Simplify SVGTextQuery::modifyStartEndPositionsRespectingLigatures

https://bugs.webkit.org/show_bug.cgi?id=262458

Reviewed by Chris Dumez.

Merge: https://src.chromium.org/viewvc/blink?view=revision&revision=177231

Simplify this method by passing it the fragment in question, and then use
SVGTextFragment::metricsListOffset to get a reasonable starting point.
Rewrite the big loop into two simpler loops - one for each end-point.
This shaves another ~15% off the TC in the bug (on top of 268656@main).

* Source/WebCore/rendering/svg/SVGTextQuery.cpp:
(SVGTextQuery::mapStartEndPositionsIntoFragmentCoordinates):
(SVGTextQuery::modifyStartEndPositionsRespectingLigatures):
* Source/WebCore/rendering/svg/SVGTextQuery.h:

Canonical link: https://commits.webkit.org/268751@main

Source/WebCore/rendering/svg/SVGTextQuery.cpp

@@ -155,82 +155,45 @@ bool SVGTextQuery::mapStartEndPositionsIntoFragmentCoordinates(Data* queryData,
     if (startPosition >= endPosition)
         return false;
 
-    modifyStartEndPositionsRespectingLigatures(queryData, startPosition, endPosition);
+    modifyStartEndPositionsRespectingLigatures(queryData, fragment, startPosition, endPosition);
     if (!queryData->textBox->mapStartEndPositionsIntoFragmentCoordinates(fragment, startPosition, endPosition))
         return false;
 
     ASSERT_WITH_SECURITY_IMPLICATION(startPosition < endPosition);
     return true;
 }
 
-void SVGTextQuery::modifyStartEndPositionsRespectingLigatures(Data* queryData, unsigned& startPosition, unsigned& endPosition) const
+void SVGTextQuery::modifyStartEndPositionsRespectingLigatures(Data* queryData, const SVGTextFragment& fragment, unsigned& startPosition, unsigned& endPosition) const
 {
     SVGTextLayoutAttributes* layoutAttributes = queryData->textRenderer->layoutAttributes();
     Vector<SVGTextMetrics>& textMetricsValues = layoutAttributes->textMetricsValues();
-    unsigned boxStart = queryData->textBox->start();
-    unsigned boxLength = queryData->textBox->len();
 
-    unsigned textMetricsOffset = 0;
-    unsigned textMetricsSize = textMetricsValues.size();
-
-    unsigned positionOffset = 0;
-    unsigned positionSize = layoutAttributes->context().text().length();
-
-    bool alterStartPosition = true;
-    bool alterEndPosition = true;
-
-    std::optional<unsigned> lastPositionOffset;
-    for (; textMetricsOffset < textMetricsSize && positionOffset < positionSize; ++textMetricsOffset) {
-        SVGTextMetrics& metrics = textMetricsValues[textMetricsOffset];
-
-        // Advance to text box start location.
-        if (positionOffset < boxStart) {
-            positionOffset += metrics.length();
-            continue;
-        }
-
-        // Stop if we've finished processing this text box.
-        if (positionOffset >= boxStart + boxLength)
-            break;
-
-        // If the start position maps to a character in the metrics list, we don't need to modify it.
-        if (startPosition == positionOffset)
-            alterStartPosition = false;
-
-        // If the start position maps to a character in the metrics list, we don't need to modify it.
-        if (endPosition == positionOffset)
-            alterEndPosition = false;
-
-        // Detect ligatures.
-        if (lastPositionOffset && lastPositionOffset.value() - positionOffset > 1) {
-            if (alterStartPosition && startPosition > lastPositionOffset.value() && startPosition < positionOffset) {
-                startPosition = lastPositionOffset.value();
-                alterStartPosition = false;
-            }
-
-            if (alterEndPosition && endPosition > lastPositionOffset.value() && endPosition < positionOffset) {
-                endPosition = positionOffset;
-                alterEndPosition = false;
-            }
-        }
+    unsigned textMetricsOffset = fragment.metricsListOffset;
 
-        if (!alterStartPosition && !alterEndPosition)
+    // Compute the offset of the fragment within the box, since that's the
+    // space <startPosition, endPosition> is in (and that's what we need).
+    unsigned fragmentOffsetInBox = fragment.characterOffset - queryData->textBox->start();
+    unsigned fragmentEndInBox = fragmentOffsetInBox + fragment.length;
+    // Find the text metrics cell that start at or contain the character startPosition.
+    while (fragmentOffsetInBox < fragmentEndInBox) {
+        auto& metrics = textMetricsValues[textMetricsOffset];
+        unsigned glyphEnd = fragmentOffsetInBox + metrics.length();
+        if (startPosition < glyphEnd)
             break;
-
-        lastPositionOffset = positionOffset;
-        positionOffset += metrics.length();
+        fragmentOffsetInBox = glyphEnd;
+        ++textMetricsOffset;
     }
+    startPosition = fragmentOffsetInBox;
 
-    if (!alterStartPosition && !alterEndPosition)
-        return;
-
-    if (lastPositionOffset && lastPositionOffset.value() - positionOffset > 1) {
-        if (alterStartPosition && startPosition > lastPositionOffset.value() && startPosition < positionOffset)
-            startPosition = lastPositionOffset.value();
-
-        if (alterEndPosition && endPosition > lastPositionOffset.value() && endPosition < positionOffset)
-            endPosition = positionOffset;
+    // Find the text metrics cell that contain or ends at the character endPosition.
+    while (fragmentOffsetInBox < fragmentEndInBox) {
+        auto& metrics = textMetricsValues[textMetricsOffset];
+        fragmentOffsetInBox += metrics.length();
+        if (fragmentOffsetInBox >= endPosition)
+            break;
+        ++textMetricsOffset;
     }
+    endPosition = fragmentOffsetInBox;
 }
 
 // numberOfCharacters() implementation

Source/WebCore/rendering/svg/SVGTextQuery.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (C) Research In Motion Limited 2010. All rights reserved.
+ * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ -51,7 +52,7 @@ class SVGTextQuery {
 
     void collectTextBoxesInFlowBox(LegacyInlineFlowBox*);
     bool mapStartEndPositionsIntoFragmentCoordinates(Data*, const SVGTextFragment&, unsigned& startPosition, unsigned& endPosition) const;
-    void modifyStartEndPositionsRespectingLigatures(Data*, unsigned& startPosition, unsigned& endPosition) const;
+    void modifyStartEndPositionsRespectingLigatures(Data*, const SVGTextFragment&, unsigned& startPosition, unsigned& endPosition) const;
 
 private:
     bool numberOfCharactersCallback(Data*, const SVGTextFragment&) const;