WebVTT: Implement 'best position' from the rendering section

Source/core/rendering/RenderVTTCue.cpp

Index: Source/core/rendering/RenderVTTCue.cpp
diff --git a/Source/core/rendering/RenderVTTCue.cpp b/Source/core/rendering/RenderVTTCue.cpp
index 5d20273e884c1250f0c8b33b183c48fef4f1e35f..c67ba49322bcf6b2aa6133909c6852eb9c17ff9d 100644
--- a/Source/core/rendering/RenderVTTCue.cpp
+++ b/Source/core/rendering/RenderVTTCue.cpp
@@ -51,6 +51,7 @@ public:
         : m_cueBox(cueBox)
         , m_cueWritingDirection(writingDirection)
         , m_linePosition(linePosition)
+        , m_bestPositionScore(std::numeric_limits<float>::infinity())
     {
     }
 
@@ -59,6 +60,7 @@ public:
 private:
     bool isOutside() const;
     bool isOverlapping() const;
+    float computePositionScore() const;
     bool shouldSwitchDirection(InlineFlowBox*, LayoutUnit) const;
 
     void moveBoxesByStep(LayoutUnit);
@@ -68,10 +70,12 @@ private:
     bool initializeLayoutParameters(InlineFlowBox*, LayoutUnit&, LayoutUnit&);
     void placeBoxInDefaultPosition(LayoutUnit, bool&);
 
-    LayoutPoint m_specifiedPosition;
     RenderVTTCue& m_cueBox;
     VTTCue::WritingDirection m_cueWritingDirection;
     float m_linePosition;
+    LayoutPoint m_specifiedPosition;
+    LayoutPoint m_bestPosition;
+    float m_bestPositionScore;
 };
 
 bool SnapToLinesLayouter::findFirstLineBox(InlineFlowBox*& firstLineBox)
@@ -143,7 +147,13 @@ void SnapToLinesLayouter::placeBoxInDefaultPosition(LayoutUnit position, bool& s
     m_specifiedPosition = m_cueBox.location();
 
     // 14. Let best position be null. It will hold a position for boxes, much like specified position in the previous step.
+    ASSERT(m_bestPosition == LayoutPoint::zero());
+
     // 15. Let best position score be null.
+    // (Setting score to positive infinite that should be guaranteed to be
+    // larger than any overlap - and easily recognizable as 'best position is
+    // null' as well.)
+    ASSERT(std::isinf(m_bestPositionScore));
 
     // 16. Let switched be false.
     switched = false;
@@ -213,16 +223,15 @@ bool SnapToLinesLayouter::switchDirection(bool& switched, LayoutUnit& step)
     // 24. Switch direction: If switched is true, then move all the boxes in
     // boxes back to their best position, and jump to the step labeled done
     // positioning below.
+    if (switched) {
+        m_cueBox.setLocation(m_bestPosition);
+        return false;
+    }
 
     // 25. Otherwise, move all the boxes in boxes back to their specified
     // position as determined in the earlier step.
     m_cueBox.setLocation(m_specifiedPosition);
 
-    // XX. If switched is true, jump to the step labeled done
-    // positioning below.
-    if (switched)
-        return false;
-
     // 26. Negate step.
     step = -step;
 
@@ -231,6 +240,22 @@ bool SnapToLinesLayouter::switchDirection(bool& switched, LayoutUnit& step)
     return true;
 }
 
+static float computeArea(const LayoutRect& rect)
+{
+    return rect.size().width().toFloat() * rect.size().height().toFloat();
+}
+
+float SnapToLinesLayouter::computePositionScore() const
+{
+    // Compute the percentage of the area of the bounding box of the boxes in
+    // boxes that is outside the title area box.
+    LayoutRect boundingBox = m_cueBox.absoluteBoundingBoxRect();
+    LayoutRect intersectingBox = intersection(m_cueBox.containingBlock()->absoluteBoundingBoxRect(), boundingBox);

[philipj_slow, 2015/01/27 10:36:09]

Extract this into a LayoutRect titleAreaBox to be very explicit? Also makes the
line shorter.

[fs, 2015/01/27 11:54:53]

Done.

+    // We know the intersection/overlap, so compute the percentage of the
+    // overlap and then invert.
+    return 1.0f - computeArea(intersectingBox) / computeArea(boundingBox);

[philipj_slow, 2015/01/27 10:36:09]

Can you assert that it's in the range [0,1]? The talk about percentages could
lead one astray. Making it [0,100] would also be fine.

[fs, 2015/01/27 11:54:53]

Modified this a bit to compute the area in LayoutUnit instead (and then remain
in LayoutUnits until the division.)

+}
+
 void SnapToLinesLayouter::layout()
 {
     // http://dev.w3.org/html5/webvtt/#dfn-apply-webvtt-cue-settings
@@ -259,6 +284,8 @@ void SnapToLinesLayouter::layout()
         // 19. Let current position score be the percentage of the area of the
         // bounding box of the boxes in boxes that is outside the title area
         // box.
+        float currentPositionScore = computePositionScore();
+
         // 20. If best position is null (i.e. this is the first run through
         // this loop, switched is still false, the boxes in boxes are at their
         // specified position, and best position score is still null), or if
@@ -266,6 +293,11 @@ void SnapToLinesLayouter::layout()
         // position score, then remember the position of all the boxes in boxes
         // as their best position, and set best position score to current
         // position score.
+        if (currentPositionScore < m_bestPositionScore) {
+            m_bestPosition = m_cueBox.location();
+            m_bestPositionScore = currentPositionScore;
+        }
+
         if (!shouldSwitchDirection(firstLineBox, step)) {
             // 22. Move all the boxes in boxes ...
             moveBoxesByStep(step);