Column balancing support in the region based multicol implementation.

Source/core/rendering/RenderMultiColumnSet.h

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 38 matching lines @@
 
     unsigned computedColumnCount() const { return m_computedColumnCount; }
     LayoutUnit computedColumnWidth() const { return m_computedColumnWidth; }
     LayoutUnit computedColumnHeight() const { return m_computedColumnHeight; }
 
     void setComputedColumnWidthAndCount(LayoutUnit width, unsigned count)
     {
         m_computedColumnWidth = width;
         m_computedColumnCount = count;
     }
-    void setComputedColumnHeight(LayoutUnit height)
-    {
-        m_computedColumnHeight = height;
-    }
+
+    LayoutUnit heightAdjustedForSetOffset(LayoutUnit height) const;
 
     void updateMinimumColumnHeight(LayoutUnit height) { m_minimumColumnHeight = std::max(height, m_minimumColumnHeight); }
     LayoutUnit minimumColumnHeight() const { return m_minimumColumnHeight; }
 
     unsigned forcedBreaksCount() const { return m_forcedBreaksCount; }
     LayoutUnit forcedBreakOffset() const { return m_forcedBreakOffset; }
     LayoutUnit maximumDistanceBetweenForcedBreaks() const { return m_maximumDistanceBetweenForcedBreaks; }
     void clearForcedBreaks()
     { 
         m_forcedBreaksCount = 0;
         m_maximumDistanceBetweenForcedBreaks = 0;
         m_forcedBreakOffset = 0;
     }
     void addForcedBreak(LayoutUnit offsetFromFirstPage)
     { 
         ASSERT(!computedColumnHeight());
         LayoutUnit distanceFromLastBreak = offsetFromFirstPage - m_forcedBreakOffset;
         if (!distanceFromLastBreak)
             return;
         m_forcedBreaksCount++;
         m_maximumDistanceBetweenForcedBreaks = std::max(m_maximumDistanceBetweenForcedBreaks, distanceFromLastBreak);
         m_forcedBreakOffset = offsetFromFirstPage;
     }
 
-    bool requiresBalancing() const { return m_requiresBalancing; }
-    void setRequiresBalancing(bool balancing) { m_requiresBalancing = balancing; }
+    // Calculate the column height when contents are supposed to be balanced. If 'initial' is set,
+    // guess an initial column height; otherwise, stretch the column height a tad. Return true if
+    // column height changed and another layout pass is required.
+    bool calculateBalancedHeight(bool initial);
+
+    // Record space shortage (the amount of space that would have been enough to prevent some
+    // element from being moved to the next column) at a column break. The smallest amount of space
+    // shortage we find is the amount with which we will stretch the column height, if it turns out
+    // after layout that the columns weren't tall enough.
+    void recordSpaceShortage(LayoutUnit spaceShortage);
 
     virtual void updateLogicalWidth() OVERRIDE;
-    virtual void updateLogicalHeight() OVERRIDE;
-    
+
+    void prepareForLayout();
+
 private:
     RenderMultiColumnSet(RenderFlowThread*);
 
     virtual void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const OVERRIDE;
 
     virtual void paintObject(PaintInfo&, const LayoutPoint& paintOffset) OVERRIDE;
 
     virtual LayoutUnit pageLogicalWidth() const OVERRIDE { return m_computedColumnWidth; }
     virtual LayoutUnit pageLogicalHeight() const OVERRIDE { return m_computedColumnHeight; }
 
@@ skipping 13 matching lines @@
     virtual const char* renderName() const;
     
     void paintColumnRules(PaintInfo&, const LayoutPoint& paintOffset);
 
     LayoutUnit columnGap() const;
     LayoutRect columnRectAt(unsigned index) const;
     unsigned columnCount() const;
 
     LayoutRect flowThreadPortionRectAt(unsigned index) const;
     LayoutRect flowThreadPortionOverflowRect(const LayoutRect& flowThreadPortion, unsigned index, unsigned colCount, LayoutUnit colGap) const;
-    
-    unsigned columnIndexAtOffset(LayoutUnit) const;
-    
+
+    enum ColumnIndexCalculationMode {
+        ClampToExistingColumns, // Stay within the range of already existing columns.
+        AssumeNewColumns // Allow column indices outside the range of already existing columns.
+    };
+    unsigned columnIndexAtOffset(LayoutUnit, ColumnIndexCalculationMode = ClampToExistingColumns) const;
+
+    void setAndConstrainColumnHeight(LayoutUnit);
+
     unsigned m_computedColumnCount;
     LayoutUnit m_computedColumnWidth;
     LayoutUnit m_computedColumnHeight;
     
     // The following variables are used when balancing the column set.
-    bool m_requiresBalancing; // Whether or not the columns in the column set have to be balanced, i.e., made to be similar logical heights.
+    LayoutUnit m_maxColumnHeight; // Maximum column height allowed.
+    LayoutUnit m_minSpaceShortage; // The smallest amout of space shortage that caused a column break.
     LayoutUnit m_minimumColumnHeight;
     unsigned m_forcedBreaksCount; // FIXME: We will ultimately need to cache more information to balance around forced breaks properly.
     LayoutUnit m_maximumDistanceBetweenForcedBreaks;
     LayoutUnit m_forcedBreakOffset;
 };
 
 inline RenderMultiColumnSet* toRenderMultiColumnSet(RenderObject* object)
 {
     ASSERT_WITH_SECURITY_IMPLICATION(!object || object->isRenderMultiColumnSet());
     return static_cast<RenderMultiColumnSet*>(object);
 }
 
 inline const RenderMultiColumnSet* toRenderMultiColumnSet(const RenderObject* object)
 {
     ASSERT_WITH_SECURITY_IMPLICATION(!object || object->isRenderMultiColumnSet());
     return static_cast<const RenderMultiColumnSet*>(object);
 }
 
 // This will catch anyone doing an unnecessary cast.
 void toRenderMultiColumnSet(const RenderMultiColumnSet*);
 
 } // namespace WebCore
 
 #endif // RenderMultiColumnSet_h