mfbt/SplayTree.h
author Petru Lingurar <petru.lingurar@softvision.ro>
Fri, 21 Dec 2018 08:56:47 +0000
changeset 501492 65621d0fe1262af0643cec37c23b2d9ec42588ad
parent 477157 ec2b59b0c7ffee3211c82df35273b4dbcbc1de44
child 508163 6f3709b3878117466168c40affa7bca0b60cf75b
permissions -rw-r--r--
Bug 1513938 - Enforce a Bundle size limit and drop `privateSession` if exceeds it. r=JanH, a=jcristau The `privateSession` key would normally allow persisting the Private Browsing session across OOMs in Activity's Bundle. We need to do that to avoid storing private, sensible data on disk like we do with the normal browsing session. In some cases `privateSession` would contain a lot of data which, along with other possible concurrent transactions could overflow Binder's buffer which has a limited fixed size, currently 1Mb. To avoid this, we will drop `privateSession` from the Bundle if the resulting size is greater than a _speculative_ size of 300KBs which would mean that in the case of an OOM all Private Browsing state would be lost. Bug 1515592 is filed to investigate for a better solution. Differential Revision: https://phabricator.services.mozilla.com/D15067

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/**
 * A sorted tree with optimal access times, where recently-accessed elements
 * are faster to access again.
 */

#ifndef mozilla_SplayTree_h
#define mozilla_SplayTree_h

#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"

namespace mozilla {

template<class T, class C>
class SplayTree;

template<typename T>
class SplayTreeNode
{
public:
  template<class A, class B>
  friend class SplayTree;

  SplayTreeNode()
    : mLeft(nullptr)
    , mRight(nullptr)
    , mParent(nullptr)
  {}

private:
  T* mLeft;
  T* mRight;
  T* mParent;
};


/**
 * Class which represents a splay tree.
 * Splay trees are balanced binary search trees for which search, insert and
 * remove are all amortized O(log n), but where accessing a node makes it
 * faster to access that node in the future.
 *
 * T indicates the type of tree elements, Comparator must have a static
 * compare(const T&, const T&) method ordering the elements. The compare
 * method must be free from side effects.
 */
template<typename T, class Comparator>
class SplayTree
{
  T* mRoot;

public:
  constexpr SplayTree()
    : mRoot(nullptr)
  {}

  bool empty() const
  {
    return !mRoot;
  }

  T* find(const T& aValue)
  {
    if (empty()) {
      return nullptr;
    }

    T* last = lookup(aValue);
    splay(last);
    return Comparator::compare(aValue, *last) == 0 ? last : nullptr;
  }

  void insert(T* aValue)
  {
    MOZ_ASSERT(!find(*aValue), "Duplicate elements are not allowed.");

    if (!mRoot) {
      mRoot = aValue;
      return;
    }
    T* last = lookup(*aValue);
    int cmp = Comparator::compare(*aValue, *last);

    finishInsertion(last, cmp, aValue);
  }

  T* findOrInsert(const T& aValue);

  T* remove(const T& aValue)
  {
    T* last = lookup(aValue);
    MOZ_ASSERT(last, "This tree must contain the element being removed.");
    MOZ_ASSERT(Comparator::compare(aValue, *last) == 0);

    // Splay the tree so that the item to remove is the root.
    splay(last);
    MOZ_ASSERT(last == mRoot);

    // Find another node which can be swapped in for the root: either the
    // rightmost child of the root's left, or the leftmost child of the
    // root's right.
    T* swap;
    T* swapChild;
    if (mRoot->mLeft) {
      swap = mRoot->mLeft;
      while (swap->mRight) {
        swap = swap->mRight;
      }
      swapChild = swap->mLeft;
    } else if (mRoot->mRight) {
      swap = mRoot->mRight;
      while (swap->mLeft) {
        swap = swap->mLeft;
      }
      swapChild = swap->mRight;
    } else {
      T* result = mRoot;
      mRoot = nullptr;
      return result;
    }

    // The selected node has at most one child, in swapChild. Detach it
    // from the subtree by replacing it with that child.
    if (swap == swap->mParent->mLeft) {
      swap->mParent->mLeft = swapChild;
    } else {
      swap->mParent->mRight = swapChild;
    }
    if (swapChild) {
      swapChild->mParent = swap->mParent;
    }

    // Make the selected node the new root.
    mRoot = swap;
    mRoot->mParent = nullptr;
    mRoot->mLeft = last->mLeft;
    mRoot->mRight = last->mRight;
    if (mRoot->mLeft) {
      mRoot->mLeft->mParent = mRoot;
    }
    if (mRoot->mRight) {
      mRoot->mRight->mParent = mRoot;
    }

    last->mLeft = nullptr;
    last->mRight = nullptr;
    return last;
  }

  T* removeMin()
  {
    MOZ_ASSERT(mRoot, "No min to remove!");

    T* min = mRoot;
    while (min->mLeft) {
      min = min->mLeft;
    }
    return remove(*min);
  }

  // For testing purposes only.
  void checkCoherency()
  {
    checkCoherency(mRoot, nullptr);
  }

private:
  /**
   * Returns the node in this comparing equal to |aValue|, or a node just
   * greater or just less than |aValue| if there is no such node.
   */
  T* lookup(const T& aValue)
  {
    MOZ_ASSERT(!empty());

    T* node = mRoot;
    T* parent;
    do {
      parent = node;
      int c = Comparator::compare(aValue, *node);
      if (c == 0) {
        return node;
      } else if (c < 0) {
        node = node->mLeft;
      } else {
        node = node->mRight;
      }
    } while (node);
    return parent;
  }

  void finishInsertion(T* aLast, int32_t aCmp, T* aNew)
  {
    MOZ_ASSERT(aCmp, "Nodes shouldn't be equal!");

    T** parentPointer = (aCmp < 0) ? &aLast->mLeft : &aLast->mRight;
    MOZ_ASSERT(!*parentPointer);
    *parentPointer = aNew;
    aNew->mParent = aLast;

    splay(aNew);
  }

  /**
   * Rotate the tree until |node| is at the root of the tree. Performing
   * the rotations in this fashion preserves the amortized balancing of
   * the tree.
   */
  void splay(T* aNode)
  {
    MOZ_ASSERT(aNode);

    while (aNode != mRoot) {
      T* parent = aNode->mParent;
      if (parent == mRoot) {
        // Zig rotation.
        rotate(aNode);
        MOZ_ASSERT(aNode == mRoot);
        return;
      }
      T* grandparent = parent->mParent;
      if ((parent->mLeft == aNode) == (grandparent->mLeft == parent)) {
        // Zig-zig rotation.
        rotate(parent);
        rotate(aNode);
      } else {
        // Zig-zag rotation.
        rotate(aNode);
        rotate(aNode);
      }
    }
  }

  void rotate(T* aNode)
  {
    // Rearrange nodes so that aNode becomes the parent of its current
    // parent, while preserving the sortedness of the tree.
    T* parent = aNode->mParent;
    if (parent->mLeft == aNode) {
      //     x          y
      //   y  c  ==>  a  x
      //  a b           b c
      parent->mLeft = aNode->mRight;
      if (aNode->mRight) {
        aNode->mRight->mParent = parent;
      }
      aNode->mRight = parent;
    } else {
      MOZ_ASSERT(parent->mRight == aNode);
      //   x             y
      //  a  y   ==>   x  c
      //    b c       a b
      parent->mRight = aNode->mLeft;
      if (aNode->mLeft) {
        aNode->mLeft->mParent = parent;
      }
      aNode->mLeft = parent;
    }
    aNode->mParent = parent->mParent;
    parent->mParent = aNode;
    if (T* grandparent = aNode->mParent) {
      if (grandparent->mLeft == parent) {
        grandparent->mLeft = aNode;
      } else {
        grandparent->mRight = aNode;
      }
    } else {
      mRoot = aNode;
    }
  }

  T* checkCoherency(T* aNode, T* aMinimum)
  {
    if (mRoot) {
      MOZ_RELEASE_ASSERT(!mRoot->mParent);
    }
    if (!aNode) {
      MOZ_RELEASE_ASSERT(!mRoot);
      return nullptr;
    }
    if (!aNode->mParent) {
      MOZ_RELEASE_ASSERT(aNode == mRoot);
    }
    if (aMinimum) {
      MOZ_RELEASE_ASSERT(Comparator::compare(*aMinimum, *aNode) < 0);
    }
    if (aNode->mLeft) {
      MOZ_RELEASE_ASSERT(aNode->mLeft->mParent == aNode);
      T* leftMaximum = checkCoherency(aNode->mLeft, aMinimum);
      MOZ_RELEASE_ASSERT(Comparator::compare(*leftMaximum, *aNode) < 0);
    }
    if (aNode->mRight) {
      MOZ_RELEASE_ASSERT(aNode->mRight->mParent == aNode);
      return checkCoherency(aNode->mRight, aNode);
    }
    return aNode;
  }

  SplayTree(const SplayTree&) = delete;
  void operator=(const SplayTree&) = delete;
};

template<typename T, class Comparator>
T*
SplayTree<T, Comparator>::findOrInsert(const T& aValue)
{
  if (!mRoot) {
    mRoot = new T(aValue);
    return mRoot;
  }

  T* last = lookup(aValue);
  int cmp = Comparator::compare(aValue, *last);
  if (!cmp) {
    return last;
  }

  T* t = new T(aValue);
  finishInsertion(last, cmp, t);
  return t;
}

}  /* namespace mozilla */

#endif /* mozilla_SplayTree_h */