flutter_flutter/sky/engine/platform/fonts/WidthCache.h

/*
 * Copyright (C) 2012 Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. 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.
 *
 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``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 APPLE INC. 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, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
 * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#ifndef SKY_ENGINE_PLATFORM_FONTS_WIDTHCACHE_H_
#define SKY_ENGINE_PLATFORM_FONTS_WIDTHCACHE_H_

#include "flutter/sky/engine/platform/geometry/IntRectExtent.h"
#include "flutter/sky/engine/platform/text/TextRun.h"
#include "flutter/sky/engine/wtf/Forward.h"
#include "flutter/sky/engine/wtf/HashFunctions.h"
#include "flutter/sky/engine/wtf/HashSet.h"
#include "flutter/sky/engine/wtf/HashTableDeletedValueType.h"
#include "flutter/sky/engine/wtf/StringHasher.h"

namespace blink {

struct WidthCacheEntry {
  WidthCacheEntry() { width = std::numeric_limits<float>::quiet_NaN(); }
  bool isValid() const { return !std::isnan(width); }
  float width;
  IntRectExtent glyphBounds;
};

class WidthCache {
 private:
  // Used to optimize small strings as hash table keys. Avoids malloc'ing an
  // out-of-line StringImpl.
  class SmallStringKey {
   public:
    static unsigned capacity() { return s_capacity; }

    SmallStringKey() : m_length(s_emptyValueLength) {}

    SmallStringKey(WTF::HashTableDeletedValueType)
        : m_length(s_deletedValueLength) {}

    template <typename CharacterType>
    SmallStringKey(CharacterType* characters, unsigned short length)
        : m_length(length) {
      ASSERT(length <= s_capacity);

      StringHasher hasher;

      bool remainder = length & 1;
      length >>= 1;

      unsigned i = 0;
      while (length--) {
        m_characters[i] = characters[i];
        m_characters[i + 1] = characters[i + 1];
        hasher.addCharactersAssumingAligned(characters[i], characters[i + 1]);
        i += 2;
      }

      if (remainder) {
        m_characters[i] = characters[i];
        hasher.addCharacter(characters[i]);
      }

      m_hash = hasher.hash();
    }

    const UChar* characters() const { return m_characters; }
    unsigned short length() const { return m_length; }
    unsigned hash() const { return m_hash; }

    bool isHashTableDeletedValue() const {
      return m_length == s_deletedValueLength;
    }
    bool isHashTableEmptyValue() const {
      return m_length == s_emptyValueLength;
    }

   private:
    static const unsigned s_capacity = 15;
    static const unsigned s_emptyValueLength = s_capacity + 1;
    static const unsigned s_deletedValueLength = s_capacity + 2;

    unsigned m_hash;
    unsigned short m_length;
    UChar m_characters[s_capacity];
  };

  struct SmallStringKeyHash {
    static unsigned hash(const SmallStringKey& key) { return key.hash(); }
    static bool equal(const SmallStringKey& a, const SmallStringKey& b) {
      return a == b;
    }
    static const bool safeToCompareToEmptyOrDeleted =
        true;  // Empty and deleted values have lengths that are not equal to
               // any valid length.
  };

  struct SmallStringKeyHashTraits : WTF::SimpleClassHashTraits<SmallStringKey> {
    static const bool hasIsEmptyValueFunction = true;
    static bool isEmptyValue(const SmallStringKey& key) {
      return key.isHashTableEmptyValue();
    }
    static const bool needsDestruction = false;
    static const unsigned minimumTableSize = 16;
  };

  friend bool operator==(const SmallStringKey&, const SmallStringKey&);

 public:
  WidthCache() : m_interval(s_maxInterval), m_countdown(m_interval) {}

  WidthCacheEntry* add(const TextRun& run, WidthCacheEntry entry) {
    if (static_cast<unsigned>(run.length()) > SmallStringKey::capacity())
      return 0;

    if (m_countdown > 0) {
      --m_countdown;
      return 0;
    }

    return addSlowCase(run, entry);
  }

  void clear() {
    m_singleCharMap.clear();
    m_map.clear();
  }

 private:
  WidthCacheEntry* addSlowCase(const TextRun& run, WidthCacheEntry entry) {
    int length = run.length();
    bool isNewEntry;
    WidthCacheEntry* value;
    if (length == 1) {
      SingleCharMap::AddResult addResult = m_singleCharMap.add(run[0], entry);
      isNewEntry = addResult.isNewEntry;
      value = &addResult.storedValue->value;
    } else {
      SmallStringKey smallStringKey;
      if (run.is8Bit())
        smallStringKey = SmallStringKey(run.characters8(), length);
      else
        smallStringKey = SmallStringKey(run.characters16(), length);

      Map::AddResult addResult = m_map.add(smallStringKey, entry);
      isNewEntry = addResult.isNewEntry;
      value = &addResult.storedValue->value;
    }

    // Cache hit: ramp up by sampling the next few words.
    if (!isNewEntry) {
      m_interval = s_minInterval;
      return value;
    }

    // Cache miss: ramp down by increasing our sampling interval.
    if (m_interval < s_maxInterval)
      ++m_interval;
    m_countdown = m_interval;

    if ((m_singleCharMap.size() + m_map.size()) < s_maxSize)
      return value;

    // No need to be fancy: we're just trying to avoid pathological growth.
    m_singleCharMap.clear();
    m_map.clear();
    return 0;
  }

  typedef HashMap<SmallStringKey,
                  WidthCacheEntry,
                  SmallStringKeyHash,
                  SmallStringKeyHashTraits>
      Map;
  typedef HashMap<uint32_t,
                  WidthCacheEntry,
                  DefaultHash<uint32_t>::Hash,
                  WTF::UnsignedWithZeroKeyHashTraits<uint32_t>>
      SingleCharMap;
  static const int s_minInterval =
      -3;  // A cache hit pays for about 3 cache misses.
  static const int s_maxInterval =
      20;  // Sampling at this interval has almost no overhead.
  static const unsigned s_maxSize =
      500000;  // Just enough to guard against pathological growth.

  int m_interval;
  int m_countdown;
  SingleCharMap m_singleCharMap;
  Map m_map;
};

inline bool operator==(const WidthCache::SmallStringKey& a,
                       const WidthCache::SmallStringKey& b) {
  if (a.length() != b.length())
    return false;
  return WTF::equal(a.characters(), b.characters(), a.length());
}

}  // namespace blink

#endif  // SKY_ENGINE_PLATFORM_FONTS_WIDTHCACHE_H_