Optimize Complex Text Shaping and Caching

Source/platform/fonts/shaping/CachingWordShapeIterator.h

Index: Source/platform/fonts/shaping/CachingWordShapeIterator.h
diff --git a/Source/platform/fonts/shaping/CachingWordShapeIterator.h b/Source/platform/fonts/shaping/CachingWordShapeIterator.h
new file mode 100644
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+/*
+ * Copyright (C) 2015 Google 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 GOOGLE 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 CachingWordShapeIterator_h
+#define CachingWordShapeIterator_h
+
+#include "platform/fonts/SimpleFontData.h"
+#include "platform/fonts/shaping/CachingWordShapeIterator.h"
+#include "platform/fonts/shaping/HarfBuzzShaper.h"
+#include "platform/fonts/shaping/ShapeCache.h"
+#include "wtf/unicode/CharacterNames.h"
+
+namespace blink {
+
+class CachingWordShapeIterator {
+public:
+    CachingWordShapeIterator(ShapeCache* cache, const TextRun& run,
+        const Font* font, HashSet<const SimpleFontData*>* fallbackFonts)
+        : m_shapeCache(cache), m_textRun(run), m_font(font)
+        , m_fallbackFonts(fallbackFonts), m_startIndex(0)
+    {
+        ASSERT(font);
+        const FontDescription& fontDescription = font->fontDescription();
+        bool hasWordSpacingOrLetterSpacing = fontDescription.wordSpacing()
+            || fontDescription.letterSpacing();
+        // Word spacing and letter spacing can change the width of a word.
+        // If a tab occurs inside a word, the width of the word varies based on
+        // its position on the line.
+        // If expansion is used (for justified text) the spacing between words
+        // change and thus we cannot use the word cache.
+        // FIXME: Move expansion handling from shaping to ShapeResult and add
+        // support for expansion to shapeWord.
+        m_isCacheable = !hasWordSpacingOrLetterSpacing && !run.allowTabs()
+            && m_textRun.expansion() == 0.0f;
+
+        // Shaping word by word is faster as each word is cached. If we cannot
+        // use the cache or if the font doesn't support word by word shaping
+        // fall back on shaping the entire run.
+        m_shapeByWord = m_font->canShapeWordByWord() && m_isCacheable;
+    }
+
+    bool next(RefPtr<ShapeResult>* wordResult)
+    {
+        if (!m_shapeByWord) {
+            if (m_startIndex)
+                return false;
+            *wordResult = shapeWord(m_textRun, m_font, m_fallbackFonts);
+            m_startIndex = 1;
+            return *wordResult;
+        }
+
+        unsigned length = m_textRun.length();
+        if (m_startIndex < length) {
+            if (m_textRun[m_startIndex] == spaceCharacter) {
+                if (!m_spaceShape) {
+                    TextRun wordRun = m_textRun.subRun(m_startIndex, 1);
+                    m_spaceShape = shapeWord(wordRun, m_font, m_fallbackFonts);
+                }
+                *wordResult = m_spaceShape;
+                m_startIndex++;
+                return true;
+            }
+
+            for (unsigned i = m_startIndex; ; i++) {
+                if (i == length || m_textRun[i] == spaceCharacter) {
+                    TextRun wordRun = m_textRun.subRun(m_startIndex,
+                        i - m_startIndex);
+                    *wordResult = shapeWord(wordRun, m_font, m_fallbackFonts);
+                    m_startIndex = i;
+                    return true;
+                }
+            }
+        }
+        return false;
+    }
+
+private:
+    PassRefPtr<ShapeResult> shapeWord(const TextRun& wordRun,
+        const Font* font, HashSet<const SimpleFontData*>* fallbackFonts)
+    {
+        ShapeCacheEntry* cacheEntry = m_isCacheable
+            ? m_shapeCache->add(wordRun, ShapeCacheEntry())
+            : nullptr;
+        if (cacheEntry && cacheEntry->m_shapeResult)
+            return cacheEntry->m_shapeResult;
+
+        HarfBuzzShaper shaper(font, wordRun, fallbackFonts);
+        RefPtr<ShapeResult> shapeResult = shaper.shapeResult();
+        if (!shapeResult)
+            return nullptr;
+
+        if (cacheEntry && (!fallbackFonts || fallbackFonts->isEmpty()))

[drott, 2015/06/23 12:45:51]

Could you add a similar FIXME + crbug for the fact that we disable caching when
the shaping process reports additional fallback fonts?

+            cacheEntry->m_shapeResult = shapeResult;
+
+        return shapeResult.release();
+    }
+
+    ShapeCache* m_shapeCache;
+    const TextRun& m_textRun;
+    const Font* m_font;
+    HashSet<const SimpleFontData*>* m_fallbackFonts;
+    unsigned m_startIndex : 30;
+    unsigned m_isCacheable : 1;

[drott, 2015/06/23 12:45:51]

When reviewing, I struggled to understand what this means from the name of the
variable. It's clear to me now, but perhaps we can find a better name?
m_paramsCacheable, or m_runAndFontCacheable?

+    unsigned m_shapeByWord : 1;
+    RefPtr<ShapeResult> m_spaceShape;
+};
+
+} // namespace blink
+
+#endif // CachingWordShapeIterator_h