Improving direction-based selection strategy.

Source/core/editing/GranularityStrategy.h

Index: Source/core/editing/GranularityStrategy.h
diff --git a/Source/core/editing/GranularityStrategy.h b/Source/core/editing/GranularityStrategy.h
index 9d31bc84eb0ad41f48f7a04ea2fa49f2e994b75c..f77bbe3f4f55923f461b1e3d5cd0c9190afedef3 100644
--- a/Source/core/editing/GranularityStrategy.h
+++ b/Source/core/editing/GranularityStrategy.h
@@ -16,9 +16,9 @@ public:
     virtual SelectionStrategy GetType() const = 0;
     virtual void Clear() = 0;
 
-    // Calculates and returns the new selection based on the updated user
-    // selection extent |extentPosition| and the granularity strategy.
-    virtual VisibleSelection updateExtent(const VisiblePosition& extentPosition, const VisibleSelection&) = 0;
+    // Calculates and returns the new selection based on the updated extent
+    // location in absolute coordinates.
+    virtual VisibleSelection updateExtent(const IntPoint&, LocalFrame*) = 0;
 
 protected:
     GranularityStrategy();
@@ -33,13 +33,46 @@ public:
     // GranularityStrategy:
     SelectionStrategy GetType() const final;
     void Clear() final;
-    VisibleSelection updateExtent(const VisiblePosition& extentPosition, const VisibleSelection&) final;
+    VisibleSelection updateExtent(const IntPoint&, LocalFrame*) final;
 };
 
 // "Expand by word, shrink by character" selection strategy.
 // Uses character granularity when selection is shrinking. If the selection is
 // expanding, granularity doesn't change until a word boundary is passed, after
 // which the granularity switches to "word".
+// In word granularity, the word is not selected until the extent point passes
+// the middle of the word.
+//
+// The "offset" feature:
+// The offset is the horizontal distance between the extent point (passed in
+// updateExtent) and the end of the selection. In character granularity the
+// offset is typically zero or near zero, however in word granularity it can be
+// significant. When the offset is positive and the extent point moved to the
+// left, the offset is preserved, i.e. the selection tracks the extent point
+// with the constant offset. When the offset is positive and the extent point
+// is moved to the right, the offset gets reduced. Selection will not grow
+// until the offset is reduced all the way to zero.
+//
+// This behavior is best illustrated by an example:
+//
+// ^ marks base, | marks extent point, > marks selection end:
+// Lorem ip^sum|> dolor sit amet, consectetur
+//
+// Move extent over the middle of "dolor". Granularity should change to word
+// granularity and the selection end should jump to the end of the word.
+// Lorem ip^sum dolo|r> sit amet, consectetur
+//
+// Move extent back one character. Granularity changes to "character". The
+// selection end should move back one character as well. Note an offset between
+// the extent and the selection end.
+// Lorem ip^sum dol|o>r sit amet, consectetur
+//
+// Move extent forward one character. The offset is reduced to 0. Selection end
+// doesn't change.
+// Lorem ip^sum dolo|>r sit amet, consectetur
+//
+// Move forward one character. End moves with extent in character granularity.
+// Lorem ip^sum dolor|> sit amet, consectetur
 class DirectionGranularityStrategy final : public GranularityStrategy {
 public:
     DirectionGranularityStrategy();
@@ -48,23 +81,37 @@ public:
     // GranularityStrategy:
     SelectionStrategy GetType() const final;
     void Clear() final;
-    VisibleSelection updateExtent(const VisiblePosition&, const VisibleSelection&) final;
+    VisibleSelection updateExtent(const IntPoint&, LocalFrame*) final;
 
 private:
-    enum class BoundAdjust {CurrentPosIfOnBound, NextBoundIfOnBound};
-    enum class SearchDirection {SearchBackwards, SearchForward};
+    enum class StrategyState {
+        // Starting state.
+        // Selection was cleared and there were no extent updates since then.
+        // One an update is performed, the strategy goes into the Expanding
+        // state unless the update shrinks the selection without changing
+        // relative base/extent order, in which case the strategy goes into the
+        // Shrinking state.
+        Cleared,
+        // Last time the selection was changed by updateExtent - it was expanded
+        // or the relative base/extent order was changed.
+        Expanding,
+        // Last time the selection was changed by updateExtent - it was shrunk
+        // (without changing relative base/extent order).
+        Shrinking
+    };
 
-    // Returns the next word boundary starting from |pos|. |direction| specifies
-    // the direction in which to search for the next bound. nextIfOnBound
-    // controls whether |pos| or the next boundary is returned when |pos| is
-    // located exactly on word boundary.
-    VisiblePosition nextWordBound(const VisiblePosition& /*pos*/, SearchDirection /*direction*/, BoundAdjust /*nextIfOnBound*/);
+    StrategyState m_state;
 
-    // Current selection granularity being used
+    // Current selection granularity being used.
     TextGranularity m_granularity;
-    // Set to true if the selection was shrunk (without changing relative
-    // base/extent order) as a result of the most recent updateExtent call.
-    bool m_lastMoveShrunkSelection;
+
+    // Horizontal offset in pixels in absolute coordinates applied to the extent point.
+    int m_offset;
+
+    // This defines location of the offset-adjusted extent point (from the
+    // latest updateExtent call) relative to the location of extent's
+    // VisiblePosition. It is used to detect sub-position extent movement.
+    IntSize m_diffExtentPointFromExtentPosition;
 };
 
 } // namespace blink