- we'll actually
  // need to compare our inline axis vs. the cross axis.
  if (mAlignSelf == NS_STYLE_ALIGN_BASELINE &&
      aAxisTracker.IsCrossAxisHorizontal()) {
    mAlignSelf = NS_STYLE_ALIGN_FLEX_START;
  }
}

// Simplified constructor for creating a special "strut" FlexItem, for a child
// with visibility:collapse. The strut has 0 main-size, and it only exists to
// impose a minimum cross size on whichever FlexLine it ends up in.
FlexItem::FlexItem(nsIFrame* aChildFrame, nscoord aCrossSize,
                   WritingMode aContainerWM)
  : mFrame(aChildFrame),
    mFlexGrow(0.0f),
    mFlexShrink(0.0f),
    mIntrinsicRatio(),
    // mBorderPadding uses default constructor,
    // mMargin uses default constructor,
    mFlexBaseSize(0),
    mMainMinSize(0),
    mMainMaxSize(0),
    mCrossMinSize(0),
    mCrossMaxSize(0),
    mMainSize(0),
    mMainPosn(0),
    mCrossSize(aCrossSize),
    mCrossPosn(0),
    mAscent(0),
    mShareOfWeightSoFar(0.0f),
    mIsFrozen(true),
    mHadMinViolation(false),
    mHadMaxViolation(false),
    mHadMeasuringReflow(false),
    mIsStretched(false),
    mIsStrut(true), // (this is the constructor for making struts, after all)
    mNeedsMinSizeAutoResolution(false),
    mWM(aContainerWM),
    mAlignSelf(NS_STYLE_ALIGN_FLEX_START)
{
  MOZ_ASSERT(mFrame, "expecting a non-null child frame");
  MOZ_ASSERT(NS_STYLE_VISIBILITY_COLLAPSE ==
             mFrame->StyleVisibility()->mVisible,
             "Should only make struts for children with 'visibility:collapse'");
  MOZ_ASSERT(mFrame->GetType() != nsGkAtoms::placeholderFrame,
             "placeholder frames should not be treated as flex items");
  MOZ_ASSERT(!(mFrame->GetStateBits() & NS_FRAME_OUT_OF_FLOW),
             "out-of-flow frames should not be treated as flex items");
}

void
FlexItem::CheckForMinSizeAuto(const ReflowInput& aFlexItemReflowInput,
                              const FlexboxAxisTracker& aAxisTracker)
{
  const nsStylePosition* pos = aFlexItemReflowInput.mStylePosition;
  const nsStyleDisplay* disp = aFlexItemReflowInput.mStyleDisplay;

  // We'll need special behavior for "min-[width|height]:auto" (whichever is in
  // the main axis) iff:
  // (a) its computed value is "auto"
  // (b) the "overflow" sub-property in the same axis (the main axis) has a
  //     computed value of "visible"
  const nsStyleCoord& minSize = GET_MAIN_COMPONENT(aAxisTracker,
                                                   pos->mMinWidth,
                                                   pos->mMinHeight);

  const uint8_t overflowVal = GET_MAIN_COMPONENT(aAxisTracker,
                                                 disp->mOverflowX,
                                                 disp->mOverflowY);

  mNeedsMinSizeAutoResolution = (minSize.GetUnit() == eStyleUnit_Auto &&
                                 overflowVal == NS_STYLE_OVERFLOW_VISIBLE);
}

nscoord
FlexItem::GetBaselineOffsetFromOuterCrossEdge(
  AxisEdgeType aEdge,
  const FlexboxAxisTracker& aAxisTracker) const
{
  // NOTE: Currently, 'mAscent' (taken from reflow) is an inherently vertical
  // measurement -- it's the distance from the border-top edge of this FlexItem
  // to its baseline. So, we can really only do baseline alignment when the
  // cross axis is vertical. (The FlexItem constructor enforces this when
  // resolving the item's "mAlignSelf" value).
  MOZ_ASSERT(!aAxisTracker.IsCrossAxisHorizontal(),
             "Only expecting to be doing baseline computations when the "
             "cross axis is vertical");

  AxisOrientationType crossAxis = aAxisTracker.GetCrossAxis();
  mozilla::Side sideToMeasureFrom = kAxisOrientationToSidesMap[crossAxis][aEdge];

  nscoord marginTopToBaseline = ResolvedAscent() + mMargin.top;

  if (sideToMeasureFrom == eSideTop) {
    // Measuring from top (normal case): the distance from the margin-box top
    // edge to the baseline is just ascent + margin-top.
    return marginTopToBaseline;
  }

  MOZ_ASSERT(sideToMeasureFrom == eSideBottom,
             "We already checked that we're dealing with a vertical axis, and "
             "we're not using the top side, so that only leaves the bottom...");

  // Measuring from bottom: The distance from the margin-box bottom edge to the
  // baseline is just the margin-box cross size (i.e. outer cross size), minus
  // the already-computed distance from margin-top to baseline.
  return GetOuterCrossSize(crossAxis) - marginTopToBaseline;
}

uint32_t
FlexItem::GetNumAutoMarginsInAxis(AxisOrientationType aAxis) const
{
  uint32_t numAutoMargins = 0;
  const nsStyleSides& styleMargin = mFrame->StyleMargin()->mMargin;
  for (uint32_t i = 0; i < eNumAxisEdges; i++) {
    mozilla::Side side = kAxisOrientationToSidesMap[aAxis][i];
    if (styleMargin.GetUnit(side) == eStyleUnit_Auto) {
      numAutoMargins++;
    }
  }

  // Mostly for clarity:
  MOZ_ASSERT(numAutoMargins <= 2,
             "We're just looking at one item along one dimension, so we "
             "should only have examined 2 margins");

  return numAutoMargins;
}

bool
FlexItem::CanMainSizeInfluenceCrossSize(
  const FlexboxAxisTracker& aAxisTracker) const
{
  if (mIsStretched) {
    // We've already had our cross-size stretched for "align-self:stretch").
    // The container is imposing its cross size on us.
    return false;
  }

  if (mIsStrut) {
    // Struts (for visibility:collapse items) have a predetermined size;
    // no need to measure anything.
    return false;
  }

  if (HasIntrinsicRatio()) {
    // For flex items that have an intrinsic ratio (and maintain it, i.e. are
    // not stretched, which we already checked above): changes to main-size
    // *do* influence the cross size.
    return true;
  }

  if (aAxisTracker.IsCrossAxisHorizontal()) {
    // If the cross axis is horizontal, then changes to the item's main size
    // (height) can't influence its cross size (width), if the item is a block
    // with a horizontal writing-mode.
    // XXXdholbert This doesn't account for vertical writing-modes, items with
    // aspect ratios, items that are multicol elements, & items that are
    // multi-line vertical flex containers. In all of those cases, a change to
    // the height could influence the width.
    return false;
  }

  // Default assumption, if we haven't proven otherwise: the resolved main size
  // *can* change the cross size.
  return true;
}

// Keeps track of our position along a particular axis (where a '0' position
// corresponds to the 'start' edge of that axis).
// This class shouldn't be instantiated directly -- rather, it should only be
// instantiated via its subclasses defined below.
class MOZ_STACK_CLASS PositionTracker {
public:
  // Accessor for the current value of the position that we're tracking.
  inline nscoord GetPosition() const { return mPosition; }
  inline AxisOrientationType GetAxis() const { return mAxis; }

  // Advances our position across the start edge of the given margin, in the
  // axis we're tracking.
  void EnterMargin(const nsMargin& aMargin)
  {
    mozilla::Side side = kAxisOrientationToSidesMap[mAxis][eAxisEdge_Start];
    mPosition += aMargin.Side(side);
  }

  // Advances our position across the end edge of the given margin, in the axis
  // we're tracking.
  void ExitMargin(const nsMargin& aMargin)
  {
    mozilla::Side side = kAxisOrientationToSidesMap[mAxis][eAxisEdge_End];
    mPosition += aMargin.Side(side);
  }

  // Advances our current position from the start side of a child frame's
  // border-box to the frame's upper or left edge (depending on our axis).
  // (Note that this is a no-op if our axis grows in the same direction as
  // the corresponding logical axis.)
  void EnterChildFrame(nscoord aChildFrameSize)
  {
    if (mIsAxisReversed) {
      mPosition += aChildFrameSize;
    }
  }

  // Advances our current position from a frame's upper or left border-box edge
  // (whichever is in the axis we're tracking) to the 'end' side of the frame
  // in the axis that we're tracking. (Note that this is a no-op if our axis
  // is reversed with respect to the corresponding logical axis.)
  void ExitChildFrame(nscoord aChildFrameSize)
  {
    if (!mIsAxisReversed) {
      mPosition += aChildFrameSize;
    }
  }

protected:
  // Protected constructor, to be sure we're only instantiated via a subclass.
  PositionTracker(AxisOrientationType aAxis, bool aIsAxisReversed)
    : mPosition(0),
      mAxis(aAxis),
      mIsAxisReversed(aIsAxisReversed)
  {}

  // Delete copy-constructor & reassignment operator, to prevent accidental
  // (unnecessary) copying.
  PositionTracker(const PositionTracker&) = delete;
  PositionTracker& operator=(const PositionTracker&) = delete;

  // Member data:
  nscoord mPosition;               // The position we're tracking
  // XXXdholbert [BEGIN DEPRECATED]
  const AxisOrientationType mAxis; // The axis along which we're moving.
  // XXXdholbert [END DEPRECATED]
  const bool mIsAxisReversed; // Is the axis along which we're moving reversed
                              // (e.g. LTR vs RTL) with respect to the
                              // corresponding axis on the flex container's WM?
};

// Tracks our position in the main axis, when we're laying out flex items.
// The "0" position represents the main-start edge of the flex container's
// content-box.
class MOZ_STACK_CLASS MainAxisPositionTracker : public PositionTracker {
public:
  MainAxisPositionTracker(const FlexboxAxisTracker& aAxisTracker,
                          const FlexLine* aLine,
                          uint8_t aJustifyContent,
                          nscoord aContentBoxMainSize);

  ~MainAxisPositionTracker() {
    MOZ_ASSERT(mNumPackingSpacesRemaining == 0,
               "miscounted the number of packing spaces");
    MOZ_ASSERT(mNumAutoMarginsInMainAxis == 0,
               "miscounted the number of auto margins");
  }

  // Advances past the packing space (if any) between two flex items
  void TraversePackingSpace();

  // If aItem has any 'auto' margins in the main axis, this method updates the
  // corresponding values in its margin.
  void ResolveAutoMarginsInMainAxis(FlexItem& aItem);

private:
  nscoord  mPackingSpaceRemaining;
  uint32_t mNumAutoMarginsInMainAxis;
  uint32_t mNumPackingSpacesRemaining;
  // XXX this should be uint16_t when we add explicit fallback handling
  uint8_t  mJustifyContent;
};

// Utility class for managing our position along the cross axis along
// the whole flex container (at a higher level than a single line).
// The "0" position represents the cross-start edge of the flex container's
// content-box.
class MOZ_STACK_CLASS CrossAxisPositionTracker : public PositionTracker {
public:
  CrossAxisPositionTracker(FlexLine* aFirstLine,
                           const ReflowInput& aReflowInput,
                           nscoord aContentBoxCrossSize,
                           bool aIsCrossSizeDefinite,
                           const FlexboxAxisTracker& aAxisTracker);

  // Advances past the packing space (if any) between two flex lines
  void TraversePackingSpace();

  // Advances past the given FlexLine
  void TraverseLine(FlexLine& aLine) { mPosition += aLine.GetLineCrossSize(); }

private:
  // Redeclare the frame-related methods from PositionTracker as private with
  // = delete, to be sure (at compile time) that no client code can invoke
  // them. (Unlike the other PositionTracker derived classes, this class here
  // deals with FlexLines, not with individual FlexItems or frames.)
  void EnterMargin(const nsMargin& aMargin) = delete;
  void ExitMargin(const nsMargin& aMargin) = delete;
  void EnterChildFrame(nscoord aChildFrameSize) = delete;
  void ExitChildFrame(nscoord aChildFrameSize) = delete;

  nscoord  mPackingSpaceRemaining;
  uint32_t mNumPackingSpacesRemaining;
  // XXX this should be uint16_t when we add explicit fallback handling
  uint8_t  mAlignContent;
};

// Utility class for managing our position along the cross axis, *within* a
// single flex line.
class MOZ_STACK_CLASS SingleLineCrossAxisPositionTracker : public PositionTracker {
public:
  explicit SingleLineCrossAxisPositionTracker(const FlexboxAxisTracker& aAxisTracker);

  void ResolveAutoMarginsInCrossAxis(const FlexLine& aLine,
                                     FlexItem& aItem);

  void EnterAlignPackingSpace(const FlexLine& aLine,
                              const FlexItem& aItem,
                              const FlexboxAxisTracker& aAxisTracker);

  // Resets our position to the cross-start edge of this line.
  inline void ResetPosition() { mPosition = 0; }
};

//----------------------------------------------------------------------

// Frame class boilerplate
// =======================

NS_QUERYFRAME_HEAD(nsFlexContainerFrame)
  NS_QUERYFRAME_ENTRY(nsFlexContainerFrame)
NS_QUERYFRAME_TAIL_INHERITING(nsContainerFrame)

NS_IMPL_FRAMEARENA_HELPERS(nsFlexContainerFrame)

nsContainerFrame*
NS_NewFlexContainerFrame(nsIPresShell* aPresShell,
                         nsStyleContext* aContext)
{
  return new (aPresShell) nsFlexContainerFrame(aContext);
}

//----------------------------------------------------------------------

// nsFlexContainerFrame Method Implementations
// ===========================================

/* virtual */
nsFlexContainerFrame::~nsFlexContainerFrame()
{
}

template<bool IsLessThanOrEqual(nsIFrame*, nsIFrame*)>
/* static */ bool
nsFlexContainerFrame::SortChildrenIfNeeded()
{
  if (nsIFrame::IsFrameListSorted<IsLessThanOrEqual>(mFrames)) {
    return false;
  }

  nsIFrame::SortFrameList<IsLessThanOrEqual>(mFrames);
  return true;
}

/* virtual */
nsIAtom*
nsFlexContainerFrame::GetType() const
{
  return nsGkAtoms::flexContainerFrame;
}

#ifdef DEBUG_FRAME_DUMP
nsresult
nsFlexContainerFrame::GetFrameName(nsAString& aResult) const
{
  return MakeFrameName(NS_LITERAL_STRING("FlexContainer"), aResult);
}
#endif

nscoord
nsFlexContainerFrame::GetLogicalBaseline(mozilla::WritingMode aWM) const
{
  NS_ASSERTION(mBaselineFromLastReflow != NS_INTRINSIC_WIDTH_UNKNOWN,
               "baseline has not been set");

  return mBaselineFromLastReflow;
}

// Helper for BuildDisplayList, to implement this special-case for flex items
// from the spec:
//    Flex items paint exactly the same as block-level elements in the
//    normal flow, except that 'z-index' values other than 'auto' create
//    a stacking context even if 'position' is 'static'.
// http://www.w3.org/TR/2012/CR-css3-flexbox-20120918/#painting
uint32_t
GetDisplayFlagsForFlexItem(nsIFrame* aFrame)
{
  MOZ_ASSERT(aFrame->IsFlexItem(), "Should only be called on flex items");

  const nsStylePosition* pos = aFrame->StylePosition();
  if (pos->mZIndex.GetUnit() == eStyleUnit_Integer) {
    return nsIFrame::DISPLAY_CHILD_FORCE_STACKING_CONTEXT;
  }
  return nsIFrame::DISPLAY_CHILD_FORCE_PSEUDO_STACKING_CONTEXT;
}

void
nsFlexContainerFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
                                       const nsRect&           aDirtyRect,
                                       const nsDisplayListSet& aLists)
{
  // XXXdholbert hacky temporary band-aid for bug 1059138: Trivially pass this
  // assertion (skip it, basically) if the first child is part of a shadow DOM.
  // (IsOrderLEQWithDOMFallback doesn't know how to compare tree-position of a
  // shadow-DOM element vs. a non-shadow-DOM element.)
  NS_ASSERTION(
    (!mFrames.IsEmpty() &&
     mFrames.FirstChild()->GetContent()->GetContainingShadow()) ||
    nsIFrame::IsFrameListSorted<IsOrderLEQWithDOMFallback>(mFrames),
    "Child frames aren't sorted correctly");

  DisplayBorderBackgroundOutline(aBuilder, aLists);

  // Our children are all block-level, so their borders/backgrounds all go on
  // the BlockBorderBackgrounds list.
  nsDisplayListSet childLists(aLists, aLists.BlockBorderBackgrounds());
  for (nsIFrame* childFrame : mFrames) {
    BuildDisplayListForChild(aBuilder, childFrame, aDirtyRect, childLists,
                             GetDisplayFlagsForFlexItem(childFrame));
  }
}

void
FlexLine::FreezeItemsEarly(bool aIsUsingFlexGrow)
{
  // After we've established the type of flexing we're doing (growing vs.
  // shrinking), and before we try to flex any items, we freeze items that
  // obviously *can't* flex.
  //
  // Quoting the spec:
  //  # Freeze, setting its target main size to its hypothetical main size...
  //  #  - any item that has a flex factor of zero
  //  #  - if using the flex grow factor: any item that has a flex base size
  //  #    greater than its hypothetical main size
  //  #  - if using the flex shrink factor: any item that has a flex base size
  //  #    smaller than its hypothetical main size
  //  http://dev.w3.org/csswg/css-flexbox/#resolve-flexible-lengths-flex-factors
  //
  // (NOTE: At this point, item->GetMainSize() *is* the item's hypothetical
  // main size, since SetFlexBaseSizeAndMainSize() sets it up that way, and the
  // item hasn't had a chance to flex away from that yet.)

  // Since this loop only operates on unfrozen flex items, we can break as
  // soon as we have seen all of them.
  uint32_t numUnfrozenItemsToBeSeen = mNumItems - mNumFrozenItems;
  for (FlexItem* item = mItems.getFirst();
       numUnfrozenItemsToBeSeen > 0; item = item->getNext()) {
    MOZ_ASSERT(item, "numUnfrozenItemsToBeSeen says items remain to be seen");

    if (!item->IsFrozen()) {
      numUnfrozenItemsToBeSeen--;
      bool shouldFreeze = (0.0f == item->GetFlexFactor(aIsUsingFlexGrow));
      if (!shouldFreeze) {
        if (aIsUsingFlexGrow) {
          if (item->GetFlexBaseSize() > item->GetMainSize()) {
            shouldFreeze = true;
          }
        } else { // using flex-shrink
          if (item->GetFlexBaseSize() < item->GetMainSize()) {
            shouldFreeze = true;
          }
        }
      }
      if (shouldFreeze) {
        // Freeze item! (at its hypothetical main size)
        item->Freeze();
        mNumFrozenItems++;
      }
    }
  }
}

// Based on the sign of aTotalViolation, this function freezes a subset of our
// flexible sizes, and restores the remaining ones to their initial pref sizes.
void
FlexLine::FreezeOrRestoreEachFlexibleSize(const nscoord aTotalViolation,
                                          bool aIsFinalIteration)
{
  enum FreezeType {
    eFreezeEverything,
    eFreezeMinViolations,
    eFreezeMaxViolations
  };

  FreezeType freezeType;
  if (aTotalViolation == 0) {
    freezeType = eFreezeEverything;
  } else if (aTotalViolation > 0) {
    freezeType = eFreezeMinViolations;
  } else { // aTotalViolation < 0
    freezeType = eFreezeMaxViolations;
  }

  // Since this loop only operates on unfrozen flex items, we can break as
  // soon as we have seen all of them.
  uint32_t numUnfrozenItemsToBeSeen = mNumItems - mNumFrozenItems;
  for (FlexItem* item = mItems.getFirst();
       numUnfrozenItemsToBeSeen > 0; item = item->getNext()) {
    MOZ_ASSERT(item, "numUnfrozenItemsToBeSeen says items remain to be seen");
    if (!item->IsFrozen()) {
      numUnfrozenItemsToBeSeen--;

      MOZ_ASSERT(!item->HadMinViolation() || !item->HadMaxViolation(),
                 "Can have either min or max violation, but not both");

      if (eFreezeEverything == freezeType ||
          (eFreezeMinViolations == freezeType && item->HadMinViolation()) ||
          (eFreezeMaxViolations == freezeType && item->HadMaxViolation())) {

        MOZ_ASSERT(item->GetMainSize() >= item->GetMainMinSize(),
                   "Freezing item at a size below its minimum");
        MOZ_ASSERT(item->GetMainSize() <= item->GetMainMaxSize(),
                   "Freezing item at a size above its maximum");

        item->Freeze();
        mNumFrozenItems++;
      } else if (MOZ_UNLIKELY(aIsFinalIteration)) {
        // XXXdholbert If & when bug 765861 is fixed, we should upgrade this
        // assertion to be fatal except in documents with enormous lengths.
        NS_ERROR("Final iteration still has unfrozen items, this shouldn't"
                 " happen unless there was nscoord under/overflow.");
        item->Freeze();
        mNumFrozenItems++;
      } // else, we'll reset this item's main size to its flex base size on the
        // next iteration of this algorithm.

      // Clear this item's violation(s), now that we've dealt with them
      item->ClearViolationFlags();
    }
  }
}

void
FlexLine::ResolveFlexibleLengths(nscoord aFlexContainerMainSize)
{
  MOZ_LOG(gFlexContainerLog, LogLevel::Debug, ("ResolveFlexibleLengths\n"));

  // Determine whether we're going to be growing or shrinking items.
  const bool isUsingFlexGrow =
    (mTotalOuterHypotheticalMainSize < aFlexContainerMainSize);

  // Do an "early freeze" for flex items that obviously can't flex in the
  // direction we've chosen:
  FreezeItemsEarly(isUsingFlexGrow);

  if (mNumFrozenItems == mNumItems) {
    // All our items are frozen, so we have no flexible lengths to resolve.
    return;
  }
  MOZ_ASSERT(!IsEmpty(), "empty lines should take the early-return above");

  // Subtract space occupied by our items' margins/borders/padding, so we can
  // just be dealing with the space available for our flex items' content
  // boxes.
  nscoord spaceReservedForMarginBorderPadding =
    mTotalOuterHypotheticalMainSize - mTotalInnerHypotheticalMainSize;

  nscoord spaceAvailableForFlexItemsContentBoxes =
    aFlexContainerMainSize - spaceReservedForMarginBorderPadding;

  nscoord origAvailableFreeSpace;
  bool isOrigAvailFreeSpaceInitialized = false;

  // NOTE: I claim that this chunk of the algorithm (the looping part) needs to
  // run the loop at MOST mNumItems times.  This claim should hold up
  // because we'll freeze at least one item on each loop iteration, and once
  // we've run out of items to freeze, there's nothing left to do.  However,
  // in most cases, we'll break out of this loop long before we hit that many
  // iterations.
  for (uint32_t iterationCounter = 0;
       iterationCounter < mNumItems; iterationCounter++) {
    // Set every not-yet-frozen item's used main size to its
    // flex base size, and subtract all the used main sizes from our
    // total amount of space to determine the 'available free space'
    // (positive or negative) to be distributed among our flexible items.
    nscoord availableFreeSpace = spaceAvailableForFlexItemsContentBoxes;
    for (FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
      if (!item->IsFrozen()) {
        item->SetMainSize(item->GetFlexBaseSize());
      }
      availableFreeSpace -= item->GetMainSize();
    }

    MOZ_LOG(gFlexContainerLog, LogLevel::Debug,
           (" available free space = %d\n", availableFreeSpace));


    // The sign of our free space should agree with the type of flexing
    // (grow/shrink) that we're doing (except if we've had integer overflow;
    // then, all bets are off). Any disagreement should've made us use the
    // other type of flexing, or should've been resolved in FreezeItemsEarly.
    // XXXdholbert If & when bug 765861 is fixed, we should upgrade this
    // assertion to be fatal except in documents with enormous lengths.
    NS_ASSERTION((isUsingFlexGrow && availableFreeSpace >= 0) ||
                 (!isUsingFlexGrow && availableFreeSpace <= 0),
                 "availableFreeSpace's sign should match isUsingFlexGrow");

    // If we have any free space available, give each flexible item a portion
    // of availableFreeSpace.
    if (availableFreeSpace != 0) {
      // The first time we do this, we initialize origAvailableFreeSpace.
      if (!isOrigAvailFreeSpaceInitialized) {
        origAvailableFreeSpace = availableFreeSpace;
        isOrigAvailFreeSpaceInitialized = true;
      }

      // STRATEGY: On each item, we compute & store its "share" of the total
      // weight that we've seen so far:
      //   curWeight / weightSum
      //
      // Then, when we go to actually distribute the space (in the next loop),
      // we can simply walk backwards through the elements and give each item
      // its "share" multiplied by the remaining available space.
      //
      // SPECIAL CASE: If the sum of the weights is larger than the
      // maximum representable float (overflowing to infinity), then we can't
      // sensibly divide out proportional shares anymore. In that case, we
      // simply treat the flex item(s) with the largest weights as if
      // their weights were infinite (dwarfing all the others), and we
      // distribute all of the available space among them.
      float weightSum = 0.0f;
      float flexFactorSum = 0.0f;
      float largestWeight = 0.0f;
      uint32_t numItemsWithLargestWeight = 0;

      // Since this loop only operates on unfrozen flex items, we can break as
      // soon as we have seen all of them.
      uint32_t numUnfrozenItemsToBeSeen = mNumItems - mNumFrozenItems;
      for (FlexItem* item = mItems.getFirst();
           numUnfrozenItemsToBeSeen > 0; item = item->getNext()) {
        MOZ_ASSERT(item,
                   "numUnfrozenItemsToBeSeen says items remain to be seen");
        if (!item->IsFrozen()) {
          numUnfrozenItemsToBeSeen--;

          float curWeight = item->GetWeight(isUsingFlexGrow);
          float curFlexFactor = item->GetFlexFactor(isUsingFlexGrow);
          MOZ_ASSERT(curWeight >= 0.0f, "weights are non-negative");
          MOZ_ASSERT(curFlexFactor >= 0.0f, "flex factors are non-negative");

          weightSum += curWeight;
          flexFactorSum += curFlexFactor;

          if (IsFinite(weightSum)) {
            if (curWeight == 0.0f) {
              item->SetShareOfWeightSoFar(0.0f);
            } else {
              item->SetShareOfWeightSoFar(curWeight / weightSum);
            }
          } // else, the sum of weights overflows to infinity, in which
            // case we don't bother with "SetShareOfWeightSoFar" since
            // we know we won't use it. (instead, we'll just give every
            // item with the largest weight an equal share of space.)

          // Update our largest-weight tracking vars
          if (curWeight > largestWeight) {
            largestWeight = curWeight;
            numItemsWithLargestWeight = 1;
          } else if (curWeight == largestWeight) {
            numItemsWithLargestWeight++;
          }
        }
      }

      if (weightSum != 0.0f) {
        MOZ_ASSERT(flexFactorSum != 0.0f,
                   "flex factor sum can't be 0, if a weighted sum "
                   "of its components (weightSum) is nonzero");
        if (flexFactorSum < 1.0f) {
          // Our unfrozen flex items don't want all of the original free space!
          // (Their flex factors add up to something less than 1.)
          // Hence, make sure we don't distribute any more than the portion of
          // our original free space that these items actually want.
          nscoord totalDesiredPortionOfOrigFreeSpace =
            NSToCoordRound(origAvailableFreeSpace * flexFactorSum);

          // Clamp availableFreeSpace to be no larger than that ^^.
          // (using min or max, depending on sign).
          // This should not change the sign of availableFreeSpace (except
          // possibly by setting it to 0), as enforced by this assertion:
          MOZ_ASSERT(totalDesiredPortionOfOrigFreeSpace == 0 ||
                     ((totalDesiredPortionOfOrigFreeSpace > 0) ==
                      (availableFreeSpace > 0)),
                     "When we reduce available free space for flex factors < 1,"
                     "we shouldn't change the sign of the free space...");

          if (availableFreeSpace > 0) {
            availableFreeSpace = std::min(availableFreeSpace,
                                          totalDesiredPortionOfOrigFreeSpace);
          } else {
            availableFreeSpace = std::max(availableFreeSpace,
                                          totalDesiredPortionOfOrigFreeSpace);
          }
        }

        MOZ_LOG(gFlexContainerLog, LogLevel::Debug,
               (" Distributing available space:"));
        // Since this loop only operates on unfrozen flex items, we can break as
        // soon as we have seen all of them.
        numUnfrozenItemsToBeSeen = mNumItems - mNumFrozenItems;

        // NOTE: It's important that we traverse our items in *reverse* order
        // here, for correct width distribution according to the items'
        // "ShareOfWeightSoFar" progressively-calculated values.
        for (FlexItem* item = mItems.getLast();
             numUnfrozenItemsToBeSeen > 0; item = item->getPrevious()) {
          MOZ_ASSERT(item,
                     "numUnfrozenItemsToBeSeen says items remain to be seen");
          if (!item->IsFrozen()) {
            numUnfrozenItemsToBeSeen--;

            // To avoid rounding issues, we compute the change in size for this
            // item, and then subtract it from the remaining available space.
            nscoord sizeDelta = 0;
            if (IsFinite(weightSum)) {
              float myShareOfRemainingSpace =
                item->GetShareOfWeightSoFar();

              MOZ_ASSERT(myShareOfRemainingSpace >= 0.0f &&
                         myShareOfRemainingSpace <= 1.0f,
                         "my share should be nonnegative fractional amount");

              if (myShareOfRemainingSpace == 1.0f) {
                // (We special-case 1.0f to avoid float error from converting
                // availableFreeSpace from integer*1.0f --> float --> integer)
                sizeDelta = availableFreeSpace;
              } else if (myShareOfRemainingSpace > 0.0f) {
                sizeDelta = NSToCoordRound(availableFreeSpace *
                                           myShareOfRemainingSpace);
              }
            } else if (item->GetWeight(isUsingFlexGrow) == largestWeight) {
              // Total flexibility is infinite, so we're just distributing
              // the available space equally among the items that are tied for
              // having the largest weight (and this is one of those items).
              sizeDelta =
                NSToCoordRound(availableFreeSpace /
                               float(numItemsWithLargestWeight));
              numItemsWithLargestWeight--;
            }

            availableFreeSpace -= sizeDelta;

            item->SetMainSize(item->GetMainSize() + sizeDelta);
            MOZ_LOG(gFlexContainerLog, LogLevel::Debug,
                   ("  child %p receives %d, for a total of %d\n",
                    item, sizeDelta, item->GetMainSize()));
          }
        }
      }
    }

    // Fix min/max violations:
    nscoord totalViolation = 0; // keeps track of adjustments for min/max
    MOZ_LOG(gFlexContainerLog, LogLevel::Debug,
           (" Checking for violations:"));

    // Since this loop only operates on unfrozen flex items, we can break as
    // soon as we have seen all of them.
    uint32_t numUnfrozenItemsToBeSeen = mNumItems - mNumFrozenItems;
    for (FlexItem* item = mItems.getFirst();
         numUnfrozenItemsToBeSeen > 0; item = item->getNext()) {
      MOZ_ASSERT(item, "numUnfrozenItemsToBeSeen says items remain to be seen");
      if (!item->IsFrozen()) {
        numUnfrozenItemsToBeSeen--;

        if (item->GetMainSize() < item->GetMainMinSize()) {
          // min violation
          totalViolation += item->GetMainMinSize() - item->GetMainSize();
          item->SetMainSize(item->GetMainMinSize());
          item->SetHadMinViolation();
        } else if (item->GetMainSize() > item->GetMainMaxSize()) {
          // max violation
          totalViolation += item->GetMainMaxSize() - item->GetMainSize();
          item->SetMainSize(item->GetMainMaxSize());
          item->SetHadMaxViolation();
        }
      }
    }

    FreezeOrRestoreEachFlexibleSize(totalViolation,
                                    iterationCounter + 1 == mNumItems);

    MOZ_LOG(gFlexContainerLog, LogLevel::Debug,
           (" Total violation: %d\n", totalViolation));

    if (mNumFrozenItems == mNumItems) {
      break;
    }

    MOZ_ASSERT(totalViolation != 0,
               "Zero violation should've made us freeze all items & break");
  }

#ifdef DEBUG
  // Post-condition: all items should've been frozen.
  // Make sure the counts match:
  MOZ_ASSERT(mNumFrozenItems == mNumItems, "All items should be frozen");

  // For good measure, check each item directly, in case our counts are busted:
  for (const FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
    MOZ_ASSERT(item->IsFrozen(), "All items should be frozen");
  }
#endif // DEBUG
}

MainAxisPositionTracker::
  MainAxisPositionTracker(const FlexboxAxisTracker& aAxisTracker,
                          const FlexLine* aLine,
                          uint8_t aJustifyContent,
                          nscoord aContentBoxMainSize)
  : PositionTracker(aAxisTracker.GetMainAxis(),
                    aAxisTracker.IsMainAxisReversed()),
    mPackingSpaceRemaining(aContentBoxMainSize), // we chip away at this below
    mNumAutoMarginsInMainAxis(0),
    mNumPackingSpacesRemaining(0),
    mJustifyContent(aJustifyContent)
{
  // 'normal' behaves as 'stretch', and 'stretch' behaves as 'flex-start',
  // in the main axis
  // https://drafts.csswg.org/css-align-3/#propdef-justify-content
  if (mJustifyContent == NS_STYLE_JUSTIFY_NORMAL ||
      mJustifyContent == NS_STYLE_JUSTIFY_STRETCH) {
    mJustifyContent = NS_STYLE_JUSTIFY_FLEX_START;
  }

  // XXX strip off the <overflow-position> bit until we implement that
  mJustifyContent &= ~NS_STYLE_JUSTIFY_FLAG_BITS;

  // mPackingSpaceRemaining is initialized to the container's main size.  Now
  // we'll subtract out the main sizes of our flex items, so that it ends up
  // with the *actual* amount of packing space.
  for (const FlexItem* item = aLine->GetFirstItem(); item;
       item = item->getNext()) {
    mPackingSpaceRemaining -= item->GetOuterMainSize(mAxis);
    mNumAutoMarginsInMainAxis += item->GetNumAutoMarginsInAxis(mAxis);
  }

  if (mPackingSpaceRemaining <= 0) {
    // No available packing space to use for resolving auto margins.
    mNumAutoMarginsInMainAxis = 0;
  }

  // If packing space is negative, 'space-between' falls back to 'flex-start',
  // and 'space-around' & 'space-evenly' fall back to 'center'. In those cases,
  // it's simplest to just pretend we have a different 'justify-content' value
  // and share code.
  if (mPackingSpaceRemaining < 0) {
    if (mJustifyContent == NS_STYLE_JUSTIFY_SPACE_BETWEEN) {
      mJustifyContent = NS_STYLE_JUSTIFY_FLEX_START;
    } else if (mJustifyContent == NS_STYLE_JUSTIFY_SPACE_AROUND ||
               mJustifyContent == NS_STYLE_JUSTIFY_SPACE_EVENLY) {
      mJustifyContent = NS_STYLE_JUSTIFY_CENTER;
    }
  }

  // Map 'left'/'right' to 'start'/'end'
  if (mJustifyContent == NS_STYLE_JUSTIFY_LEFT ||
      mJustifyContent == NS_STYLE_JUSTIFY_RIGHT) {
    if (aAxisTracker.IsColumnOriented()) {
      // Container's alignment axis is not parallel to the inline axis,
      // so we map both 'left' and 'right' to 'start'.
      mJustifyContent = NS_STYLE_JUSTIFY_START;
    } else {
      // Row-oriented, so we map 'left' and 'right' to 'start' or 'end',
      // depending on left-to-right writing mode.
      const bool isLTR = aAxisTracker.GetWritingMode().IsBidiLTR();
      const bool isJustifyLeft = (mJustifyContent == NS_STYLE_JUSTIFY_LEFT);
      mJustifyContent = (isJustifyLeft == isLTR) ? NS_STYLE_JUSTIFY_START
                                                 : NS_STYLE_JUSTIFY_END;
    }
  }

  // Map 'start'/'end' to 'flex-start'/'flex-end'.
  if (mJustifyContent == NS_STYLE_JUSTIFY_START) {
    mJustifyContent = NS_STYLE_JUSTIFY_FLEX_START;
  } else if (mJustifyContent == NS_STYLE_JUSTIFY_END) {
    mJustifyContent = NS_STYLE_JUSTIFY_FLEX_END;
  }

  // If our main axis is (internally) reversed, swap the justify-content
  // "flex-start" and "flex-end" behaviors:
  if (aAxisTracker.AreAxesInternallyReversed()) {
    if (mJustifyContent == NS_STYLE_JUSTIFY_FLEX_START) {
      mJustifyContent = NS_STYLE_JUSTIFY_FLEX_END;
    } else if (mJustifyContent == NS_STYLE_JUSTIFY_FLEX_END) {
      mJustifyContent = NS_STYLE_JUSTIFY_FLEX_START;
    }
  }

  // Figure out how much space we'll set aside for auto margins or
  // packing spaces, and advance past any leading packing-space.
  if (mNumAutoMarginsInMainAxis == 0 &&
      mPackingSpaceRemaining != 0 &&
      !aLine->IsEmpty()) {
    switch (mJustifyContent) {
      case NS_STYLE_JUSTIFY_BASELINE:
      case NS_STYLE_JUSTIFY_LAST_BASELINE:
        NS_WARNING("NYI: justify-content:left/right/baseline/last-baseline");
        MOZ_FALLTHROUGH;
      case NS_STYLE_JUSTIFY_FLEX_START:
        // All packing space should go at the end --> nothing to do here.
        break;
      case NS_STYLE_JUSTIFY_FLEX_END:
        // All packing space goes at the beginning
        mPosition += mPackingSpaceRemaining;
        break;
      case NS_STYLE_JUSTIFY_CENTER:
        // Half the packing space goes at the beginning
        mPosition += mPackingSpaceRemaining / 2;
        break;
      case NS_STYLE_JUSTIFY_SPACE_BETWEEN:
      case NS_STYLE_JUSTIFY_SPACE_AROUND:
      case NS_STYLE_JUSTIFY_SPACE_EVENLY:
        nsFlexContainerFrame::CalculatePackingSpace(aLine->NumItems(),
                                                    mJustifyContent,
                                                    &mPosition,
                                                    &mNumPackingSpacesRemaining,
                                                    &mPackingSpaceRemaining);
        break;
      default:
        MOZ_ASSERT_UNREACHABLE("Unexpected justify-content value");
    }
  }

  MOZ_ASSERT(mNumPackingSpacesRemaining == 0 ||
             mNumAutoMarginsInMainAxis == 0,
             "extra space should either go to packing space or to "
             "auto margins, but not to both");
}

void
MainAxisPositionTracker::ResolveAutoMarginsInMainAxis(FlexItem& aItem)
{
  if (mNumAutoMarginsInMainAxis) {
    const nsStyleSides& styleMargin = aItem.Frame()->StyleMargin()->mMargin;
    for (uint32_t i = 0; i < eNumAxisEdges; i++) {
      mozilla::Side side = kAxisOrientationToSidesMap[mAxis][i];
      if (styleMargin.GetUnit(side) == eStyleUnit_Auto) {
        // NOTE: This integer math will skew the distribution of remainder
        // app-units towards the end, which is fine.
        nscoord curAutoMarginSize =
          mPackingSpaceRemaining / mNumAutoMarginsInMainAxis;

        MOZ_ASSERT(aItem.GetMarginComponentForSide(side) == 0,
                   "Expecting auto margins to have value '0' before we "
                   "resolve them");
        aItem.SetMarginComponentForSide(side, curAutoMarginSize);

        mNumAutoMarginsInMainAxis--;
        mPackingSpaceRemaining -= curAutoMarginSize;
      }
    }
  }
}

void
MainAxisPositionTracker::TraversePackingSpace()
{
  if (mNumPackingSpacesRemaining) {
    MOZ_ASSERT(mJustifyContent == NS_STYLE_JUSTIFY_SPACE_BETWEEN ||
               mJustifyContent == NS_STYLE_JUSTIFY_SPACE_AROUND ||
               mJustifyContent == NS_STYLE_JUSTIFY_SPACE_EVENLY,
               "mNumPackingSpacesRemaining only applies for "
               "space-between/space-around/space-evenly");

    MOZ_ASSERT(mPackingSpaceRemaining >= 0,
               "ran out of packing space earlier than we expected");

    // NOTE: This integer math will skew the distribution of remainder
    // app-units towards the end, which is fine.
    nscoord curPackingSpace =
      mPackingSpaceRemaining / mNumPackingSpacesRemaining;

    mPosition += curPackingSpace;
    mNumPackingSpacesRemaining--;
    mPackingSpaceRemaining -= curPackingSpace;
  }
}

CrossAxisPositionTracker::
  CrossAxisPositionTracker(FlexLine* aFirstLine,
                           const ReflowInput& aReflowInput,
                           nscoord aContentBoxCrossSize,
                           bool aIsCrossSizeDefinite,
                           const FlexboxAxisTracker& aAxisTracker)
  : PositionTracker(aAxisTracker.GetCrossAxis(),
                    aAxisTracker.IsCrossAxisReversed()),
    mPackingSpaceRemaining(0),
    mNumPackingSpacesRemaining(0),
    mAlignContent(aReflowInput.mStylePosition->mAlignContent)
{
  MOZ_ASSERT(aFirstLine, "null first line pointer");

  // 'normal' behaves as 'stretch'
  if (mAlignContent == NS_STYLE_ALIGN_NORMAL) {
    mAlignContent = NS_STYLE_ALIGN_STRETCH;
  }

  // XXX strip of the <overflow-position> bit until we implement that
  mAlignContent &= ~NS_STYLE_ALIGN_FLAG_BITS;

  const bool isSingleLine =
    NS_STYLE_FLEX_WRAP_NOWRAP == aReflowInput.mStylePosition->mFlexWrap;
  if (isSingleLine) {
    MOZ_ASSERT(!aFirstLine->getNext(),
               "If we're styled as single-line, we should only have 1 line");
    // "If the flex container is single-line and has a definite cross size, the
    // cross size of the flex line is the flex container's inner cross size."
    //
    // SOURCE: https://drafts.csswg.org/css-flexbox/#algo-cross-line
    // NOTE: This means (by definition) that there's no packing space, which
    // means we don't need to be concerned with "align-conent" at all and we
    // can return early. This is handy, because this is the usual case (for
    // single-line flexbox).
    if (aIsCrossSizeDefinite) {
      aFirstLine->SetLineCrossSize(aContentBoxCrossSize);
      return;
    }

    // "If the flex container is single-line, then clamp the line's
    // cross-size to be within the container's computed min and max cross-size
    // properties."
    aFirstLine->SetLineCrossSize(NS_CSS_MINMAX(aFirstLine->GetLineCrossSize(),
                                               aReflowInput.ComputedMinBSize(),
                                               aReflowInput.ComputedMaxBSize()));
  }

  // NOTE: The rest of this function should essentially match
  // MainAxisPositionTracker's constructor, though with FlexLines instead of
  // FlexItems, and with the additional value "stretch" (and of course with
  // cross sizes instead of main sizes.)

  // Figure out how much packing space we have (container's cross size minus
  // all the lines' cross sizes).  Also, share this loop to count how many
  // lines we have. (We need that count in some cases below.)
  mPackingSpaceRemaining = aContentBoxCrossSize;
  uint32_t numLines = 0;
  for (FlexLine* line = aFirstLine; line; line = line->getNext()) {
    mPackingSpaceRemaining -= line->GetLineCrossSize();
    numLines++;
  }

  // If packing space is negative, 'space-between' and 'stretch' behave like
  // 'flex-start', and 'space-around' and 'space-evenly' behave like 'center'.
  // In those cases, it's simplest to just pretend we have a different
  // 'align-content' value and share code.
  if (mPackingSpaceRemaining < 0) {
    if (mAlignContent == NS_STYLE_ALIGN_SPACE_BETWEEN ||
        mAlignContent == NS_STYLE_ALIGN_STRETCH) {
      mAlignContent = NS_STYLE_ALIGN_FLEX_START;
    } else if (mAlignContent == NS_STYLE_ALIGN_SPACE_AROUND ||
               mAlignContent == NS_STYLE_ALIGN_SPACE_EVENLY) {
      mAlignContent = NS_STYLE_ALIGN_CENTER;
    }
  }

  // Map 'left'/'right' to 'start'/'end'
  if (mAlignContent == NS_STYLE_ALIGN_LEFT ||
      mAlignContent == NS_STYLE_ALIGN_RIGHT) {
    if (aAxisTracker.IsRowOriented()) {
      // Container's alignment axis is not parallel to the inline axis,
      // so we map both 'left' and 'right' to 'start'.
      mAlignContent = NS_STYLE_ALIGN_START;
    } else {
      // Column-oriented, so we map 'left' and 'right' to 'start' or 'end',
      // depending on left-to-right writing mode.
      const bool isLTR = aAxisTracker.GetWritingMode().IsBidiLTR();
      const bool isAlignLeft = (mAlignContent == NS_STYLE_ALIGN_LEFT);
      mAlignContent = (isAlignLeft == isLTR) ? NS_STYLE_ALIGN_START
                                             : NS_STYLE_ALIGN_END;
    }
  }

  // Map 'start'/'end' to 'flex-start'/'flex-end'.
  if (mAlignContent == NS_STYLE_ALIGN_START) {
    mAlignContent = NS_STYLE_ALIGN_FLEX_START;
  } else if (mAlignContent == NS_STYLE_ALIGN_END) {
    mAlignContent = NS_STYLE_ALIGN_FLEX_END;
  }

  // If our cross axis is (internally) reversed, swap the align-content
  // "flex-start" and "flex-end" behaviors:
  if (aAxisTracker.AreAxesInternallyReversed()) {
    if (mAlignContent == NS_STYLE_ALIGN_FLEX_START) {
      mAlignContent = NS_STYLE_ALIGN_FLEX_END;
    } else if (mAlignContent == NS_STYLE_ALIGN_FLEX_END) {
      mAlignContent = NS_STYLE_ALIGN_FLEX_START;
    }
  }

  // Figure out how much space we'll set aside for packing spaces, and advance
  // past any leading packing-space.
  if (mPackingSpaceRemaining != 0) {
    switch (mAlignContent) {
      case NS_STYLE_ALIGN_SELF_START:
      case NS_STYLE_ALIGN_SELF_END:
      case NS_STYLE_ALIGN_BASELINE:
      case NS_STYLE_ALIGN_LAST_BASELINE:
        NS_WARNING("NYI: align-items/align-self:left/right/self-start/self-end/baseline/last-baseline");
        MOZ_FALLTHROUGH;
      case NS_STYLE_ALIGN_FLEX_START:
        // All packing space should go at the end --> nothing to do here.
        break;
      case NS_STYLE_ALIGN_FLEX_END:
        // All packing space goes at the beginning
        mPosition += mPackingSpaceRemaining;
        break;
      case NS_STYLE_ALIGN_CENTER:
        // Half the packing space goes at the beginning
        mPosition += mPackingSpaceRemaining / 2;
        break;
      case NS_STYLE_ALIGN_SPACE_BETWEEN:
      case NS_STYLE_ALIGN_SPACE_AROUND:
      case NS_STYLE_ALIGN_SPACE_EVENLY:
        nsFlexContainerFrame::CalculatePackingSpace(numLines,
                                                    mAlignContent,
                                                    &mPosition,
                                                    &mNumPackingSpacesRemaining,
                                                    &mPackingSpaceRemaining);
        break;
      case NS_STYLE_ALIGN_STRETCH: {
        // Split space equally between the lines:
        MOZ_ASSERT(mPackingSpaceRemaining > 0,
                   "negative packing space should make us use 'flex-start' "
                   "instead of 'stretch' (and we shouldn't bother with this "
                   "code if we have 0 packing space)");

        uint32_t numLinesLeft = numLines;
        for (FlexLine* line = aFirstLine; line; line = line->getNext()) {
          // Our share is the amount of space remaining, divided by the number
          // of lines remainig.
          MOZ_ASSERT(numLinesLeft > 0, "miscalculated num lines");
          nscoord shareOfExtraSpace = mPackingSpaceRemaining / numLinesLeft;
          nscoord newSize = line->GetLineCrossSize() + shareOfExtraSpace;
          line->SetLineCrossSize(newSize);

          mPackingSpaceRemaining -= shareOfExtraSpace;
          numLinesLeft--;
        }
        MOZ_ASSERT(numLinesLeft == 0, "miscalculated num lines");
        break;
      }
      default:
        MOZ_ASSERT_UNREACHABLE("Unexpected align-content value");
    }
  }
}

void
CrossAxisPositionTracker::TraversePackingSpace()
{
  if (mNumPackingSpacesRemaining) {
    MOZ_ASSERT(mAlignContent == NS_STYLE_ALIGN_SPACE_BETWEEN ||
               mAlignContent == NS_STYLE_ALIGN_SPACE_AROUND ||
               mAlignContent == NS_STYLE_ALIGN_SPACE_EVENLY,
               "mNumPackingSpacesRemaining only applies for "
               "space-between/space-around/space-evenly");

    MOZ_ASSERT(mPackingSpaceRemaining >= 0,
               "ran out of packing space earlier than we expected");

    // NOTE: This integer math will skew the distribution of remainder
    // app-units towards the end, which is fine.
    nscoord curPackingSpace =
      mPackingSpaceRemaining / mNumPackingSpacesRemaining;

    mPosition += curPackingSpace;
    mNumPackingSpacesRemaining--;
    mPackingSpaceRemaining -= curPackingSpace;
  }
}

SingleLineCrossAxisPositionTracker::
  SingleLineCrossAxisPositionTracker(const FlexboxAxisTracker& aAxisTracker)
  : PositionTracker(aAxisTracker.GetCrossAxis(),
                    aAxisTracker.IsCrossAxisReversed())
{
}

void
FlexLine::ComputeCrossSizeAndBaseline(const FlexboxAxisTracker& aAxisTracker)
{
  nscoord crossStartToFurthestBaseline = nscoord_MIN;
  nscoord crossEndToFurthestBaseline = nscoord_MIN;
  nscoord largestOuterCrossSize = 0;
  for (const FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
    nscoord curOuterCrossSize =
      item->GetOuterCrossSize(aAxisTracker.GetCrossAxis());

    if (item->GetAlignSelf() == NS_STYLE_ALIGN_BASELINE &&
        item->GetNumAutoMarginsInAxis(aAxisTracker.GetCrossAxis()) == 0) {
      // FIXME: Once we support "writing-mode", we'll have to do baseline
      // alignment in vertical flex containers here (w/ horizontal cross-axes).

      // Find distance from our item's cross-start and cross-end margin-box
      // edges to its baseline.
      //
      // Here's a diagram of a flex-item that we might be doing this on.
      // "mmm" is the margin-box, "bbb" is the border-box. The bottom of
      // the text "BASE" is the baseline.
      //
      // ---(cross-start)---
      //                ___              ___            ___
      //   mmmmmmmmmmmm  |                |margin-start  |
      //   m          m  |               _|_   ___       |
      //   m bbbbbbbb m  |curOuterCrossSize     |        |crossStartToBaseline
      //   m b      b m  |                      |ascent  |
      //   m b BASE b m  |                     _|_      _|_
      //   m b      b m  |                               |
      //   m bbbbbbbb m  |                               |crossEndToBaseline
      //   m          m  |                               |
      //   mmmmmmmmmmmm _|_                             _|_
      //
      // ---(cross-end)---
      //
      // We already have the curOuterCrossSize, margin-start, and the ascent.
      // * We can get crossStartToBaseline by adding margin-start + ascent.
      // * If we subtract that from the curOuterCrossSize, we get
      //   crossEndToBaseline.

      nscoord crossStartToBaseline =
        item->GetBaselineOffsetFromOuterCrossEdge(eAxisEdge_Start,
                                                  aAxisTracker);
      nscoord crossEndToBaseline = curOuterCrossSize - crossStartToBaseline;

      // Now, update our "largest" values for these (across all the flex items
      // in this flex line), so we can use them in computing the line's cross
      // size below:
      crossStartToFurthestBaseline = std::max(crossStartToFurthestBaseline,
                                              crossStartToBaseline);
      crossEndToFurthestBaseline = std::max(crossEndToFurthestBaseline,
                                            crossEndToBaseline);
    } else {
      largestOuterCrossSize = std::max(largestOuterCrossSize, curOuterCrossSize);
    }
  }

  // The line's baseline offset is the distance from the line's edge (start or
  // end, depending on whether we've flipped the axes) to the furthest
  // item-baseline. The item(s) with that baseline will be exactly aligned with
  // the line's edge.
  mBaselineOffset = aAxisTracker.AreAxesInternallyReversed() ?
    crossEndToFurthestBaseline : crossStartToFurthestBaseline;

  // The line's cross-size is the larger of:
  //  (a) [largest cross-start-to-baseline + largest baseline-to-cross-end] of
  //      all baseline-aligned items with no cross-axis auto margins...
  // and
  //  (b) largest cross-size of all other children.
  mLineCrossSize = std::max(crossStartToFurthestBaseline +
                            crossEndToFurthestBaseline,
                            largestOuterCrossSize);
}

void
FlexItem::ResolveStretchedCrossSize(nscoord aLineCrossSize,
                                    const FlexboxAxisTracker& aAxisTracker)
{
  AxisOrientationType crossAxis = aAxisTracker.GetCrossAxis();
  // We stretch IFF we are align-self:stretch, have no auto margins in
  // cross axis, and have cross-axis size property == "auto". If any of those
  // conditions don't hold up, we won't stretch.
  if (mAlignSelf != NS_STYLE_ALIGN_STRETCH ||
      GetNumAutoMarginsInAxis(crossAxis) != 0 ||
      eStyleUnit_Auto != aAxisTracker.ComputedCrossSize(mFrame).GetUnit()) {
    return;
  }

  // If we've already been stretched, we can bail out early, too.
  // No need to redo the calculation.
  if (mIsStretched) {
    return;
  }

  // Reserve space for margins & border & padding, and then use whatever
  // remains as our item's cross-size (clamped to its min/max range).
  nscoord stretchedSize = aLineCrossSize -
    GetMarginBorderPaddingSizeInAxis(crossAxis);

  stretchedSize = NS_CSS_MINMAX(stretchedSize, mCrossMinSize, mCrossMaxSize);

  // Update the cross-size & make a note that it's stretched, so we know to
  // override the reflow state's computed cross-size in our final reflow.
  SetCrossSize(stretchedSize);
  mIsStretched = true;
}

void
SingleLineCrossAxisPositionTracker::
  ResolveAutoMarginsInCrossAxis(const FlexLine& aLine,
                                FlexItem& aItem)
{
  // Subtract the space that our item is already occupying, to see how much
  // space (if any) is available for its auto margins.
  nscoord spaceForAutoMargins = aLine.GetLineCrossSize() -
    aItem.GetOuterCrossSize(mAxis);

  if (spaceForAutoMargins <= 0) {
    return; // No available space  --> nothing to do
  }

  uint32_t numAutoMargins = aItem.GetNumAutoMarginsInAxis(mAxis);
  if (numAutoMargins == 0) {
    return; // No auto margins --> nothing to do.
  }

  // OK, we have at least one auto margin and we have some available space.
  // Give each auto margin a share of the space.
  const nsStyleSides& styleMargin = aItem.Frame()->StyleMargin()->mMargin;
  for (uint32_t i = 0; i < eNumAxisEdges; i++) {
    mozilla::Side side = kAxisOrientationToSidesMap[mAxis][i];
    if (styleMargin.GetUnit(side) == eStyleUnit_Auto) {
      MOZ_ASSERT(aItem.GetMarginComponentForSide(side) == 0,
                 "Expecting auto margins to have value '0' before we "
                 "update them");

      // NOTE: integer divison is fine here; numAutoMargins is either 1 or 2.
      // If it's 2 & spaceForAutoMargins is odd, 1st margin gets smaller half.
      nscoord curAutoMarginSize = spaceForAutoMargins / numAutoMargins;
      aItem.SetMarginComponentForSide(side, curAutoMarginSize);
      numAutoMargins--;
      spaceForAutoMargins -= curAutoMarginSize;
    }
  }
}

void
SingleLineCrossAxisPositionTracker::
  EnterAlignPackingSpace(const FlexLine& aLine,
                         const FlexItem& aItem,
                         const FlexboxAxisTracker& aAxisTracker)
{
  // We don't do align-self alignment on items that have auto margins
  // in the cross axis.
  if (aItem.GetNumAutoMarginsInAxis(mAxis)) {
    return;
  }

  uint8_t alignSelf = aItem.GetAlignSelf();
  // NOTE: 'stretch' behaves like 'flex-start' once we've stretched any
  // auto-sized items (which we've already done).
  if (alignSelf == NS_STYLE_ALIGN_STRETCH) {
    alignSelf = NS_STYLE_ALIGN_FLEX_START;
  }

  // Map 'left'/'right' to 'start'/'end'
  if (alignSelf == NS_STYLE_ALIGN_LEFT || alignSelf == NS_STYLE_ALIGN_RIGHT) {
    if (aAxisTracker.IsRowOriented()) {
      // Container's alignment axis is not parallel to the inline axis,
      // so we map both 'left' and 'right' to 'start'.
      alignSelf = NS_STYLE_ALIGN_START;
    } else {
      // Column-oriented, so we map 'left' and 'right' to 'start' or 'end',
      // depending on left-to-right writing mode.
      const bool isLTR = aAxisTracker.GetWritingMode().IsBidiLTR();
      const bool isAlignLeft = (alignSelf == NS_STYLE_ALIGN_LEFT);
      alignSelf = (isAlignLeft == isLTR) ? NS_STYLE_ALIGN_START
                                         : NS_STYLE_ALIGN_END;
    }
  }

  // Map 'start'/'end' to 'flex-start'/'flex-end'.
  if (alignSelf == NS_STYLE_ALIGN_START) {
    alignSelf = NS_STYLE_ALIGN_FLEX_START;
  } else if (alignSelf == NS_STYLE_ALIGN_END) {
    alignSelf = NS_STYLE_ALIGN_FLEX_END;
  }

  // If our cross axis is (internally) reversed, swap the align-self
  // "flex-start" and "flex-end" behaviors:
  if (aAxisTracker.AreAxesInternallyReversed()) {
    if (alignSelf == NS_STYLE_ALIGN_FLEX_START) {
      alignSelf = NS_STYLE_ALIGN_FLEX_END;
    } else if (alignSelf == NS_STYLE_ALIGN_FLEX_END) {
      alignSelf = NS_STYLE_ALIGN_FLEX_START;
    }
  }

  switch (alignSelf) {
    case NS_STYLE_ALIGN_SELF_START:
    case NS_STYLE_ALIGN_SELF_END:
    case NS_STYLE_ALIGN_LAST_BASELINE:
      NS_WARNING("NYI: align-items/align-self:left/right/self-start/self-end/last-baseline");
      MOZ_FALLTHROUGH;
    case NS_STYLE_ALIGN_FLEX_START:
      // No space to skip over -- we're done.
      break;
    case NS_STYLE_ALIGN_FLEX_END:
      mPosition += aLine.GetLineCrossSize() - aItem.GetOuterCrossSize(mAxis);
      break;
    case NS_STYLE_ALIGN_CENTER:
      // Note: If cross-size is odd, the "after" space will get the extra unit.
      mPosition +=
        (aLine.GetLineCrossSize() - aItem.GetOuterCrossSize(mAxis)) / 2;
      break;
    case NS_STYLE_ALIGN_BASELINE: {
      // Normally, baseline-aligned items are collectively aligned with the
      // line's cross-start edge; however, if our cross axis is (internally)
      // reversed, we instead align them with the cross-end edge.
      AxisEdgeType baselineAlignEdge =
        aAxisTracker.AreAxesInternallyReversed() ?
        eAxisEdge_End : eAxisEdge_Start;

      nscoord itemBaselineOffset =
        aItem.GetBaselineOffsetFromOuterCrossEdge(baselineAlignEdge,
                                                  aAxisTracker);

      nscoord lineBaselineOffset = aLine.GetBaselineOffset();

      NS_ASSERTION(lineBaselineOffset >= itemBaselineOffset,
                   "failed at finding largest baseline offset");

      // How much do we need to adjust our position (from the line edge),
      // to get the item's baseline to hit the line's baseline offset:
      nscoord baselineDiff = lineBaselineOffset - itemBaselineOffset;

      if (aAxisTracker.AreAxesInternallyReversed()) {
        // Advance to align item w/ line's flex-end edge (as in FLEX_END case):
        mPosition += aLine.GetLineCrossSize() - aItem.GetOuterCrossSize(mAxis);
        // ...and step *back* by the baseline adjustment:
        mPosition -= baselineDiff;
      } else {
        // mPosition is already at line's flex-start edge.
        // From there, we step *forward* by the baseline adjustment:
        mPosition += baselineDiff;
      }
      break;
    }
    default:
      MOZ_ASSERT_UNREACHABLE("Unexpected align-self value");
      break;
  }
}

// Utility function to convert an InlineDir to an AxisOrientationType
static inline AxisOrientationType
InlineDirToAxisOrientation(WritingMode::InlineDir aInlineDir)
{
  switch (aInlineDir) {
    case WritingMode::eInlineLTR:
      return eAxis_LR;
    case WritingMode::eInlineRTL:
      return eAxis_RL;
    case WritingMode::eInlineTTB:
      return eAxis_TB;
    case WritingMode::eInlineBTT:
      return eAxis_BT;
  }

  MOZ_ASSERT_UNREACHABLE("Unhandled InlineDir");
  return eAxis_LR; // in case of unforseen error, assume English LTR text flow.
}

// Utility function to convert a BlockDir to an AxisOrientationType
static inline AxisOrientationType
BlockDirToAxisOrientation(WritingMode::BlockDir aBlockDir)
{
  switch (aBlockDir) {
    case WritingMode::eBlockLR:
      return eAxis_LR;
    case WritingMode::eBlockRL:
      return eAxis_RL;
    case WritingMode::eBlockTB:
      return eAxis_TB;
    // NOTE: WritingMode::eBlockBT (bottom-to-top) does not exist.
  }

  MOZ_ASSERT_UNREACHABLE("Unhandled BlockDir");
  return eAxis_TB; // in case of unforseen error, assume English TTB block-flow
}

FlexboxAxisTracker::FlexboxAxisTracker(
  const nsFlexContainerFrame* aFlexContainer,
  const WritingMode& aWM)
  : mWM(aWM),
    mAreAxesInternallyReversed(false)
{
  if (IsLegacyBox(aFlexContainer)) {
    InitAxesFromLegacyProps(aFlexContainer);
  } else {
    InitAxesFromModernProps(aFlexContainer);
  }

  // Master switch to enable/disable bug 983427's code for reversing our axes
  // and reversing some logic, to avoid reflowing children in bottom-to-top
  // order. (This switch can be removed eventually, but for now, it allows
  // this special-case code path to be compared against the normal code path.)
  static bool sPreventBottomToTopChildOrdering = true;

  if (sPreventBottomToTopChildOrdering) {
    // If either axis is bottom-to-top, we flip both axes (and set a flag
    // so that we can flip some logic to make the reversal transparent).
    if (eAxis_BT == mMainAxis || eAxis_BT == mCrossAxis) {
      mMainAxis = GetReverseAxis(mMainAxis);
      mCrossAxis = GetReverseAxis(mCrossAxis);
      mAreAxesInternallyReversed = true;
      mIsMainAxisReversed = !mIsMainAxisReversed;
      mIsCrossAxisReversed = !mIsCrossAxisReversed;
    }
  }
}

void
FlexboxAxisTracker::InitAxesFromLegacyProps(
  const nsFlexContainerFrame* aFlexContainer)
{
  const nsStyleXUL* styleXUL = aFlexContainer->StyleXUL();

  const bool boxOrientIsVertical = (styleXUL->mBoxOrient ==
                                    StyleBoxOrient::Vertical);
  const bool wmIsVertical = mWM.IsVertical();

  // If box-orient agrees with our writing-mode, then we're "row-oriented"
  // (i.e. the flexbox main axis is the same as our writing mode's inline
  // direction).  Otherwise, we're column-oriented (i.e. the flexbox's main
  // axis is perpendicular to the writing-mode's inline direction).
  mIsRowOriented = (boxOrientIsVertical == wmIsVertical);

  // XXXdholbert BEGIN CODE TO SET DEPRECATED MEMBER-VARS
  if (boxOrientIsVertical) {
    mMainAxis = eAxis_TB;
    mCrossAxis = eAxis_LR;
  } else {
    mMainAxis = eAxis_LR;
    mCrossAxis = eAxis_TB;
  }
  // "direction: rtl" reverses the writing-mode's inline axis.
  // So, we need to reverse the corresponding flex axis to match.
  // (Note this we don't toggle "mIsMainAxisReversed" for this condition,
  // because the main axis will still match mWM's inline direction.)
  if (!mWM.IsBidiLTR()) {
    AxisOrientationType& axisToFlip = mIsRowOriented ? mMainAxis : mCrossAxis;
    axisToFlip = GetReverseAxis(axisToFlip);
  }
  // XXXdholbert END CODE TO SET DEPRECATED MEMBER-VARS

  // Legacy flexbox can use "-webkit-box-direction: reverse" to reverse the
  // main axis (so it runs in the reverse direction of the inline axis):
  if (styleXUL->mBoxDirection == StyleBoxDirection::Reverse) {
    mMainAxis = GetReverseAxis(mMainAxis);
    mIsMainAxisReversed = true;
  } else {
    mIsMainAxisReversed = false;
  }

  // Legacy flexbox does not support reversing the cross axis -- it has no
  // equivalent of modern flexbox's "flex-wrap: wrap-reverse".
  mIsCrossAxisReversed = false;
}

void
FlexboxAxisTracker::InitAxesFromModernProps(
  const nsFlexContainerFrame* aFlexContainer)
{
  const nsStylePosition* stylePos = aFlexContainer->StylePosition();
  uint32_t flexDirection = stylePos->mFlexDirection;

  // Inline dimension ("start-to-end"):
  // (NOTE: I'm intentionally not calling these "inlineAxis"/"blockAxis", since
  // those terms have explicit definition in the writing-modes spec, which are
  // the opposite of how I'd be using them here.)
  AxisOrientationType inlineDimension =
    InlineDirToAxisOrientation(mWM.GetInlineDir());
  AxisOrientationType blockDimension =
    BlockDirToAxisOrientation(mWM.GetBlockDir());

  // Determine main axis:
  switch (flexDirection) {
    case NS_STYLE_FLEX_DIRECTION_ROW:
      mMainAxis = inlineDimension;
      mIsRowOriented = true;
      mIsMainAxisReversed = false;
      break;
    case NS_STYLE_FLEX_DIRECTION_ROW_REVERSE:
      mMainAxis = GetReverseAxis(inlineDimension);
      mIsRowOriented = true;
      mIsMainAxisReversed = true;
      break;
    case NS_STYLE_FLEX_DIRECTION_COLUMN:
      mMainAxis = blockDimension;
      mIsRowOriented = false;
      mIsMainAxisReversed = false;
      break;
    case NS_STYLE_FLEX_DIRECTION_COLUMN_REVERSE:
      mMainAxis = GetReverseAxis(blockDimension);
      mIsRowOriented = false;
      mIsMainAxisReversed = true;
      break;
    default:
      MOZ_ASSERT_UNREACHABLE("Unexpected flex-direction value");
  }

  // Determine cross axis:
  // (This is set up so that a bogus |flexDirection| value will
  // give us blockDimension.
  if (flexDirection == NS_STYLE_FLEX_DIRECTION_COLUMN ||
      flexDirection == NS_STYLE_FLEX_DIRECTION_COLUMN_REVERSE) {
    mCrossAxis = inlineDimension;
  } else {
    mCrossAxis = blockDimension;
  }

  // "flex-wrap: wrap-reverse" reverses our cross axis.
  if (stylePos->mFlexWrap == NS_STYLE_FLEX_WRAP_WRAP_REVERSE) {
    mCrossAxis = GetReverseAxis(mCrossAxis);
    mIsCrossAxisReversed = true;
  } else {
    mIsCrossAxisReversed = false;
  }
}

// Allocates a new FlexLine, adds it to the given LinkedList (at the front or
// back depending on aShouldInsertAtFront), and returns a pointer to it.
static FlexLine*
AddNewFlexLineToList(LinkedList<FlexLine>& aLines,
                     bool aShouldInsertAtFront)
{
  FlexLine* newLine = new FlexLine();
  if (aShouldInsertAtFront) {
    aLines.insertFront(newLine);
  } else {
    aLines.insertBack(newLine);
  }
  return newLine;
}

void
nsFlexContainerFrame::GenerateFlexLines(
  nsPresContext* aPresContext,
  const ReflowInput& aReflowInput,
  nscoord aContentBoxMainSize,
  nscoord aAvailableBSizeForContent,
  const nsTArray<StrutInfo>& aStruts,
  const FlexboxAxisTracker& aAxisTracker,
  LinkedList<FlexLine>& aLines)
{
  MOZ_ASSERT(aLines.isEmpty(), "Expecting outparam to start out empty");

  const bool isSingleLine =
    NS_STYLE_FLEX_WRAP_NOWRAP == aReflowInput.mStylePosition->mFlexWrap;

  // If we're transparently reversing axes, then we'll need to link up our
  // FlexItems and FlexLines in the reverse order, so that the rest of flex
  // layout (with flipped axes) will still produce the correct result.
  // Here, we declare a convenience bool that we'll pass when adding a new
  // FlexLine or FlexItem, to make us insert it at the beginning of its list
  // (so the list ends up reversed).
  const bool shouldInsertAtFront = aAxisTracker.AreAxesInternallyReversed();

  // We have at least one FlexLine. Even an empty flex container has a single
  // (empty) flex line.
  FlexLine* curLine = AddNewFlexLineToList(aLines, shouldInsertAtFront);

  nscoord wrapThreshold;
  if (isSingleLine) {
    // Not wrapping. Set threshold to sentinel value that tells us not to wrap.
    wrapThreshold = NS_UNCONSTRAINEDSIZE;
  } else {
    // Wrapping! Set wrap threshold to flex container's content-box main-size.
    wrapThreshold = aContentBoxMainSize;

    // If the flex container doesn't have a definite content-box main-size
    // (e.g. if main axis is vertical & 'height' is 'auto'), make sure we at
    // least wrap when we hit its max main-size.
    if (wrapThreshold == NS_UNCONSTRAINEDSIZE) {
      const nscoord flexContainerMaxMainSize =
        GET_MAIN_COMPONENT_LOGICAL(aAxisTracker, aAxisTracker.GetWritingMode(),
                                   aReflowInput.ComputedMaxISize(),
                                   aReflowInput.ComputedMaxBSize());

      wrapThreshold = flexContainerMaxMainSize;
    }

    // Also: if we're column-oriented and paginating in the block dimension,
    // we may need to wrap to a new flex line sooner (before we grow past the
    // available BSize, potentially running off the end of the page).
    if (aAxisTracker.IsColumnOriented() &&
        aAvailableBSizeForContent != NS_UNCONSTRAINEDSIZE) {
      wrapThreshold = std::min(wrapThreshold, aAvailableBSizeForContent);
    }
  }

  // Tracks the index of the next strut, in aStruts (and when this hits
  // aStruts.Length(), that means there are no more struts):
  uint32_t nextStrutIdx = 0;

  // Overall index of the current flex item in the flex container. (This gets
  // checked against entries in aStruts.)
  uint32_t itemIdxInContainer = 0;

  for (nsIFrame* childFrame : mFrames) {
    // Honor "page-break-before", if we're multi-line and this line isn't empty:
    if (!isSingleLine && !curLine->IsEmpty() &&
        childFrame->StyleDisplay()->mBreakBefore) {
      curLine = AddNewFlexLineToList(aLines, shouldInsertAtFront);
    }

    UniquePtr<FlexItem> item;
    if (nextStrutIdx < aStruts.Length() &&
        aStruts[nextStrutIdx].mItemIdx == itemIdxInContainer) {

      // Use the simplified "strut" FlexItem constructor:
      item = MakeUnique<FlexItem>(childFrame, aStruts[nextStrutIdx].mStrutCrossSize,
                                  aReflowInput.GetWritingMode());
      nextStrutIdx++;
    } else {
      item = GenerateFlexItemForChild(aPresContext, childFrame,
                                      aReflowInput, aAxisTracker);
    }

    nscoord itemInnerHypotheticalMainSize = item->GetMainSize();
    nscoord itemOuterHypotheticalMainSize =
      item->GetOuterMainSize(aAxisTracker.GetMainAxis());

    // Check if we need to wrap |item| to a new line
    // (i.e. check if its outer hypothetical main size pushes our line over
    // the threshold)
    if (wrapThreshold != NS_UNCONSTRAINEDSIZE &&
        !curLine->IsEmpty() && // No need to wrap at start of a line.
        wrapThreshold < (curLine->GetTotalOuterHypotheticalMainSize() +
                         itemOuterHypotheticalMainSize)) {
      curLine = AddNewFlexLineToList(aLines, shouldInsertAtFront);
    }

    // Add item to current flex line (and update the line's bookkeeping about
    // how large its items collectively are).
    curLine->AddItem(item.release(), shouldInsertAtFront,
                     itemInnerHypotheticalMainSize,
                     itemOuterHypotheticalMainSize);

    // Honor "page-break-after", if we're multi-line and have more children:
    if (!isSingleLine && childFrame->GetNextSibling() &&
        childFrame->StyleDisplay()->mBreakAfter) {
      curLine = AddNewFlexLineToList(aLines, shouldInsertAtFront);
    }
    itemIdxInContainer++;
  }
}

// Retrieves the content-box main-size of our flex container from the
// reflow state (specifically, the main-size of *this continuation* of the
// flex container).
nscoord
nsFlexContainerFrame::GetMainSizeFromReflowInput(
  const ReflowInput& aReflowInput,
  const FlexboxAxisTracker& aAxisTracker)
{
  if (aAxisTracker.IsRowOriented()) {
    // Row-oriented --> our main axis is the inline axis, so our main size
    // is our inline size (which should already be resolved).
    NS_WARNING_ASSERTION(
      aReflowInput.ComputedISize() != NS_UNCONSTRAINEDSIZE,
      "Unconstrained inline size; this should only result from huge sizes "
      "(not intrinsic sizing w/ orthogonal flows)");
    return aReflowInput.ComputedISize();
  }

  // Note: This may be unconstrained, if our block size is "auto":
  return GetEffectiveComputedBSize(aReflowInput);
}

// Returns the largest outer hypothetical main-size of any line in |aLines|.
// (i.e. the hypothetical main-size of the largest line)
static nscoord
GetLargestLineMainSize(const FlexLine* aFirstLine)
{
  nscoord largestLineOuterSize = 0;
  for (const FlexLine* line = aFirstLine; line; line = line->getNext()) {
    largestLineOuterSize = std::max(largestLineOuterSize,
                                    line->GetTotalOuterHypotheticalMainSize());
  }
  return largestLineOuterSize;
}

/* Resolves the content-box main-size of a flex container frame,
 * primarily based on:
 * - the "tentative" main size, taken from the reflow state ("tentative"
 *    because it may be unconstrained or may run off the page).
 * - the available BSize (needed if the main axis is the block axis).
 * - the sizes of our lines of flex items.
 *
 * Guaranteed to return a definite length, i.e. not NS_UNCONSTRAINEDSIZE,
 * aside from cases with huge lengths which happen to compute to that value.
 *
 * (Note: This function should be structurally similar to 'ComputeCrossSize()',
 * except that here, the caller has already grabbed the tentative size from the
 * reflow state.)
 */
static nscoord
ResolveFlexContainerMainSize(const ReflowInput& aReflowInput,
                             const FlexboxAxisTracker& aAxisTracker,
                             nscoord aTentativeMainSize,
                             nscoord aAvailableBSizeForContent,
                             const FlexLine* aFirstLine,
                             nsReflowStatus& aStatus)
{
  MOZ_ASSERT(aFirstLine, "null first line pointer");

  if (aAxisTracker.IsRowOriented()) {
    // Row-oriented --> our main axis is the inline axis, so our main size
    // is our inline size (which should already be resolved).
    return aTentativeMainSize;
  }

  if (aTentativeMainSize != NS_INTRINSICSIZE) {
    // Column-oriented case, with fixed BSize:
    if (aAvailableBSizeForContent == NS_UNCONSTRAINEDSIZE ||
        aTentativeMainSize < aAvailableBSizeForContent) {
      // Not in a fragmenting context, OR no need to fragment because we have
      // more available BSize than we need. Either way, we don't need to clamp.
      // (Note that the reflow state has already done the appropriate
      // min/max-BSize clamping.)
      return aTentativeMainSize;
    }

    // Fragmenting *and* our fixed BSize is larger than available BSize:
    // Mark incomplete so we get a next-in-flow, and take up all of the
    // available BSize (or the amount of BSize required by our children, if
    // that's larger; but of course not more than our own computed BSize).
    // XXXdholbert For now, we don't support pushing children to our next
    // continuation or splitting children, so "amount of BSize required by
    // our children" is just the main-size (BSize) of our longest flex line.
    NS_FRAME_SET_INCOMPLETE(aStatus);
    nscoord largestLineOuterSize = GetLargestLineMainSize(aFirstLine);

    if (largestLineOuterSize <= aAvailableBSizeForContent) {
      return aAvailableBSizeForContent;
    }
    return std::min(aTentativeMainSize, largestLineOuterSize);
  }

  // Column-oriented case, with auto BSize:
  // Resolve auto BSize to the largest FlexLine length, clamped to our
  // computed min/max main-size properties.
  // XXXdholbert Handle constrained-aAvailableBSizeForContent case here.
  nscoord largestLineOuterSize = GetLargestLineMainSize(aFirstLine);
  return NS_CSS_MINMAX(largestLineOuterSize,
                       aReflowInput.ComputedMinBSize(),
                       aReflowInput.ComputedMaxBSize());
}

nscoord
nsFlexContainerFrame::ComputeCrossSize(const ReflowInput& aReflowInput,
                                       const FlexboxAxisTracker& aAxisTracker,
                                       nscoord aSumLineCrossSizes,
                                       nscoord aAvailableBSizeForContent,
                                       bool* aIsDefinite,
                                       nsReflowStatus& aStatus)
{
  MOZ_ASSERT(aIsDefinite, "outparam pointer must be non-null");

  if (aAxisTracker.IsColumnOriented()) {
    // Column-oriented --> our cross axis is the inline axis, so our cross size
    // is our inline size (which should already be resolved).
    NS_WARNING_ASSERTION(
      aReflowInput.ComputedISize() != NS_UNCONSTRAINEDSIZE,
      "Unconstrained inline size; this should only result from huge sizes "
      "(not intrinsic sizing w/ orthogonal flows)");
    *aIsDefinite = true;
    return aReflowInput.ComputedISize();
  }

  nscoord effectiveComputedBSize = GetEffectiveComputedBSize(aReflowInput);
  if (effectiveComputedBSize != NS_INTRINSICSIZE) {
    // Row-oriented case (cross axis is block-axis), with fixed BSize:
    *aIsDefinite = true;
    if (aAvailableBSizeForContent == NS_UNCONSTRAINEDSIZE ||
        effectiveComputedBSize < aAvailableBSizeForContent) {
      // Not in a fragmenting context, OR no need to fragment because we have
      // more available BSize than we need. Either way, just use our fixed
      // BSize.  (Note that the reflow state has already done the appropriate
      // min/max-BSize clamping.)
      return effectiveComputedBSize;
    }

    // Fragmenting *and* our fixed BSize is too tall for available BSize:
    // Mark incomplete so we get a next-in-flow, and take up all of the
    // available BSize (or the amount of BSize required by our children, if
    // that's larger; but of course not more than our own computed BSize).
    // XXXdholbert For now, we don't support pushing children to our next
    // continuation or splitting children, so "amount of BSize required by
    // our children" is just the sum of our FlexLines' BSizes (cross sizes).
    NS_FRAME_SET_INCOMPLETE(aStatus);
    if (aSumLineCrossSizes <= aAvailableBSizeForContent) {
      return aAvailableBSizeForContent;
    }
    return std::min(effectiveComputedBSize, aSumLineCrossSizes);
  }

  // Row-oriented case (cross axis is block axis), with auto BSize:
  // Shrink-wrap our line(s), subject to our min-size / max-size
  // constraints in that (block) axis.
  // XXXdholbert Handle constrained-aAvailableBSizeForContent case here.
  *aIsDefinite = false;
  return NS_CSS_MINMAX(aSumLineCrossSizes,
                       aReflowInput.ComputedMinBSize(),
                       aReflowInput.ComputedMaxBSize());
}

void
FlexLine::PositionItemsInMainAxis(uint8_t aJustifyContent,
                                  nscoord aContentBoxMainSize,
                                  const FlexboxAxisTracker& aAxisTracker)
{
  MainAxisPositionTracker mainAxisPosnTracker(aAxisTracker, this,
                                              aJustifyContent,
                                              aContentBoxMainSize);
  for (FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
    nscoord itemMainBorderBoxSize =
      item->GetMainSize() +
      item->GetBorderPaddingSizeInAxis(mainAxisPosnTracker.GetAxis());

    // Resolve any main-axis 'auto' margins on aChild to an actual value.
    mainAxisPosnTracker.ResolveAutoMarginsInMainAxis(*item);

    // Advance our position tracker to child's upper-left content-box corner,
    // and use that as its position in the main axis.
    mainAxisPosnTracker.EnterMargin(item->GetMargin());
    mainAxisPosnTracker.EnterChildFrame(itemMainBorderBoxSize);

    item->SetMainPosition(mainAxisPosnTracker.GetPosition());

    mainAxisPosnTracker.ExitChildFrame(itemMainBorderBoxSize);
    mainAxisPosnTracker.ExitMargin(item->GetMargin());
    mainAxisPosnTracker.TraversePackingSpace();
  }
}

/**
 * Given the flex container's "flex-relative ascent" (i.e. distance from the
 * flex container's content-box cross-start edge to its baseline), returns
 * its actual physical ascent value (the distance from the *border-box* top
 * edge to its baseline).
 */
static nscoord
ComputePhysicalAscentFromFlexRelativeAscent(
  nscoord aFlexRelativeAscent,
  nscoord aContentBoxCrossSize,
  const ReflowInput& aReflowInput,
  const FlexboxAxisTracker& aAxisTracker)
{
  return aReflowInput.ComputedPhysicalBorderPadding().top +
    PhysicalCoordFromFlexRelativeCoord(aFlexRelativeAscent,
                                       aContentBoxCrossSize,
                                       aAxisTracker.GetCrossAxis());
}

void
nsFlexContainerFrame::SizeItemInCrossAxis(
  nsPresContext* aPresContext,
  const FlexboxAxisTracker& aAxisTracker,
  ReflowInput& aChildReflowInput,
  FlexItem& aItem)
{
  if (aAxisTracker.IsCrossAxisHorizontal()) {
    MOZ_ASSERT(aItem.HasIntrinsicRatio(),
               "For now, caller's CanMainSizeInfluenceCrossSize check should "
               "only allow us to get here for items with intrinsic ratio");
    // XXXdholbert When we finish support for vertical writing-modes,
    // (in bug 1079155 or a dependency), we'll relax the horizontal check in
    // CanMainSizeInfluenceCrossSize, and this function will need to be able
    // to measure the baseline & width (given our resolved height)
    // of vertical-writing-mode flex items here.
    // For now, we only expect to get here for items with an intrinsic aspect
    // ratio; and for those items, we can just read the size off of the reflow
    // state, without performing reflow.
    aItem.SetCrossSize(aChildReflowInput.ComputedWidth());
    return;
  }

  MOZ_ASSERT(!aItem.HadMeasuringReflow(),
             "We shouldn't need more than one measuring reflow");

  if (aItem.GetAlignSelf() == NS_STYLE_ALIGN_STRETCH) {
    // This item's got "align-self: stretch", so we probably imposed a
    // stretched computed height on it during its previous reflow. We're
    // not imposing that height for *this* measuring reflow, so we need to
    // tell it to treat this reflow as a vertical resize (regardless of
    // whether any of its ancestors are being resized).
    aChildReflowInput.SetVResize(true);
  }
  ReflowOutput childDesiredSize(aChildReflowInput);
  nsReflowStatus childReflowStatus;
  const uint32_t flags = NS_FRAME_NO_MOVE_FRAME;
  ReflowChild(aItem.Frame(), aPresContext,
              childDesiredSize, aChildReflowInput,
              0, 0, flags, childReflowStatus);
  aItem.SetHadMeasuringReflow();

  // XXXdholbert Once we do pagination / splitting, we'll need to actually
  // handle incomplete childReflowStatuses. But for now, we give our kids
  // unconstrained available height, which means they should always complete.
  MOZ_ASSERT(NS_FRAME_IS_COMPLETE(childReflowStatus),
             "We gave flex item unconstrained available height, so it "
             "should be complete");

  // Tell the child we're done with its initial reflow.
  // (Necessary for e.g. GetBaseline() to work below w/out asserting)
  FinishReflowChild(aItem.Frame(), aPresContext,
                    childDesiredSize, &aChildReflowInput, 0, 0, flags);

  // Save the sizing info that we learned from this reflow
  // -----------------------------------------------------

  // Tentatively store the child's desired content-box cross-size.
  // Note that childDesiredSize is the border-box size, so we have to
  // subtract border & padding to get the content-box size.
  // (Note that at this point in the code, we know our cross axis is vertical,
  // so we don't bother with making aAxisTracker pick the cross-axis component
  // for us.)
  nscoord crossAxisBorderPadding = aItem.GetBorderPadding().TopBottom();
  if (childDesiredSize.Height() < crossAxisBorderPadding) {
    // Child's requested size isn't large enough for its border/padding!
    // This is OK for the trivial nsFrame::Reflow() impl, but other frame
    // classes should know better. So, if we get here, the child had better be
    // an instance of nsFrame (i.e. it should return null from GetType()).
    // XXXdholbert Once we've fixed bug 765861, we should upgrade this to an
    // assertion that trivially passes if bug 765861's flag has been flipped.
    NS_WARNING_ASSERTION(
      !aItem.Frame()->GetType(),
      "Child should at least request space for border/padding");
    aItem.SetCrossSize(0);
  } else {
    // (normal case)
    aItem.SetCrossSize(childDesiredSize.Height() - crossAxisBorderPadding);
  }

  aItem.SetAscent(childDesiredSize.BlockStartAscent());
}

void
FlexLine::PositionItemsInCrossAxis(nscoord aLineStartPosition,
                                   const FlexboxAxisTracker& aAxisTracker)
{
  SingleLineCrossAxisPositionTracker lineCrossAxisPosnTracker(aAxisTracker);

  for (FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
    // First, stretch the item's cross size (if appropriate), and resolve any
    // auto margins in this axis.
    item->ResolveStretchedCrossSize(mLineCrossSize, aAxisTracker);
    lineCrossAxisPosnTracker.ResolveAutoMarginsInCrossAxis(*this, *item);

    // Compute the cross-axis position of this item
    nscoord itemCrossBorderBoxSize =
      item->GetCrossSize() +
      item->GetBorderPaddingSizeInAxis(aAxisTracker.GetCrossAxis());
    lineCrossAxisPosnTracker.EnterAlignPackingSpace(*this, *item, aAxisTracker);
    lineCrossAxisPosnTracker.EnterMargin(item->GetMargin());
    lineCrossAxisPosnTracker.EnterChildFrame(itemCrossBorderBoxSize);

    item->SetCrossPosition(aLineStartPosition +
                           lineCrossAxisPosnTracker.GetPosition());

    // Back out to cross-axis edge of the line.
    lineCrossAxisPosnTracker.ResetPosition();
  }
}

void
nsFlexContainerFrame::Reflow(nsPresContext* aPresContext,
                             ReflowOutput& aDesiredSize,
                             const ReflowInput& aReflowInput,
                             nsReflowStatus& aStatus)
{
  MarkInReflow();
  DO_GLOBAL_REFLOW_COUNT("nsFlexContainerFrame");
  DISPLAY_REFLOW(aPresContext, this, aReflowInput, aDesiredSize, aStatus);
  MOZ_LOG(gFlexContainerLog, LogLevel::Debug,
         ("Reflow() for nsFlexContainerFrame %p\n", this));

  if (IsFrameTreeTooDeep(aReflowInput, aDesiredSize, aStatus)) {
    return;
  }

  // We (and our children) can only depend on our ancestor's bsize if we have
  // a percent-bsize, or if we're positioned and we have "block-start" and "block-end"
  // set and have block-size:auto.  (There are actually other cases, too -- e.g. if
  // our parent is itself a block-dir flex container and we're flexible -- but
  // we'll let our ancestors handle those sorts of cases.)
  WritingMode wm = aReflowInput.GetWritingMode();
  const nsStylePosition* stylePos = StylePosition();
  const nsStyleCoord& bsize = stylePos->BSize(wm);
  if (bsize.HasPercent() ||
      (StyleDisplay()->IsAbsolutelyPositionedStyle() &&
       eStyleUnit_Auto == bsize.GetUnit() &&
       eStyleUnit_Auto != stylePos->mOffset.GetBStartUnit(wm) &&
       eStyleUnit_Auto != stylePos->mOffset.GetBEndUnit(wm))) {
    AddStateBits(NS_FRAME_CONTAINS_RELATIVE_BSIZE);
  }

  // If we've never reordered our children, then we can trust that they're
  // already in DOM-order, and we only need to consider their "order" property
  // when checking them for sortedness & sorting them.
  //
  // After we actually sort them, though, we can't trust that they're in DOM
  // order anymore.  So, from that point on, our sort & sorted-order-checking
  // operations need to use a fancier LEQ function that also takes DOM order
  // into account, so that we can honor the spec's requirement that frames w/
  // equal "order" values are laid out in DOM order.

  if (!HasAnyStateBits(NS_STATE_FLEX_CHILDREN_REORDERED)) {
    if (SortChildrenIfNeeded<IsOrderLEQ>()) {
      AddStateBits(NS_STATE_FLEX_CHILDREN_REORDERED);
    }
  } else {
    SortChildrenIfNeeded<IsOrderLEQWithDOMFallback>();
  }

  RenumberList();

  const FlexboxAxisTracker axisTracker(this, aReflowInput.GetWritingMode());

  // If we're being fragmented into a constrained BSize, then subtract off
  // borderpadding BStart from that constrained BSize, to get the available
  // BSize for our content box. (No need to subtract the borderpadding BStart
  // if we're already skipping it via GetLogicalSkipSides, though.)
  nscoord availableBSizeForContent = aReflowInput.AvailableBSize();
  if (availableBSizeForContent != NS_UNCONSTRAINEDSIZE &&
      !(GetLogicalSkipSides(&aReflowInput).BStart())) {
    availableBSizeForContent -=
      aReflowInput.ComputedLogicalBorderPadding().BStart(wm);
    // (Don't let that push availableBSizeForContent below zero, though):
    availableBSizeForContent = std::max(availableBSizeForContent, 0);
  }

  nscoord contentBoxMainSize = GetMainSizeFromReflowInput(aReflowInput,
                                                          axisTracker);

  AutoTArray<StrutInfo, 1> struts;
  DoFlexLayout(aPresContext, aDesiredSize, aReflowInput, aStatus,
               contentBoxMainSize, availableBSizeForContent,
               struts, axisTracker);

  if (!struts.IsEmpty()) {
    // We're restarting flex layout, with new knowledge of collapsed items.
    DoFlexLayout(aPresContext, aDesiredSize, aReflowInput, aStatus,
                 contentBoxMainSize, availableBSizeForContent,
                 struts, axisTracker);
  }
}

// RAII class to clean up a list of FlexLines.
// Specifically, this removes each line from the list, deletes all the
// FlexItems in its list, and deletes the FlexLine.
class MOZ_RAII AutoFlexLineListClearer
{
public:
  explicit AutoFlexLineListClearer(LinkedList<FlexLine>& aLines
                                   MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
  : mLines(aLines)
  {
    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
  }

  ~AutoFlexLineListClearer()
  {
    while (FlexLine* line = mLines.popFirst()) {
      while (FlexItem* item = line->mItems.popFirst()) {
        delete item;
      }
      delete line;
    }
  }

private:
  LinkedList<FlexLine>& mLines;
  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
};

// Class to let us temporarily provide an override value for the the main-size
// CSS property ('width' or 'height') on a flex item, for use in
// nsLayoutUtils::ComputeSizeWithIntrinsicDimensions.
// (We could use this overridden size more broadly, too, but it's probably
// better to avoid property-table accesses.  So, where possible, we communicate
// the resolved main-size to the child via modifying its reflow state directly,
// instead of using this class.)
class MOZ_RAII AutoFlexItemMainSizeOverride final
{
public:
  explicit AutoFlexItemMainSizeOverride(FlexItem& aItem
                                        MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
    : mItemProps(aItem.Frame()->Properties())
  {
    MOZ_GUARD_OBJECT_NOTIFIER_INIT;

    MOZ_ASSERT(!mItemProps.Has(nsIFrame::FlexItemMainSizeOverride()),
               "FlexItemMainSizeOverride prop shouldn't be set already; "
               "it should only be set temporarily (& not recursively)");
    NS_ASSERTION(aItem.HasIntrinsicRatio(),
                 "This should only be needed for items with an aspect ratio");

    mItemProps.Set(nsIFrame::FlexItemMainSizeOverride(), aItem.GetMainSize());
  }

  ~AutoFlexItemMainSizeOverride() {
    mItemProps.Remove(nsIFrame::FlexItemMainSizeOverride());
  }

private:
  const FrameProperties mItemProps;
  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
};

void
nsFlexContainerFrame::CalculatePackingSpace(uint32_t aNumThingsToPack,
                                            uint8_t aAlignVal,
                                            nscoord* aFirstSubjectOffset,
                                            uint32_t* aNumPackingSpacesRemaining,
                                            nscoord* aPackingSpaceRemaining)
{
  MOZ_ASSERT(NS_STYLE_ALIGN_SPACE_BETWEEN == NS_STYLE_JUSTIFY_SPACE_BETWEEN &&
             NS_STYLE_ALIGN_SPACE_AROUND == NS_STYLE_JUSTIFY_SPACE_AROUND &&
             NS_STYLE_ALIGN_SPACE_EVENLY == NS_STYLE_JUSTIFY_SPACE_EVENLY,
             "CalculatePackingSpace assumes that NS_STYLE_ALIGN_SPACE and "
             "NS_STYLE_JUSTIFY_SPACE constants are interchangeable");

  MOZ_ASSERT(aAlignVal == NS_STYLE_ALIGN_SPACE_BETWEEN ||
             aAlignVal == NS_STYLE_ALIGN_SPACE_AROUND ||
             aAlignVal == NS_STYLE_ALIGN_SPACE_EVENLY,
             "Unexpected alignment value");

  MOZ_ASSERT(*aPackingSpaceRemaining >= 0,
             "Should not be called with negative packing space");

  MOZ_ASSERT(aNumThingsToPack >= 1,
             "Should not be called with less than 1 thing to pack");

  // Packing spaces between items:
  *aNumPackingSpacesRemaining = aNumThingsToPack - 1;

  if (aAlignVal == NS_STYLE_ALIGN_SPACE_BETWEEN) {
    // No need to reserve space at beginning/end, so we're done.
    return;
  }

  // We need to add 1 or 2 packing spaces, split between beginning/end, for
  // space-around / space-evenly:
  size_t numPackingSpacesForEdges =
    aAlignVal == NS_STYLE_JUSTIFY_SPACE_AROUND ? 1 : 2;

  // How big will each "full" packing space be:
  nscoord packingSpaceSize = *aPackingSpaceRemaining /
    (*aNumPackingSpacesRemaining + numPackingSpacesForEdges);
  // How much packing-space are we allocating to the edges:
  nscoord totalEdgePackingSpace = numPackingSpacesForEdges * packingSpaceSize;

  // Use half of that edge packing space right now:
  *aFirstSubjectOffset += totalEdgePackingSpace / 2;
  // ...but we need to subtract all of it right away, so that we won't
  // hand out any of it to intermediate packing spaces.
  *aPackingSpaceRemaining -= totalEdgePackingSpace;
}

void
nsFlexContainerFrame::DoFlexLayout(nsPresContext*           aPresContext,
                                   ReflowOutput&     aDesiredSize,
                                   const ReflowInput& aReflowInput,
                                   nsReflowStatus&          aStatus,
                                   nscoord aContentBoxMainSize,
                                   nscoord aAvailableBSizeForContent,
                                   nsTArray<StrutInfo>& aStruts,
                                   const FlexboxAxisTracker& aAxisTracker)
{
  aStatus = NS_FRAME_COMPLETE;

  LinkedList<FlexLine> lines;
  AutoFlexLineListClearer cleanupLines(lines);

  GenerateFlexLines(aPresContext, aReflowInput,
                    aContentBoxMainSize,
                    aAvailableBSizeForContent,
                    aStruts, aAxisTracker, lines);

  aContentBoxMainSize =
    ResolveFlexContainerMainSize(aReflowInput, aAxisTracker,
                                 aContentBoxMainSize, aAvailableBSizeForContent,
                                 lines.getFirst(), aStatus);

  for (FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
    line->ResolveFlexibleLengths(aContentBoxMainSize);
  }

  // Cross Size Determination - Flexbox spec section 9.4
  // ===================================================
  // Calculate the hypothetical cross size of each item:
  nscoord sumLineCrossSizes = 0;
  for (FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
    for (FlexItem* item = line->GetFirstItem(); item; item = item->getNext()) {
      // The item may already have the correct cross-size; only recalculate
      // if the item's main size resolution (flexing) could have influenced it:
      if (item->CanMainSizeInfluenceCrossSize(aAxisTracker)) {
        Maybe<AutoFlexItemMainSizeOverride> sizeOverride;
        if (item->HasIntrinsicRatio()) {
          // For flex items with an aspect ratio, we have to impose an override
          // for the main-size property *before* we even instantiate the reflow
          // state, in order for aspect ratio calculations to produce the right
          // cross size in the reflow state. (For other flex items, it's OK
          // (and cheaper) to impose our main size *after* the reflow state has
          // been constructed, since the main size shouldn't influence anything
          // about cross-size measurement until we actually reflow the child.)
          sizeOverride.emplace(*item);
        }

        WritingMode wm = item->Frame()->GetWritingMode();
        LogicalSize availSize = aReflowInput.ComputedSize(wm);
        availSize.BSize(wm) = NS_UNCONSTRAINEDSIZE;
        ReflowInput childReflowInput(aPresContext, aReflowInput,
                                           item->Frame(), availSize);
        if (!sizeOverride) {
          // Directly override the computed main-size, by tweaking reflow state:
          if (aAxisTracker.IsMainAxisHorizontal()) {
            childReflowInput.SetComputedWidth(item->GetMainSize());
          } else {
            childReflowInput.SetComputedHeight(item->GetMainSize());
          }
        }

        SizeItemInCrossAxis(aPresContext, aAxisTracker,
                            childReflowInput, *item);
      }
    }
    // Now that we've finished with this line's items, size the line itself:
    line->ComputeCrossSizeAndBaseline(aAxisTracker);
    sumLineCrossSizes += line->GetLineCrossSize();
  }

  bool isCrossSizeDefinite;
  const nscoord contentBoxCrossSize =
    ComputeCrossSize(aReflowInput, aAxisTracker, sumLineCrossSizes,
                     aAvailableBSizeForContent, &isCrossSizeDefinite, aStatus);

  // Set up state for cross-axis alignment, at a high level (outside the
  // scope of a particular flex line)
  CrossAxisPositionTracker
    crossAxisPosnTracker(lines.getFirst(),
                         aReflowInput, contentBoxCrossSize,
                         isCrossSizeDefinite, aAxisTracker);

  // Now that we know the cross size of each line (including
  // "align-content:stretch" adjustments, from the CrossAxisPositionTracker
  // constructor), we can create struts for any flex items with
  // "visibility: collapse" (and restart flex layout).
  if (aStruts.IsEmpty()) { // (Don't make struts if we already did)
    BuildStrutInfoFromCollapsedItems(lines.getFirst(), aStruts);
    if (!aStruts.IsEmpty()) {
      // Restart flex layout, using our struts.
      return;
    }
  }

  // If the container should derive its baseline from the first FlexLine,
  // do that here (while crossAxisPosnTracker is conveniently pointing
  // at the cross-start edge of that line, which the line's baseline offset is
  // measured from):
  nscoord flexContainerAscent;
  if (!aAxisTracker.AreAxesInternallyReversed()) {
    nscoord firstLineBaselineOffset = lines.getFirst()->GetBaselineOffset();
    if (firstLineBaselineOffset == nscoord_MIN) {
      // No baseline-aligned items in line. Use sentinel value to prompt us to
      // get baseline from the first FlexItem after we've reflowed it.
      flexContainerAscent = nscoord_MIN;
    } else  {
      flexContainerAscent =
        ComputePhysicalAscentFromFlexRelativeAscent(
          crossAxisPosnTracker.GetPosition() + firstLineBaselineOffset,
          contentBoxCrossSize, aReflowInput, aAxisTracker);
    }
  }

  const auto justifyContent = IsLegacyBox(aReflowInput.mFrame) ?
    ConvertLegacyStyleToJustifyContent(StyleXUL()) :
    aReflowInput.mStylePosition->mJustifyContent;

  for (FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
    // Main-Axis Alignment - Flexbox spec section 9.5
    // ==============================================
    line->PositionItemsInMainAxis(justifyContent,
                                  aContentBoxMainSize,
                                  aAxisTracker);

    // Cross-Axis Alignment - Flexbox spec section 9.6
    // ===============================================
    line->PositionItemsInCrossAxis(crossAxisPosnTracker.GetPosition(),
                                   aAxisTracker);
    crossAxisPosnTracker.TraverseLine(*line);
    crossAxisPosnTracker.TraversePackingSpace();
  }

  // If the container should derive its baseline from the last FlexLine,
  // do that here (while crossAxisPosnTracker is conveniently pointing
  // at the cross-end edge of that line, which the line's baseline offset is
  // measured from):
  if (aAxisTracker.AreAxesInternallyReversed()) {
    nscoord lastLineBaselineOffset = lines.getLast()->GetBaselineOffset();
    if (lastLineBaselineOffset == nscoord_MIN) {
      // No baseline-aligned items in line. Use sentinel value to prompt us to
      // get baseline from the last FlexItem after we've reflowed it.
      flexContainerAscent = nscoord_MIN;
    } else {
      flexContainerAscent =
        ComputePhysicalAscentFromFlexRelativeAscent(
          crossAxisPosnTracker.GetPosition() - lastLineBaselineOffset,
          contentBoxCrossSize, aReflowInput, aAxisTracker);
    }
  }

  // Before giving each child a final reflow, calculate the origin of the
  // flex container's content box (with respect to its border-box), so that
  // we can compute our flex item's final positions.
  WritingMode flexWM = aReflowInput.GetWritingMode();
  LogicalMargin containerBP = aReflowInput.ComputedLogicalBorderPadding();

  // Unconditionally skip block-end border & padding for now, regardless of
  // writing-mode/GetLogicalSkipSides.  We add it lower down, after we've
  // established baseline and decided whether bottom border-padding fits (if
  // we're fragmented).
  const nscoord blockEndContainerBP = containerBP.BEnd(flexWM);
  const LogicalSides skipSides =
    GetLogicalSkipSides(&aReflowInput) | LogicalSides(eLogicalSideBitsBEnd);
  containerBP.ApplySkipSides(skipSides);

  const LogicalPoint containerContentBoxOrigin(flexWM,
                                               containerBP.IStart(flexWM),
                                               containerBP.BStart(flexWM));

  // Determine flex container's border-box size (used in positioning children):
  LogicalSize logSize =
    aAxisTracker.LogicalSizeFromFlexRelativeSizes(aContentBoxMainSize,
                                                  contentBoxCrossSize);
  logSize += aReflowInput.ComputedLogicalBorderPadding().Size(flexWM);
  nsSize containerSize = logSize.GetPhysicalSize(flexWM);

  // If the flex container has no baseline-aligned items, it will use this item
  // (the first item, discounting any under-the-hood reversing that we've done)
  // to determine its baseline:
  const FlexItem* const firstItem =
    aAxisTracker.AreAxesInternallyReversed()
    ? lines.getLast()->GetLastItem()
    : lines.getFirst()->GetFirstItem();

  // FINAL REFLOW: Give each child frame another chance to reflow, now that
  // we know its final size and position.
  for (const FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
    for (const FlexItem* item = line->GetFirstItem(); item;
         item = item->getNext()) {
      LogicalPoint framePos = aAxisTracker.LogicalPointFromFlexRelativePoint(
                               item->GetMainPosition(),
                               item->GetCrossPosition(),
                               aContentBoxMainSize,
                               contentBoxCrossSize);
      // Adjust framePos to be relative to the container's border-box
      // (i.e. its frame rect), instead of the container's content-box:
      framePos += containerContentBoxOrigin;

      // (Intentionally snapshotting this before ApplyRelativePositioning, to
      // maybe use for setting the flex container's baseline.)
      const nscoord itemNormalBPos = framePos.B(flexWM);

      // Check if we actually need to reflow the item -- if we already reflowed
      // it with the right size, we can just reposition it as-needed.
      bool itemNeedsReflow = true; // (Start out assuming the worst.)
      if (item->HadMeasuringReflow()) {
        LogicalSize finalFlexItemCBSize =
          aAxisTracker.LogicalSizeFromFlexRelativeSizes(item->GetMainSize(),
                                                        item->GetCrossSize());
        // We've already reflowed the child once. Was the size we gave it in
        // that reflow the same as its final (post-flexing/stretching) size?
        if (finalFlexItemCBSize ==
            LogicalSize(flexWM,
                        item->Frame()->GetContentRectRelativeToSelf().Size())) {
          // Even if our size hasn't changed, some of our descendants might
          // care that our bsize is now considered "definite" (whereas it
          // wasn't in our previous "measuring" reflow), if they have a
          // relative bsize.
          if (!(item->Frame()->GetStateBits() &
                NS_FRAME_CONTAINS_RELATIVE_BSIZE)) {
            // Item has the correct size (and its children don't care that
            // it's now "definite"). Let's just make sure it's at the right
            // position.
            itemNeedsReflow = false;
            MoveFlexItemToFinalPosition(aReflowInput, *item, framePos,
                                        containerSize);
          }
        }
      }
      if (itemNeedsReflow) {
        ReflowFlexItem(aPresContext, aAxisTracker, aReflowInput,
                       *item, framePos, containerSize);
      }

      // If this is our first item and we haven't established a baseline for
      // the container yet (i.e. if we don't have 'align-self: baseline' on any
      // children), then use this child's baseline as the container's baseline.
      if (item == firstItem &&
          flexContainerAscent == nscoord_MIN) {
        flexContainerAscent = itemNormalBPos + item->ResolvedAscent();
      }
    }
  }

  // Compute flex container's desired size (in its own writing-mode),
  // starting w/ content-box size & growing from there:
  LogicalSize desiredSizeInFlexWM =
    aAxisTracker.LogicalSizeFromFlexRelativeSizes(aContentBoxMainSize,
                                                  contentBoxCrossSize);
  // Add border/padding (w/ skipSides already applied):
  desiredSizeInFlexWM.ISize(flexWM) += containerBP.IStartEnd(flexWM);
  desiredSizeInFlexWM.BSize(flexWM) += containerBP.BStartEnd(flexWM);

  if (flexContainerAscent == nscoord_MIN) {
    // Still don't have our baseline set -- this happens if we have no
    // children (or if our children are huge enough that they have nscoord_MIN
    // as their baseline... in which case, we'll use the wrong baseline, but no
    // big deal)
    NS_WARNING_ASSERTION(
      lines.getFirst()->IsEmpty(),
      "Have flex items but didn't get an ascent - that's odd (or there are "
      "just gigantic sizes involved)");
    // Per spec, synthesize baseline from the flex container's content box
    // (i.e. use block-end side of content-box)
    // XXXdholbert This only makes sense if parent's writing mode is
    // horizontal (& even then, really we should be using the BSize in terms
    // of the parent's writing mode, not ours). Clean up in bug 1155322.
    flexContainerAscent = desiredSizeInFlexWM.BSize(flexWM);
  }

  // XXXdholbert flexContainerAscent needs to be in terms of
  // our parent's writing-mode here. See bug 1155322.
  aDesiredSize.SetBlockStartAscent(flexContainerAscent);

  // Cache this baseline for use outside of this call.
  mBaselineFromLastReflow = flexContainerAscent;

  // Now: If we're complete, add bottom border/padding to desired height (which
  // we skipped via skipSides) -- unless that pushes us over available height,
  // in which case we become incomplete (unless we already weren't asking for
  // any height, in which case we stay complete to avoid looping forever).
  // NOTE: If we're auto-height, we allow our bottom border/padding to push us
  // over the available height without requesting a continuation, for
  // consistency with the behavior of "display:block" elements.
  if (NS_FRAME_IS_COMPLETE(aStatus)) {
    nscoord desiredBSizeWithBEndBP =
      desiredSizeInFlexWM.BSize(flexWM) + blockEndContainerBP;

    if (aReflowInput.AvailableBSize() == NS_UNCONSTRAINEDSIZE ||
        desiredSizeInFlexWM.BSize(flexWM) == 0 ||
        desiredBSizeWithBEndBP <= aReflowInput.AvailableBSize() ||
        aReflowInput.ComputedBSize() == NS_INTRINSICSIZE) {
      // Update desired height to include block-end border/padding
      desiredSizeInFlexWM.BSize(flexWM) = desiredBSizeWithBEndBP;
    } else {
      // We couldn't fit bottom border/padding, so we'll need a continuation.
      NS_FRAME_SET_INCOMPLETE(aStatus);
    }
  }

  // Convert flex container's final desired size to parent's WM, for outparam.
  aDesiredSize.SetSize(flexWM, desiredSizeInFlexWM);

  // Overflow area = union(my overflow area, kids' overflow areas)
  aDesiredSize.SetOverflowAreasToDesiredBounds();
  for (nsIFrame* childFrame : mFrames) {
    ConsiderChildOverflow(aDesiredSize.mOverflowAreas, childFrame);
  }

  FinishReflowWithAbsoluteFrames(aPresContext, aDesiredSize,
                                 aReflowInput, aStatus);

  NS_FRAME_SET_TRUNCATION(aStatus, aReflowInput, aDesiredSize)
}

void
nsFlexContainerFrame::MoveFlexItemToFinalPosition(
  const ReflowInput& aReflowInput,
  const FlexItem& aItem,
  LogicalPoint& aFramePos,
  const nsSize& aContainerSize)
{
  WritingMode outerWM = aReflowInput.GetWritingMode();

  // If item is relpos, look up its offsets (cached from prev reflow)
  LogicalMargin logicalOffsets(outerWM);
  if (NS_STYLE_POSITION_RELATIVE == aItem.Frame()->StyleDisplay()->mPosition) {
    FrameProperties props = aItem.Frame()->Properties();
    nsMargin* cachedOffsets = props.Get(nsIFrame::ComputedOffsetProperty());
    MOZ_ASSERT(cachedOffsets,
               "relpos previously-reflowed frame should've cached its offsets");
    logicalOffsets = LogicalMargin(outerWM, *cachedOffsets);
  }
  ReflowInput::ApplyRelativePositioning(aItem.Frame(), outerWM,
                                              logicalOffsets, &aFramePos,
                                              aContainerSize);
  aItem.Frame()->SetPosition(outerWM, aFramePos, aContainerSize);
  PositionFrameView(aItem.Frame());
  PositionChildViews(aItem.Frame());
}

void
nsFlexContainerFrame::ReflowFlexItem(nsPresContext* aPresContext,
                                     const FlexboxAxisTracker& aAxisTracker,
                                     const ReflowInput& aReflowInput,
                                     const FlexItem& aItem,
                                     LogicalPoint& aFramePos,
                                     const nsSize& aContainerSize)
{
  WritingMode outerWM = aReflowInput.GetWritingMode();
  WritingMode wm = aItem.Frame()->GetWritingMode();
  LogicalSize availSize = aReflowInput.ComputedSize(wm);
  availSize.BSize(wm) = NS_UNCONSTRAINEDSIZE;
  ReflowInput childReflowInput(aPresContext, aReflowInput,
                                     aItem.Frame(), availSize);

  // Keep track of whether we've overriden the child's computed height
  // and/or width, so we can set its resize flags accordingly.
  bool didOverrideComputedWidth = false;
  bool didOverrideComputedHeight = false;

  // Override computed main-size
  if (aAxisTracker.IsMainAxisHorizontal()) {
    childReflowInput.SetComputedWidth(aItem.GetMainSize());
    didOverrideComputedWidth = true;
  } else {
    childReflowInput.SetComputedHeight(aItem.GetMainSize());
    didOverrideComputedHeight = true;
  }

  // Override reflow state's computed cross-size if either:
  // - the item was stretched (in which case we're imposing a cross size)
  // ...or...
  // - the item it has an aspect ratio (in which case the cross-size that's
  // currently in the reflow state is based on arithmetic involving a stale
  // main-size value that we just stomped on above). (Note that we could handle
  // this case using an AutoFlexItemMainSizeOverride, as we do elsewhere; but
  // given that we *already know* the correct cross size to use here, it's
  // cheaper to just directly set it instead of setting a frame property.)
  if (aItem.IsStretched() ||
      aItem.HasIntrinsicRatio()) {
    if (aAxisTracker.IsCrossAxisHorizontal()) {
      childReflowInput.SetComputedWidth(aItem.GetCrossSize());
      didOverrideComputedWidth = true;
    } else {
      childReflowInput.SetComputedHeight(aItem.GetCrossSize());
      didOverrideComputedHeight = true;
    }
  }
  if (aItem.IsStretched() && !aAxisTracker.IsCrossAxisHorizontal()) {
    // If this item's height is stretched, it's a relative height.
    aItem.Frame()->AddStateBits(NS_FRAME_CONTAINS_RELATIVE_BSIZE);
  }

  // XXXdholbert Might need to actually set the correct margins in the
  // reflow state at some point, so that they can be saved on the frame for
  // UsedMarginProperty().  Maybe doesn't matter though...?

  // If we're overriding the computed width or height, *and* we had an
  // earlier "measuring" reflow, then this upcoming reflow needs to be
  // treated as a resize.
  if (aItem.HadMeasuringReflow()) {
    if (didOverrideComputedWidth) {
      // (This is somewhat redundant, since the reflow state already
      // sets mHResize whenever our computed width has changed since the
      // previous reflow. Still, it's nice for symmetry, and it may become
      // necessary once we support orthogonal flows.)
      childReflowInput.SetHResize(true);
    }
    if (didOverrideComputedHeight) {
      childReflowInput.SetVResize(true);
    }
  }
  // NOTE: Be very careful about doing anything else with childReflowInput
  // after this point, because some of its methods (e.g. SetComputedWidth)
  // internally call InitResizeFlags and stomp on mVResize & mHResize.

  ReflowOutput childDesiredSize(childReflowInput);
  nsReflowStatus childReflowStatus;
  ReflowChild(aItem.Frame(), aPresContext,
              childDesiredSize, childReflowInput,
              outerWM, aFramePos, aContainerSize,
              0, childReflowStatus);

  // XXXdholbert Once we do pagination / splitting, we'll need to actually
  // handle incomplete childReflowStatuses. But for now, we give our kids
  // unconstrained available height, which means they should always
  // complete.
  MOZ_ASSERT(NS_FRAME_IS_COMPLETE(childReflowStatus),
             "We gave flex item unconstrained available height, so it "
             "should be complete");

  LogicalMargin offsets =
    childReflowInput.ComputedLogicalOffsets().ConvertTo(outerWM, wm);
  ReflowInput::ApplyRelativePositioning(aItem.Frame(), outerWM,
                                              offsets, &aFramePos,
                                              aContainerSize);

  FinishReflowChild(aItem.Frame(), aPresContext,
                    childDesiredSize, &childReflowInput,
                    outerWM, aFramePos, aContainerSize, 0);

  aItem.SetAscent(childDesiredSize.BlockStartAscent());
}

/* virtual */ nscoord
nsFlexContainerFrame::GetMinISize(nsRenderingContext* aRenderingContext)
{
  nscoord minISize = 0;
  DISPLAY_MIN_WIDTH(this, minISize);

  RenumberList();

  const nsStylePosition* stylePos = StylePosition();
  const FlexboxAxisTracker axisTracker(this, GetWritingMode());

  for (nsIFrame* childFrame : mFrames) {
    nscoord childMinISize =
      nsLayoutUtils::IntrinsicForContainer(aRenderingContext, childFrame,
                                           nsLayoutUtils::MIN_ISIZE);
    // For a horizontal single-line flex container, the intrinsic min
    // isize is the sum of its items' min isizes.
    // For a column-oriented flex container, or for a multi-line row-
    // oriented flex container, the intrinsic min isize is the max of
    // its items' min isizes.
    if (axisTracker.IsRowOriented() &&
        NS_STYLE_FLEX_WRAP_NOWRAP == stylePos->mFlexWrap) {
      minISize += childMinISize;
    } else {
      minISize = std::max(minISize, childMinISize);
    }
  }
  return minISize;
}

/* virtual */ nscoord
nsFlexContainerFrame::GetPrefISize(nsRenderingContext* aRenderingContext)
{
  nscoord prefISize = 0;
  DISPLAY_PREF_WIDTH(this, prefISize);

  RenumberList();

  // XXXdholbert Optimization: We could cache our intrinsic widths like
  // nsBlockFrame does (and return it early from this function if it's set).
  // Whenever anything happens that might change it, set it to
  // NS_INTRINSIC_WIDTH_UNKNOWN (like nsBlockFrame::MarkIntrinsicISizesDirty
  // does)
  const FlexboxAxisTracker axisTracker(this, GetWritingMode());

  for (nsIFrame* childFrame : mFrames) {
    nscoord childPrefISize =
      nsLayoutUtils::IntrinsicForContainer(aRenderingContext, childFrame,
                                           nsLayoutUtils::PREF_ISIZE);
    if (axisTracker.IsRowOriented()) {
      prefISize += childPrefISize;
    } else {
      prefISize = std::max(prefISize, childPrefISize);
    }
  }
  return prefISize;
}