mfbt/LinkedList.h
author Bryce Van Dyk <bvandyk@mozilla.com>
Mon, 27 Aug 2018 16:25:54 +0000
changeset 488616 259675bd67f96acf1ae0f2ed6115de4331a94d32
parent 483927 9c5f1bec200521f3d605604ff99beab5fbd8a1d7
child 505383 6f3709b3878117466168c40affa7bca0b60cf75b
permissions -rw-r--r--
Bug 1486502 - Add widevine CDM headers to third party paths, clang-format ignore. r=sylvestre We wish to keep the widevine headers in the same formatting as upstream to ease comparison and as we do not modify these files. This patch adds the existing headers, as well as another we anticipate pulling down for our next bump (content_decryption_module_proxy.h) to the ignored paths. These files are ignored individually rather than the whole directory they're in, as we also have Mozilla code in that dir. Differential Revision: https://phabricator.services.mozilla.com/D4347

/* -*- 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 type-safe doubly-linked list class. */

/*
 * The classes LinkedList<T> and LinkedListElement<T> together form a
 * convenient, type-safe doubly-linked list implementation.
 *
 * The class T which will be inserted into the linked list must inherit from
 * LinkedListElement<T>.  A given object may be in only one linked list at a
 * time.
 *
 * A LinkedListElement automatically removes itself from the list upon
 * destruction, and a LinkedList will fatally assert in debug builds if it's
 * non-empty when it's destructed.
 *
 * For example, you might use LinkedList in a simple observer list class as
 * follows.
 *
 *   class Observer : public LinkedListElement<Observer>
 *   {
 *   public:
 *     void observe(char* aTopic) { ... }
 *   };
 *
 *   class ObserverContainer
 *   {
 *   private:
 *     LinkedList<Observer> list;
 *
 *   public:
 *     void addObserver(Observer* aObserver)
 *     {
 *       // Will assert if |aObserver| is part of another list.
 *       list.insertBack(aObserver);
 *     }
 *
 *     void removeObserver(Observer* aObserver)
 *     {
 *       // Will assert if |aObserver| is not part of some list.
 *       aObserver.remove();
 *       // Or, will assert if |aObserver| is not part of |list| specifically.
 *       // aObserver.removeFrom(list);
 *     }
 *
 *     void notifyObservers(char* aTopic)
 *     {
 *       for (Observer* o = list.getFirst(); o != nullptr; o = o->getNext()) {
 *         o->observe(aTopic);
 *       }
 *     }
 *   };
 *
 * Additionally, the class AutoCleanLinkedList<T> is a LinkedList<T> that will
 * remove and delete each element still within itself upon destruction. Note
 * that because each element is deleted, elements must have been allocated
 * using |new|.
 */

#ifndef mozilla_LinkedList_h
#define mozilla_LinkedList_h

#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/MemoryReporting.h"
#include "mozilla/Move.h"
#include "mozilla/RefPtr.h"

#ifdef __cplusplus

namespace mozilla {

template<typename T>
class LinkedListElement;

namespace detail {

/**
 * LinkedList supports refcounted elements using this adapter class. Clients
 * using LinkedList<RefPtr<T>> will get a data structure that holds a strong
 * reference to T as long as T is in the list.
 */
template<typename T>
struct LinkedListElementTraits
{
  typedef T* RawType;
  typedef const T* ConstRawType;
  typedef T* ClientType;
  typedef const T* ConstClientType;

  // These static methods are called when an element is added to or removed from
  // a linked list. It can be used to keep track ownership in lists that are
  // supposed to own their elements. If elements are transferred from one list
  // to another, no enter or exit calls happen since the elements still belong
  // to a list.
  static void enterList(LinkedListElement<T>* elt) {}
  static void exitList(LinkedListElement<T>* elt) {}

  // This method is called when AutoCleanLinkedList cleans itself
  // during destruction. It can be used to call delete on elements if
  // the list is the sole owner.
  static void cleanElement(LinkedListElement<T>* elt) { delete elt->asT(); }
};

template<typename T>
struct LinkedListElementTraits<RefPtr<T>>
{
  typedef T* RawType;
  typedef const T* ConstRawType;
  typedef RefPtr<T> ClientType;
  typedef RefPtr<const T> ConstClientType;

  static void enterList(LinkedListElement<RefPtr<T>>* elt) { elt->asT()->AddRef(); }
  static void exitList(LinkedListElement<RefPtr<T>>* elt) { elt->asT()->Release(); }
  static void cleanElement(LinkedListElement<RefPtr<T>>* elt) {}
};

} /* namespace detail */

template<typename T>
class LinkedList;

template<typename T>
class LinkedListElement
{
  typedef typename detail::LinkedListElementTraits<T> Traits;
  typedef typename Traits::RawType RawType;
  typedef typename Traits::ConstRawType ConstRawType;
  typedef typename Traits::ClientType ClientType;
  typedef typename Traits::ConstClientType ConstClientType;

  /*
   * It's convenient that we return nullptr when getNext() or getPrevious()
   * hits the end of the list, but doing so costs an extra word of storage in
   * each linked list node (to keep track of whether |this| is the sentinel
   * node) and a branch on this value in getNext/getPrevious.
   *
   * We could get rid of the extra word of storage by shoving the "is
   * sentinel" bit into one of the pointers, although this would, of course,
   * have performance implications of its own.
   *
   * But the goal here isn't to win an award for the fastest or slimmest
   * linked list; rather, we want a *convenient* linked list.  So we won't
   * waste time guessing which micro-optimization strategy is best.
   *
   *
   * Speaking of unnecessary work, it's worth addressing here why we wrote
   * mozilla::LinkedList in the first place, instead of using stl::list.
   *
   * The key difference between mozilla::LinkedList and stl::list is that
   * mozilla::LinkedList stores the mPrev/mNext pointers in the object itself,
   * while stl::list stores the mPrev/mNext pointers in a list element which
   * itself points to the object being stored.
   *
   * mozilla::LinkedList's approach makes it harder to store an object in more
   * than one list.  But the upside is that you can call next() / prev() /
   * remove() directly on the object.  With stl::list, you'd need to store a
   * pointer to its iterator in the object in order to accomplish this.  Not
   * only would this waste space, but you'd have to remember to update that
   * pointer every time you added or removed the object from a list.
   *
   * In-place, constant-time removal is a killer feature of doubly-linked
   * lists, and supporting this painlessly was a key design criterion.
   */

private:
  LinkedListElement* mNext;
  LinkedListElement* mPrev;
  const bool mIsSentinel;

public:
  LinkedListElement()
    : mNext(this),
      mPrev(this),
      mIsSentinel(false)
  { }

  /*
   * Moves |aOther| into |*this|. If |aOther| is already in a list, then
   * |aOther| is removed from the list and replaced by |*this|.
   */
  LinkedListElement(LinkedListElement<T>&& aOther)
    : mIsSentinel(aOther.mIsSentinel)
  {
    adjustLinkForMove(std::move(aOther));
  }

  LinkedListElement& operator=(LinkedListElement<T>&& aOther)
  {
    MOZ_ASSERT(mIsSentinel == aOther.mIsSentinel, "Mismatch NodeKind!");
    MOZ_ASSERT(!isInList(),
               "Assigning to an element in a list messes up that list!");
    adjustLinkForMove(std::move(aOther));
    return *this;
  }

  ~LinkedListElement()
  {
    if (!mIsSentinel && isInList()) {
      remove();
    }
  }

  /*
   * Get the next element in the list, or nullptr if this is the last element
   * in the list.
   */
  RawType getNext()            { return mNext->asT(); }
  ConstRawType getNext() const { return mNext->asT(); }

  /*
   * Get the previous element in the list, or nullptr if this is the first
   * element in the list.
   */
  RawType getPrevious()            { return mPrev->asT(); }
  ConstRawType getPrevious() const { return mPrev->asT(); }

  /*
   * Insert aElem after this element in the list.  |this| must be part of a
   * linked list when you call setNext(); otherwise, this method will assert.
   */
  void setNext(RawType aElem)
  {
    MOZ_ASSERT(isInList());
    setNextUnsafe(aElem);
  }

  /*
   * Insert aElem before this element in the list.  |this| must be part of a
   * linked list when you call setPrevious(); otherwise, this method will
   * assert.
   */
  void setPrevious(RawType aElem)
  {
    MOZ_ASSERT(isInList());
    setPreviousUnsafe(aElem);
  }

  /*
   * Remove this element from the list which contains it.  If this element is
   * not currently part of a linked list, this method asserts.
   */
  void remove()
  {
    MOZ_ASSERT(isInList());

    mPrev->mNext = mNext;
    mNext->mPrev = mPrev;
    mNext = this;
    mPrev = this;

    Traits::exitList(this);
  }

  /*
   * Remove this element from the list containing it.  Returns a pointer to the
   * element that follows this element (before it was removed).  This method
   * asserts if the element does not belong to a list. Note: In a refcounted list,
   * |this| may be destroyed.
   */
  RawType removeAndGetNext()
  {
    RawType r = getNext();
    remove();
    return r;
  }

  /*
   * Remove this element from the list containing it.  Returns a pointer to the
   * previous element in the containing list (before the removal).  This method
   * asserts if the element does not belong to a list. Note: In a refcounted list,
   * |this| may be destroyed.
   */
  RawType removeAndGetPrevious()
  {
    RawType r = getPrevious();
    remove();
    return r;
  }

  /*
   * Identical to remove(), but also asserts in debug builds that this element
   * is in aList.
   */
  void removeFrom(const LinkedList<T>& aList)
  {
    aList.assertContains(asT());
    remove();
  }

  /*
   * Return true if |this| part is of a linked list, and false otherwise.
   */
  bool isInList() const
  {
    MOZ_ASSERT((mNext == this) == (mPrev == this));
    return mNext != this;
  }

private:
  friend class LinkedList<T>;
  friend struct detail::LinkedListElementTraits<T>;

  enum class NodeKind {
    Normal,
    Sentinel
  };

  explicit LinkedListElement(NodeKind nodeKind)
    : mNext(this),
      mPrev(this),
      mIsSentinel(nodeKind == NodeKind::Sentinel)
  { }

  /*
   * Return |this| cast to T* if we're a normal node, or return nullptr if
   * we're a sentinel node.
   */
  RawType asT()
  {
    return mIsSentinel ? nullptr : static_cast<RawType>(this);
  }
  ConstRawType asT() const
  {
    return mIsSentinel ? nullptr : static_cast<ConstRawType>(this);
  }

  /*
   * Insert aElem after this element, but don't check that this element is in
   * the list.  This is called by LinkedList::insertFront().
   */
  void setNextUnsafe(RawType aElem)
  {
    LinkedListElement *listElem = static_cast<LinkedListElement*>(aElem);
    MOZ_ASSERT(!listElem->isInList());

    listElem->mNext = this->mNext;
    listElem->mPrev = this;
    this->mNext->mPrev = listElem;
    this->mNext = listElem;

    Traits::enterList(aElem);
  }

  /*
   * Insert aElem before this element, but don't check that this element is in
   * the list.  This is called by LinkedList::insertBack().
   */
  void setPreviousUnsafe(RawType aElem)
  {
    LinkedListElement<T>* listElem = static_cast<LinkedListElement<T>*>(aElem);
    MOZ_ASSERT(!listElem->isInList());

    listElem->mNext = this;
    listElem->mPrev = this->mPrev;
    this->mPrev->mNext = listElem;
    this->mPrev = listElem;

    Traits::enterList(aElem);
  }

  /*
   * Adjust mNext and mPrev for implementing move constructor and move
   * assignment.
   */
  void adjustLinkForMove(LinkedListElement<T>&& aOther)
  {
    if (!aOther.isInList()) {
      mNext = this;
      mPrev = this;
      return;
    }

    if (!mIsSentinel) {
      Traits::enterList(this);
    }

    MOZ_ASSERT(aOther.mNext->mPrev == &aOther);
    MOZ_ASSERT(aOther.mPrev->mNext == &aOther);

    /*
     * Initialize |this| with |aOther|'s mPrev/mNext pointers, and adjust those
     * element to point to this one.
     */
    mNext = aOther.mNext;
    mPrev = aOther.mPrev;

    mNext->mPrev = this;
    mPrev->mNext = this;

    /*
     * Adjust |aOther| so it doesn't think it's in a list.  This makes it
     * safely destructable.
     */
    aOther.mNext = &aOther;
    aOther.mPrev = &aOther;

    if (!mIsSentinel) {
      Traits::exitList(&aOther);
    }
  }

  LinkedListElement& operator=(const LinkedListElement<T>& aOther) = delete;
  LinkedListElement(const LinkedListElement<T>& aOther) = delete;
};

template<typename T>
class LinkedList
{
private:
  typedef typename detail::LinkedListElementTraits<T> Traits;
  typedef typename Traits::RawType RawType;
  typedef typename Traits::ConstRawType ConstRawType;
  typedef typename Traits::ClientType ClientType;
  typedef typename Traits::ConstClientType ConstClientType;
  typedef LinkedListElement<T>* ElementType;
  typedef const LinkedListElement<T>* ConstElementType;

  LinkedListElement<T> sentinel;

public:
  template <typename Type, typename Element>
  class Iterator {
    Type mCurrent;

  public:
    explicit Iterator(Type aCurrent) : mCurrent(aCurrent) {}

    Type operator *() const {
      return mCurrent;
    }

    const Iterator& operator++() {
      mCurrent = static_cast<Element>(mCurrent)->getNext();
      return *this;
    }

    bool operator!=(const Iterator& aOther) const {
      return mCurrent != aOther.mCurrent;
    }
  };

  LinkedList() : sentinel(LinkedListElement<T>::NodeKind::Sentinel) { }

  LinkedList(LinkedList<T>&& aOther)
    : sentinel(std::move(aOther.sentinel))
  { }

  LinkedList& operator=(LinkedList<T>&& aOther)
  {
    MOZ_ASSERT(isEmpty(), "Assigning to a non-empty list leaks elements in that list!");
    sentinel = std::move(aOther.sentinel);
    return *this;
  }

  ~LinkedList() {
    MOZ_ASSERT(isEmpty(),
               "failing this assertion means this LinkedList's creator is "
               "buggy: it should have removed all this list's elements before "
               "the list's destruction");
  }

  /*
   * Add aElem to the front of the list.
   */
  void insertFront(RawType aElem)
  {
    /* Bypass setNext()'s this->isInList() assertion. */
    sentinel.setNextUnsafe(aElem);
  }

  /*
   * Add aElem to the back of the list.
   */
  void insertBack(RawType aElem)
  {
    sentinel.setPreviousUnsafe(aElem);
  }

  /*
   * Get the first element of the list, or nullptr if the list is empty.
   */
  RawType getFirst()            { return sentinel.getNext(); }
  ConstRawType getFirst() const { return sentinel.getNext(); }

  /*
   * Get the last element of the list, or nullptr if the list is empty.
   */
  RawType getLast()            { return sentinel.getPrevious(); }
  ConstRawType getLast() const { return sentinel.getPrevious(); }

  /*
   * Get and remove the first element of the list.  If the list is empty,
   * return nullptr.
   */
  ClientType popFirst()
  {
    ClientType ret = sentinel.getNext();
    if (ret) {
      static_cast<LinkedListElement<T>*>(RawType(ret))->remove();
    }
    return ret;
  }

  /*
   * Get and remove the last element of the list.  If the list is empty,
   * return nullptr.
   */
  ClientType popLast()
  {
    ClientType ret = sentinel.getPrevious();
    if (ret) {
      static_cast<LinkedListElement<T>*>(RawType(ret))->remove();
    }
    return ret;
  }

  /*
   * Return true if the list is empty, or false otherwise.
   */
  bool isEmpty() const
  {
    return !sentinel.isInList();
  }

  /*
   * Remove all the elements from the list.
   *
   * This runs in time linear to the list's length, because we have to mark
   * each element as not in the list.
   */
  void clear()
  {
    while (popFirst()) {
    }
  }

  /*
   * Allow range-based iteration:
   *
   *     for (MyElementType* elt : myList) { ... }
   */
  Iterator<RawType, ElementType> begin() {
    return Iterator<RawType, ElementType>(getFirst());
  }
  Iterator<ConstRawType, ConstElementType> begin() const {
    return Iterator<ConstRawType, ConstElementType>(getFirst());
  }
  Iterator<RawType, ElementType> end() {
    return Iterator<RawType, ElementType>(nullptr);
  }
  Iterator<ConstRawType, ConstElementType> end() const {
    return Iterator<ConstRawType, ConstElementType>(nullptr);
  }

  /*
   * Measures the memory consumption of the list excluding |this|.  Note that
   * it only measures the list elements themselves.  If the list elements
   * contain pointers to other memory blocks, those blocks must be measured
   * separately during a subsequent iteration over the list.
   */
  size_t sizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const
  {
    size_t n = 0;
    ConstRawType t = getFirst();
    while (t) {
      n += aMallocSizeOf(t);
      t = static_cast<const LinkedListElement<T>*>(t)->getNext();
    }
    return n;
  }

  /*
   * Like sizeOfExcludingThis(), but measures |this| as well.
   */
  size_t sizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
  {
    return aMallocSizeOf(this) + sizeOfExcludingThis(aMallocSizeOf);
  }

  /*
   * In a debug build, make sure that the list is sane (no cycles, consistent
   * mNext/mPrev pointers, only one sentinel).  Has no effect in release builds.
   */
  void debugAssertIsSane() const
  {
#ifdef DEBUG
    const LinkedListElement<T>* slow;
    const LinkedListElement<T>* fast1;
    const LinkedListElement<T>* fast2;

    /*
     * Check for cycles in the forward singly-linked list using the
     * tortoise/hare algorithm.
     */
    for (slow = sentinel.mNext,
         fast1 = sentinel.mNext->mNext,
         fast2 = sentinel.mNext->mNext->mNext;
         slow != &sentinel && fast1 != &sentinel && fast2 != &sentinel;
         slow = slow->mNext, fast1 = fast2->mNext, fast2 = fast1->mNext) {
      MOZ_ASSERT(slow != fast1);
      MOZ_ASSERT(slow != fast2);
    }

    /* Check for cycles in the backward singly-linked list. */
    for (slow = sentinel.mPrev,
         fast1 = sentinel.mPrev->mPrev,
         fast2 = sentinel.mPrev->mPrev->mPrev;
         slow != &sentinel && fast1 != &sentinel && fast2 != &sentinel;
         slow = slow->mPrev, fast1 = fast2->mPrev, fast2 = fast1->mPrev) {
      MOZ_ASSERT(slow != fast1);
      MOZ_ASSERT(slow != fast2);
    }

    /*
     * Check that |sentinel| is the only node in the list with
     * mIsSentinel == true.
     */
    for (const LinkedListElement<T>* elem = sentinel.mNext;
         elem != &sentinel;
         elem = elem->mNext) {
      MOZ_ASSERT(!elem->mIsSentinel);
    }

    /* Check that the mNext/mPrev pointers match up. */
    const LinkedListElement<T>* prev = &sentinel;
    const LinkedListElement<T>* cur = sentinel.mNext;
    do {
        MOZ_ASSERT(cur->mPrev == prev);
        MOZ_ASSERT(prev->mNext == cur);

        prev = cur;
        cur = cur->mNext;
    } while (cur != &sentinel);
#endif /* ifdef DEBUG */
  }

private:
  friend class LinkedListElement<T>;

  void assertContains(const RawType aValue) const
  {
#ifdef DEBUG
    for (ConstRawType elem = getFirst(); elem; elem = elem->getNext()) {
      if (elem == aValue) {
        return;
      }
    }
    MOZ_CRASH("element wasn't found in this list!");
#endif
  }

  LinkedList& operator=(const LinkedList<T>& aOther) = delete;
  LinkedList(const LinkedList<T>& aOther) = delete;
};

template <typename T>
class AutoCleanLinkedList : public LinkedList<T>
{
private:
  using Traits = detail::LinkedListElementTraits<T>;
  using ClientType = typename detail::LinkedListElementTraits<T>::ClientType;
public:
  ~AutoCleanLinkedList()
  {
    clear();
  }

  AutoCleanLinkedList& operator=(AutoCleanLinkedList&& aOther)
  {
    LinkedList<T>::operator=(std::forward<LinkedList<T>>(aOther));
    return *this;
  }

  void clear()
  {
    while (ClientType element = this->popFirst()) {
      Traits::cleanElement(element);
    }
  }
};

} /* namespace mozilla */

#endif /* __cplusplus */

#endif /* mozilla_LinkedList_h */