third_party/WebKit/Source/core/html/CollectionIndexCache.h

/*
 * Copyright (C) 2013 Apple Inc. All rights reserved.
 * Copyright (C) 2014 Samsung Electronics. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *     * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above
 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
 * distribution.
 *     * Neither the name of Google Inc. nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#ifndef CollectionIndexCache_h
#define CollectionIndexCache_h

namespace blink {

template <typename Collection, typename NodeType>
class CollectionIndexCache {
  DISALLOW_NEW();

 public:
  CollectionIndexCache();

  bool isEmpty(const Collection& collection) {
    if (isCachedNodeCountValid())
      return !cachedNodeCount();
    if (cachedNode())
      return false;
    return !nodeAt(collection, 0);
  }
  bool hasExactlyOneNode(const Collection& collection) {
    if (isCachedNodeCountValid())
      return cachedNodeCount() == 1;
    if (cachedNode())
      return !cachedNodeIndex() && !nodeAt(collection, 1);
    return nodeAt(collection, 0) && !nodeAt(collection, 1);
  }

  unsigned nodeCount(const Collection&);
  NodeType* nodeAt(const Collection&, unsigned index);

  void invalidate();

  DEFINE_INLINE_VIRTUAL_TRACE() { visitor->trace(m_currentNode); }

 protected:
  ALWAYS_INLINE NodeType* cachedNode() const { return m_currentNode; }
  ALWAYS_INLINE unsigned cachedNodeIndex() const {
    DCHECK(cachedNode());
    return m_cachedNodeIndex;
  }
  ALWAYS_INLINE void setCachedNode(NodeType* node, unsigned index) {
    DCHECK(node);
    m_currentNode = node;
    m_cachedNodeIndex = index;
  }

  ALWAYS_INLINE bool isCachedNodeCountValid() const {
    return m_isLengthCacheValid;
  }
  ALWAYS_INLINE unsigned cachedNodeCount() const { return m_cachedNodeCount; }
  ALWAYS_INLINE void setCachedNodeCount(unsigned length) {
    m_cachedNodeCount = length;
    m_isLengthCacheValid = true;
  }

 private:
  NodeType* nodeBeforeCachedNode(const Collection&, unsigned index);
  NodeType* nodeAfterCachedNode(const Collection&, unsigned index);

  Member<NodeType> m_currentNode;
  unsigned m_cachedNodeCount;
  unsigned m_cachedNodeIndex : 31;
  unsigned m_isLengthCacheValid : 1;
};

template <typename Collection, typename NodeType>
CollectionIndexCache<Collection, NodeType>::CollectionIndexCache()
    : m_currentNode(nullptr),
      m_cachedNodeCount(0),
      m_cachedNodeIndex(0),
      m_isLengthCacheValid(false) {}

template <typename Collection, typename NodeType>
void CollectionIndexCache<Collection, NodeType>::invalidate() {
  m_currentNode = nullptr;
  m_isLengthCacheValid = false;
}

template <typename Collection, typename NodeType>
inline unsigned CollectionIndexCache<Collection, NodeType>::nodeCount(
    const Collection& collection) {
  if (isCachedNodeCountValid())
    return cachedNodeCount();

  nodeAt(collection, UINT_MAX);
  DCHECK(isCachedNodeCountValid());

  return cachedNodeCount();
}

template <typename Collection, typename NodeType>
inline NodeType* CollectionIndexCache<Collection, NodeType>::nodeAt(
    const Collection& collection,
    unsigned index) {
  if (isCachedNodeCountValid() && index >= cachedNodeCount())
    return nullptr;

  if (cachedNode()) {
    if (index > cachedNodeIndex())
      return nodeAfterCachedNode(collection, index);
    if (index < cachedNodeIndex())
      return nodeBeforeCachedNode(collection, index);
    return cachedNode();
  }

  // No valid cache yet, let's find the first matching element.
  DCHECK(!isCachedNodeCountValid());
  NodeType* firstNode = collection.traverseToFirst();
  if (!firstNode) {
    // The collection is empty.
    setCachedNodeCount(0);
    return nullptr;
  }
  setCachedNode(firstNode, 0);
  return index ? nodeAfterCachedNode(collection, index) : firstNode;
}

template <typename Collection, typename NodeType>
inline NodeType*
CollectionIndexCache<Collection, NodeType>::nodeBeforeCachedNode(
    const Collection& collection,
    unsigned index) {
  DCHECK(cachedNode());  // Cache should be valid.
  unsigned currentIndex = cachedNodeIndex();
  DCHECK_GT(currentIndex, index);

  // Determine if we should traverse from the beginning of the collection
  // instead of the cached node.
  bool firstIsCloser = index < currentIndex - index;
  if (firstIsCloser || !collection.canTraverseBackward()) {
    NodeType* firstNode = collection.traverseToFirst();
    DCHECK(firstNode);
    setCachedNode(firstNode, 0);
    return index ? nodeAfterCachedNode(collection, index) : firstNode;
  }

  // Backward traversal from the cached node to the requested index.
  DCHECK(collection.canTraverseBackward());
  NodeType* currentNode =
      collection.traverseBackwardToOffset(index, *cachedNode(), currentIndex);
  DCHECK(currentNode);
  setCachedNode(currentNode, currentIndex);
  return currentNode;
}

template <typename Collection, typename NodeType>
inline NodeType*
CollectionIndexCache<Collection, NodeType>::nodeAfterCachedNode(
    const Collection& collection,
    unsigned index) {
  DCHECK(cachedNode());  // Cache should be valid.
  unsigned currentIndex = cachedNodeIndex();
  DCHECK_LT(currentIndex, index);

  // Determine if we should traverse from the end of the collection instead of
  // the cached node.
  bool lastIsCloser = isCachedNodeCountValid() &&
                      cachedNodeCount() - index < index - currentIndex;
  if (lastIsCloser && collection.canTraverseBackward()) {
    NodeType* lastItem = collection.traverseToLast();
    DCHECK(lastItem);
    setCachedNode(lastItem, cachedNodeCount() - 1);
    if (index < cachedNodeCount() - 1)
      return nodeBeforeCachedNode(collection, index);
    return lastItem;
  }

  // Forward traversal from the cached node to the requested index.
  NodeType* currentNode =
      collection.traverseForwardToOffset(index, *cachedNode(), currentIndex);
  if (!currentNode) {
    // Did not find the node. On plus side, we now know the length.
    if (isCachedNodeCountValid())
      DCHECK_EQ(currentIndex + 1, cachedNodeCount());
    setCachedNodeCount(currentIndex + 1);
    return nullptr;
  }
  setCachedNode(currentNode, currentIndex);
  return currentNode;
}

}  // namespace blink

#endif  // CollectionIndexCache_h