*** DO NOT LAND *** Attempt to understand Regions complexity

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
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-/*
- * 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
- *    documentation and/or other materials provided with the distribution.
- *
- * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
- * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
- * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
- * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE COMPUTER, INC. OR
- * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
- * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
- * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
- * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
- * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
- * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
- * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
- */
-
-
-#ifndef RenderMultiColumnSet_h
-#define RenderMultiColumnSet_h
-
-#include "core/rendering/RenderRegionSet.h"
-
-namespace WebCore {
-
-// RenderMultiColumnSet represents a set of columns that all have the same width and height. By combining runs of same-size columns into a single
-// object, we significantly reduce the number of unique RenderObjects required to represent columns.
-//
-// A simple multi-column block will have exactly one RenderMultiColumnSet child. A simple paginated multi-column block will have three
-// RenderMultiColumnSet children: one for the content at the bottom of the first page (whose columns will have a shorter height), one
-// for the 2nd to n-1 pages, and then one last column set that will hold the shorter columns on the final page (that may have to be balanced
-// as well).
-//
-// Column spans result in the creation of new column sets as well, since a spanning region has to be placed in between the column sets that
-// come before and after the span.
-class RenderMultiColumnSet FINAL : public RenderRegionSet {
-public:
-    static RenderMultiColumnSet* createAnonymous(RenderFlowThread*);
-
-    virtual bool isRenderMultiColumnSet() const OVERRIDE { return true; }
-
-    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;
-    }
-
-    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;
-    }
-
-    // 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;
-
-    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; }
-
-    virtual LayoutUnit pageLogicalTopForOffset(LayoutUnit offset) const OVERRIDE;
-
-    // FIXME: This will change once we have column sets constrained by enclosing pages, etc.
-    virtual LayoutUnit logicalHeightOfAllFlowThreadContent() const OVERRIDE { return m_computedColumnHeight; }
-
-    // FIXME: For now we return false, but it's likely we will leverage the auto height region code to do column
-    // balancing. That's why we have an override of this function that is distinct from RenderRegionSet's override.
-    virtual bool shouldHaveAutoLogicalHeight() const OVERRIDE { return false; }
-
-    virtual void repaintFlowThreadContent(const LayoutRect& repaintRect) const OVERRIDE;
-
-    virtual void collectLayerFragments(LayerFragments&, const LayoutRect& layerBoundingBox, const LayoutRect& dirtyRect) OVERRIDE;
-
-    virtual const char* renderName() const OVERRIDE;
-
-    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;
-
-    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.
-    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;
-};
-
-DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderMultiColumnSet, isRenderMultiColumnSet());
-
-} // namespace WebCore
-
-#endif // RenderMultiColumnSet_h
-