Determine the right SVGTextMetrics using the correct approach

Source/core/layout/svg/SVGTextQuery.cpp

Index: Source/core/layout/svg/SVGTextQuery.cpp
diff --git a/Source/core/layout/svg/SVGTextQuery.cpp b/Source/core/layout/svg/SVGTextQuery.cpp
index e152abb11612fd7119e2655a8fa04c34ee95344d..2fa358df026b86ceada57817e3276995fbf90361 100644
--- a/Source/core/layout/svg/SVGTextQuery.cpp
+++ b/Source/core/layout/svg/SVGTextQuery.cpp
@@ -420,6 +420,21 @@ struct ExtentOfCharacterData : SVGTextQuery::Data {
     FloatRect extent;
 };
 
+const SVGTextMetrics& findMetricsForCharacter(const Vector<SVGTextMetrics>& textMetricsValues, const SVGTextFragment& fragment, unsigned startInFragment)
+{
+    // Find the text metrics cell that start at or contain the character at |startInFragment|.
+    unsigned textMetricsOffset = fragment.metricsListOffset;
+    unsigned fragmentOffset = 0;
+    while (fragmentOffset < fragment.length) {
+        const SVGTextMetrics& metrics = textMetricsValues[textMetricsOffset++];
+        unsigned glyphEnd = fragmentOffset + metrics.length();
+        if (startInFragment < glyphEnd)
+            break;
+        fragmentOffset = glyphEnd;
+    }
+    return textMetricsValues[textMetricsOffset - 1];

[Erik Dahlström (inactive), 2015/03/30 12:28:45]

Any chance we'll get an OOB index here?

[fs, 2015/03/30 13:26:40]

Only if the invariants don't hold. (And OOB here means RELEASE_ASSERT.) There's
a lot of different ways to write this function - none of them which is great
overall (IMHO).

[fs, 2015/03/30 14:00:56]

Non-BMP works too. Musical symbols to the rescue! (Test added.)

+}
+
 static inline void calculateGlyphBoundaries(SVGTextQuery::Data* queryData, const SVGTextFragment& fragment, int startPosition, FloatRect& extent)
 {
     float scalingFactor = queryData->textLayoutObject->scalingFactor();
@@ -432,8 +447,8 @@ static inline void calculateGlyphBoundaries(SVGTextQuery::Data* queryData, const
     // Use the SVGTextMetrics computed by SVGTextMetricsBuilder (which spends
     // time attempting to compute more correct glyph bounds already, handling
     // cursive scripts to some degree.)
-    Vector<SVGTextMetrics>& textMetricsValues = queryData->textLayoutObject->layoutAttributes()->textMetricsValues();
-    const SVGTextMetrics& metrics = textMetricsValues[fragment.characterOffset + startPosition];
+    const Vector<SVGTextMetrics>& textMetricsValues = queryData->textLayoutObject->layoutAttributes()->textMetricsValues();
+    const SVGTextMetrics& metrics = findMetricsForCharacter(textMetricsValues, fragment, startPosition);
 
     // TODO(fs): Negative glyph extents seems kind of weird to have, but
     // presently it can occur in some cases (like Arabic.)