tubestation/layout/generic/CSSOrderAwareFrameIterator.h

/* -*- 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/. */

/* Iterator class for frame lists that respect CSS "order" during layout */

#ifndef mozilla_CSSOrderAwareFrameIterator_h
#define mozilla_CSSOrderAwareFrameIterator_h

#include <algorithm>
#include "nsFrameList.h"
#include "nsIFrame.h"
#include "mozilla/Maybe.h"
#include "mozilla/Assertions.h"

#if defined(__clang__) && __clang_major__ == 3 && __clang_minor__ <= 8
#define CLANG_CRASH_BUG 1
#endif

namespace mozilla {

template<typename Iterator>
class CSSOrderAwareFrameIteratorT
{
public:
  enum OrderState { eUnknownOrder, eKnownOrdered, eKnownUnordered };
  enum ChildFilter { eSkipPlaceholders, eIncludeAll };
  CSSOrderAwareFrameIteratorT(nsIFrame* aContainer,
                              nsIFrame::ChildListID aListID,
                              ChildFilter aFilter = eSkipPlaceholders,
                              OrderState aState = eUnknownOrder)
    : mChildren(aContainer->GetChildList(aListID))
    , mArrayIndex(0)
    , mItemIndex(0)
    , mSkipPlaceholders(aFilter == eSkipPlaceholders)
#ifdef DEBUG
    , mContainer(aContainer)
    , mListID(aListID)
#endif
  {
    size_t count = 0;
    bool isOrdered = aState != eKnownUnordered;
    if (aState == eUnknownOrder) {
      auto maxOrder = std::numeric_limits<int32_t>::min();
      for (auto child : mChildren) {
        ++count;
        int32_t order = child->StylePosition()->mOrder;
        if (order < maxOrder) {
          isOrdered = false;
          break;
        }
        maxOrder = order;
      }
    }
    if (isOrdered) {
      mIter.emplace(begin(mChildren));
      mIterEnd.emplace(end(mChildren));
    } else {
      count *= 2; // XXX somewhat arbitrary estimate for now...
      mArray.emplace(count);
      for (Iterator i(begin(mChildren)), iEnd(end(mChildren)); i != iEnd; ++i) {
        mArray->AppendElement(*i);
      }
      // XXX replace this with nsTArray::StableSort when bug 1147091 is fixed.
      std::stable_sort(mArray->begin(), mArray->end(), CSSOrderComparator);
    }

    if (mSkipPlaceholders) {
      SkipPlaceholders();
    }
  }
  ~CSSOrderAwareFrameIteratorT()
  {
    MOZ_ASSERT(IsForward() == mItemCount.isNothing());
  }

  bool IsForward() const;
  Iterator begin(const nsFrameList& aList);
  Iterator end(const nsFrameList& aList);

  nsIFrame* operator*() const
  {
    MOZ_ASSERT(!AtEnd());
    if (mIter.isSome()) {
      return **mIter;
    }
    return (*mArray)[mArrayIndex];
  }

  /**
   * Return the child index of the current item, placeholders not counted.
   * It's forbidden to call this method when the current frame is placeholder.
   */
  size_t ItemIndex() const
  {
    MOZ_ASSERT(!AtEnd());
    MOZ_ASSERT((**this)->GetType() != nsGkAtoms::placeholderFrame,
               "MUST not call this when at a placeholder");
    MOZ_ASSERT(IsForward() || mItemIndex < *mItemCount,
               "Returning an out-of-range mItemIndex...");
    return mItemIndex;
  }

  void SetItemCount(size_t aItemCount)
  {
#ifndef CLANG_CRASH_BUG
    MOZ_ASSERT(mIter.isSome() || aItemCount <= mArray->Length(),
               "item count mismatch");
#endif
    mItemCount.emplace(aItemCount);
    // Note: it's OK if mItemIndex underflows -- ItemIndex()
    // will not be called unless there is at least one item.
    mItemIndex = IsForward() ? 0 : *mItemCount - 1;
  }

  /**
   * Skip over placeholder children.
   */
  void SkipPlaceholders()
  {
    if (mIter.isSome()) {
      for (; *mIter != *mIterEnd; ++*mIter) {
        nsIFrame* child = **mIter;
        if (child->GetType() != nsGkAtoms::placeholderFrame) {
          return;
        }
      }
    } else {
      for (; mArrayIndex < mArray->Length(); ++mArrayIndex) {
        nsIFrame* child = (*mArray)[mArrayIndex];
        if (child->GetType() != nsGkAtoms::placeholderFrame) {
          return;
        }
      }
    }
  }

  bool AtEnd() const
  {
#ifndef CLANG_CRASH_BUG
    // Clang 3.6.2 crashes when compiling this assertion:
    MOZ_ASSERT(mIter.isSome() || mArrayIndex <= mArray->Length());
#endif
    return mIter ? (*mIter == *mIterEnd) : mArrayIndex >= mArray->Length();
  }

  void Next()
  {
#ifdef DEBUG
    MOZ_ASSERT(!AtEnd());
    nsFrameList list = mContainer->GetChildList(mListID);
    MOZ_ASSERT(list.FirstChild() == mChildren.FirstChild() &&
               list.LastChild() == mChildren.LastChild(),
               "the list of child frames must not change while iterating!");
#endif
    if (mSkipPlaceholders ||
        (**this)->GetType() != nsGkAtoms::placeholderFrame) {
      IsForward() ? ++mItemIndex : --mItemIndex;
    }
    if (mIter.isSome()) {
      ++*mIter;
    } else {
      ++mArrayIndex;
    }
    if (mSkipPlaceholders) {
      SkipPlaceholders();
    }
  }

  void Reset(ChildFilter aFilter = eSkipPlaceholders)
  {
    if (mIter.isSome()) {
      mIter.reset();
      mIter.emplace(begin(mChildren));
      mIterEnd.reset();
      mIterEnd.emplace(end(mChildren));
    } else {
      mArrayIndex = 0;
    }
    mItemIndex = IsForward() ? 0 : *mItemCount - 1;
    mSkipPlaceholders = aFilter == eSkipPlaceholders;
    if (mSkipPlaceholders) {
      SkipPlaceholders();
    }
  }

  bool IsValid() const { return mIter.isSome() || mArray.isSome(); }

  void Invalidate()
  {
    mIter.reset();
    mArray.reset();
    mozWritePoison(&mChildren, sizeof(mChildren));
  }

  bool ItemsAreAlreadyInOrder() const { return mIter.isSome(); }

  static bool CSSOrderComparator(nsIFrame* const& a, nsIFrame* const& b);
private:
  nsFrameList mChildren;
  // Used if child list is already in ascending 'order'.
  Maybe<Iterator> mIter;
  Maybe<Iterator> mIterEnd;
  // Used if child list is *not* in ascending 'order'.
  // This array is pre-sorted in reverse order for a reverse iterator.
  Maybe<nsTArray<nsIFrame*>> mArray;
  size_t mArrayIndex;
  // The index of the current item (placeholders excluded).
  size_t mItemIndex;
  // The number of items (placeholders excluded).
  // It's only initialized and used in a reverse iterator.
  Maybe<size_t> mItemCount;
  // Skip placeholder children in the iteration?
  bool mSkipPlaceholders;
#ifdef DEBUG
  nsIFrame* mContainer;
  nsIFrame::ChildListID mListID;
#endif
};

typedef CSSOrderAwareFrameIteratorT<nsFrameList::iterator>
  CSSOrderAwareFrameIterator;
typedef CSSOrderAwareFrameIteratorT<nsFrameList::reverse_iterator>
  ReverseCSSOrderAwareFrameIterator;

} // namespace mozilla

#endif // mozilla_CSSOrderAwareFrameIterator_h