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
 *    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
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 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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#ifndef RenderMultiColumnSet_h
#define RenderMultiColumnSet_h

#include "core/rendering/RenderMultiColumnFlowThread.h"
#include "core/rendering/RenderRegion.h"
#include "wtf/Vector.h"

namespace blink {

// 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.
//
// Column sets are inserted as anonymous children of the actual multicol container (i.e. the
// renderer whose style computes to non-auto column-count and/or column-width).
//
// Being a "region", a column set has no children on its own, but is merely used to slice a portion
// of the tall "single-column" flow thread into actual columns visually, to convert from flow thread
// coordinates to visual ones. It is in charge of both positioning columns correctly relatively to
// the parent multicol container, and to calculate the correct translation for each column's
// contents, and to paint any rules between them. RenderMultiColumnSet objects are used for
// painting, hit testing, and any other type of operation that requires mapping from flow thread
// coordinates to visual coordinates.
//
// Column spans result in the creation of new column sets, since a spanning renderer has to be
// placed in between the column sets that come before and after the span.
class RenderMultiColumnSet final : public RenderRegion {
public:
    enum BalancedHeightCalculation { GuessFromFlowThreadPortion, StretchBySpaceShortage };

    static RenderMultiColumnSet* createAnonymous(RenderFlowThread*, RenderStyle* parentStyle);

    virtual bool isOfType(RenderObjectType type) const override { return type == RenderObjectRenderMultiColumnSet || RenderRegion::isOfType(type); }

    virtual LayoutUnit pageLogicalWidth() const override final { return flowThread()->logicalWidth(); }
    virtual LayoutUnit pageLogicalHeight() const override final { return m_columnHeight; }

    RenderBlockFlow* multiColumnBlockFlow() const { return toRenderBlockFlow(parent()); }
    RenderMultiColumnFlowThread* multiColumnFlowThread() const
    {
        ASSERT_WITH_SECURITY_IMPLICATION(!flowThread() || flowThread()->isRenderMultiColumnFlowThread());
        return static_cast<RenderMultiColumnFlowThread*>(flowThread());
    }

    RenderMultiColumnSet* nextSiblingMultiColumnSet() const;
    RenderMultiColumnSet* previousSiblingMultiColumnSet() const;

    LayoutUnit logicalTopInFlowThread() const { return isHorizontalWritingMode() ? flowThreadPortionRect().y() : flowThreadPortionRect().x(); }
    LayoutUnit logicalBottomInFlowThread() const { return isHorizontalWritingMode() ? flowThreadPortionRect().maxY() : flowThreadPortionRect().maxX(); }

    LayoutUnit logicalHeightInFlowThread() const { return isHorizontalWritingMode() ? flowThreadPortionRect().height() : flowThreadPortionRect().width(); }

    // The used CSS value of column-count, i.e. how many columns there are room for without overflowing.
    unsigned usedColumnCount() const { return multiColumnFlowThread()->columnCount(); }

    // Find the column that contains the given block offset, and return the translation needed to
    // get from flow thread coordinates to visual coordinates.
    LayoutSize flowThreadTranslationAtOffset(LayoutUnit) const;

    LayoutUnit heightAdjustedForSetOffset(LayoutUnit height) const;

    void updateMinimumColumnHeight(LayoutUnit height) { m_minimumColumnHeight = std::max(height, m_minimumColumnHeight); }
    LayoutUnit minimumColumnHeight() const { return m_minimumColumnHeight; }

    // Add a content run, specified by its end position. A content run is appended at every
    // forced/explicit break and at the end of the column set. The content runs are used to
    // determine where implicit/soft breaks will occur, in order to calculate an initial column
    // height.
    void addContentRun(LayoutUnit endOffsetFromFirstPage);

    // (Re-)calculate the column height if it's auto.
    bool recalculateColumnHeight(BalancedHeightCalculation);

    // 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);

    // Reset previously calculated column height. Will mark for layout if needed.
    void resetColumnHeight();

    // Expand this set's flow thread portion rectangle to contain all trailing flow thread
    // overflow. Only to be called on the last set.
    void expandToEncompassFlowThreadContentsIfNeeded();

    void attachRegion();
    void detachRegion();

    void paintInvalidationForFlowThreadContent(const LayoutRect& paintInvalidationRect) const;

    // The top of the nearest page inside the region. For RenderRegions, this is just the logical top of the
    // flow thread portion we contain. For sets, we have to figure out the top of the nearest column or
    // page.
    LayoutUnit pageLogicalTopForOffset(LayoutUnit offset) const;

    void collectLayerFragments(LayerFragments&, const LayoutRect& layerBoundingBox, const LayoutRect& dirtyRect);

    LayoutUnit columnGap() const;

    // The "CSS actual" value of column-count. This includes overflowing columns, if any.
    unsigned actualColumnCount() const;

private:
    RenderMultiColumnSet(RenderFlowThread*);

    virtual void insertedIntoTree() override final;
    virtual void willBeRemovedFromTree() override final;

    virtual void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const override;

    virtual void paintObject(PaintInfo&, const LayoutPoint& paintOffset) override;

    virtual void addOverflowFromChildren() override;

    virtual const char* renderName() const override;

    LayoutUnit calculateMaxColumnHeight() const;
    LayoutRect columnRectAt(unsigned index) 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);

    // 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 calculateColumnHeight(BalancedHeightCalculation) const;

    LayoutUnit m_columnHeight;

    // 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;

    // 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
    // thread portion start and flow thread portion 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((m_breakOffset - startOffset).toFloat() / 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());

} // namespace blink

#endif // RenderMultiColumnSet_h