Bug 1123917 part 1 - Add line-relative direction mapping methods to writing modes. r=jfkthame

--HG--
extra : rebase_source : b560e4956cef826d5d7c55fd2c644aa52857dfca
extra : source : ae961e0f6c20b5a9bfec10a99568d0282e437c9e

layout/generic/WritingModes.h

@@ -67,12 +67,23 @@ inline LogicalEdge GetEdge(LogicalSide aSide)
   return IsEnd(aSide) ? eLogicalEdgeEnd : eLogicalEdgeStart;
 }
 
+inline LogicalEdge GetOppositeEdge(LogicalEdge aEdge)
+{
+  // This relies on the only two LogicalEdge enum values being 0 and 1.
+  return LogicalEdge(1 - aEdge);
+}
+
 inline LogicalSide
 MakeLogicalSide(LogicalAxis aAxis, LogicalEdge aEdge)
 {
   return LogicalSide((aAxis << 1) | aEdge);
 }
 
+inline LogicalSide GetOppositeSide(LogicalSide aSide)
+{
+  return MakeLogicalSide(GetAxis(aSide), GetOppositeEdge(GetEdge(aSide)));
+}
+
 enum LogicalSideBits {
   eLogicalSideBitsNone   = 0,
   eLogicalSideBitsBStart = 1 << eLogicalSideBStart,
@@ -84,6 +95,13 @@ enum LogicalSideBits {
   eLogicalSideBitsAll = eLogicalSideBitsBBoth | eLogicalSideBitsIBoth
 };
 
+enum LineRelativeDir {
+  eLineRelativeDirOver  = eLogicalSideBStart,
+  eLineRelativeDirUnder = eLogicalSideBEnd,
+  eLineRelativeDirLeft  = eLogicalSideIStart,
+  eLineRelativeDirRight = eLogicalSideIEnd
+};
+
 /**
  * LogicalSides represents a set of logical sides.
  */
@@ -350,6 +368,19 @@ public:
     return PhysicalSideForInlineAxis(GetEdge(aSide));
   }
 
+  /**
+   * Returns the physical side corresponding to the specified
+   * line-relative direction, given the current writing mode.
+   */
+  mozilla::Side PhysicalSide(LineRelativeDir aDir) const
+  {
+    LogicalSide side = static_cast<LogicalSide>(aDir);
+    if (IsLineInverted()) {
+      side = GetOppositeSide(side);
+    }
+    return PhysicalSide(side);
+  }
+
   /**
    * Default constructor gives us a horizontal, LTR writing mode.
    * XXX We will probably eliminate this and require explicit initialization