Region based multicol: support explicit column breaks

Source/core/rendering/RenderMultiColumnSet.h

Index: Source/core/rendering/RenderMultiColumnSet.h
diff --git a/Source/core/rendering/RenderMultiColumnSet.h b/Source/core/rendering/RenderMultiColumnSet.h
index 057f4bf260b6f1691628c3c51f0fffc748845d51..4ece7e3b9f68adf3fe2944548ecc159dc4e3b440 100644
--- a/Source/core/rendering/RenderMultiColumnSet.h
+++ b/Source/core/rendering/RenderMultiColumnSet.h
@@ -28,6 +28,7 @@
 #define RenderMultiColumnSet_h
 
 #include "core/rendering/RenderRegionSet.h"
+#include "wtf/Vector.h"
 
 namespace WebCore {
 
@@ -62,30 +63,14 @@ public:
     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;
-    }
+    unsigned forcedBreaksCount() const { return m_contentRuns.size(); }
+    void clearForcedBreaks();
+    void addForcedBreak(LayoutUnit offsetFromFirstPage);
 
-    // 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);
+    // (Re-)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 recalculateBalancedHeight(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
@@ -139,17 +124,52 @@ private:
 
     void setAndConstrainColumnHeight(LayoutUnit);
 
-    unsigned m_computedColumnCount;
-    LayoutUnit m_computedColumnWidth;
+    // Return the index of the content run with the currently tallest columns, taking all implicit
+    // breaks assumed so far into account.
+    unsigned findRunWithTallestColumns() const;
+
+    // Given the current list of content runs, make assumptions about where we need to insert
+    // implicit breaks (if there's room for any at all; depending on the number of explicit breaks),
+    // and store the results. This is needed in order to balance the columns.
+    void distributeImplicitBreaks();
+
+    LayoutUnit calculateBalancedHeight(bool initial) const;
+
+    unsigned m_computedColumnCount; // Used column count (the resulting 'N' from the pseudo-algorithm in the multicol spec)
+    LayoutUnit m_computedColumnWidth; // Used column width (the resulting 'W' from the pseudo-algorithm in the multicol spec)
     LayoutUnit m_computedColumnHeight;
 
     // The following variables are used when balancing the column set.
     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;
+
+    // A run of content without explicit (forced) breaks; i.e. a flow thread portion between two
+    // explicit breaks, between flow thread start and an explicit break, between an explicit break
+    // and flow thread end, or, in cases when there are no explicit breaks at all: between flow flow
+    // thread start and flow thread end. We need to know where the explicit breaks are, in order to
+    // figure out where the implicit breaks will end up, so that we get the columns properly
+    // balanced. A content run starts out as representing one single column, and will represent one
+    // additional column for each implicit break "inserted" there.
+    class ContentRun {
+    public:
+        ContentRun(LayoutUnit breakOffset)
+            : m_breakOffset(breakOffset)
+            , m_assumedImplicitBreaks(0) { }
+
+        unsigned assumedImplicitBreaks() const { return m_assumedImplicitBreaks; }
+        void assumeAnotherImplicitBreak() { m_assumedImplicitBreaks++; }
+        LayoutUnit breakOffset() const { return m_breakOffset; }
+
+        // Return the column height that this content run would require, considering the implicit
+        // breaks assumed so far.
+        LayoutUnit columnLogicalHeight(LayoutUnit startOffset) const { return ceilf(float(m_breakOffset - startOffset) / float(m_assumedImplicitBreaks + 1)); }
+
+    private:
+        LayoutUnit m_breakOffset; // Flow thread offset where this run ends.
+        unsigned m_assumedImplicitBreaks; // Number of implicit breaks in this run assumed so far.
+    };
+    Vector<ContentRun, 1> m_contentRuns;
 };
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderMultiColumnSet, isRenderMultiColumnSet());