Column balancing refactoring. Don't propagate data during layout.

third_party/WebKit/Source/core/layout/ColumnBalancer.cpp

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "config.h"
+
+#include "core/layout/ColumnBalancer.h"
+
+#include "core/layout/LayoutMultiColumnSet.h"
+
+namespace blink {
+
+ColumnBalancer::ColumnBalancer(const MultiColumnFragmentainerGroup& group)
+    : m_group(group)
+{
+}
+
+void ColumnBalancer::traverse()
+{
+    traverseSubtree(*m_group.columnSet().flowThread());
+    ASSERT(!flowThreadOffset());
+}
+
+void ColumnBalancer::traverseSubtree(const LayoutBox& box)
+{
+    if (box.childrenInline() && box.isLayoutBlockFlow()) {
+        // Look for breaks between lines.
+        for (const RootInlineBox* line = toLayoutBlockFlow(box).firstRootBox(); line; line = line->nextRootBox()) {
+            LayoutUnit lineTopInFlowThread = m_flowThreadOffset + line->lineTopWithLeading();
+            if (lineTopInFlowThread < group().logicalTopInFlowThread())
+                continue;
+            if (lineTopInFlowThread >= group().logicalBottomInFlowThread())
+                break;
+            examineLine(*line);
+        }
+        return;
+    }
+
+    const LayoutFlowThread* flowThread = group().columnSet().flowThread();
+    bool isHorizontalWritingMode = flowThread->isHorizontalWritingMode();
+
+    // Look for breaks between and inside children.
+    for (const LayoutObject* child = box.slowFirstChild(); child; child = child->nextSibling()) {
+        if (!child->isBox())
+            continue;
+        const LayoutBox& childBox = toLayoutBox(*child);
+        LayoutRect overflowRect = childBox.layoutOverflowRect();
+        LayoutUnit childLogicalBottomWithOverflow = childBox.logicalTop() + (isHorizontalWritingMode ? overflowRect.maxY() : overflowRect.maxX());
+        if (m_flowThreadOffset + childLogicalBottomWithOverflow <= group().logicalTopInFlowThread()) {
+            // This child is fully above the fragmentainer group we're examining.
+            continue;
+        }
+        LayoutUnit childLogicalTopWithOverflow = childBox.logicalTop() + (isHorizontalWritingMode ? overflowRect.y() : overflowRect.x());
+        if (m_flowThreadOffset + childLogicalTopWithOverflow >= group().logicalBottomInFlowThread()) {
+            // This child is fully below the fragmentainer group we're examining. We cannot just
+            // stop here, though, thanks to negative margins. So keep looking.
+            continue;
+        }
+        if (childBox.isOutOfFlowPositioned() || childBox.isColumnSpanAll())
+            continue;
+
+        // Tables are wicked. Both table rows and table cells are relative to their table section.
+        LayoutUnit offsetForThisChild = childBox.isTableRow() ? LayoutUnit() : childBox.logicalTop();
+        m_flowThreadOffset += offsetForThisChild;
+
+        examineBoxAfterEntering(childBox);
+        // Unless the child is unsplittable, or if the child establishes an inner multicol
+        // container, we descend into its subtree for further examination.
+        if (!childBox.isUnsplittableForPagination()
+            && (!childBox.isLayoutBlockFlow() || !toLayoutBlockFlow(childBox).multiColumnFlowThread()))
+            traverseSubtree(childBox);
+        examineBoxBeforeLeaving(childBox);
+
+        m_flowThreadOffset -= offsetForThisChild;
+    }
+}
+
+InitialColumnHeightFinder::InitialColumnHeightFinder(const MultiColumnFragmentainerGroup& group)
+    : ColumnBalancer(group)
+{
+    traverse();
+    // We have now found each explicit / forced break, and their location. Now we need to figure out
+    // how many additional implicit / soft breaks we need and guess where they will occur, in order
+    // to provide an initial column height.
+    distributeImplicitBreaks();
+}
+
+LayoutUnit InitialColumnHeightFinder::initialMinimalBalancedHeight() const
+{
+    unsigned index = contentRunIndexWithTallestColumns();
+    LayoutUnit startOffset = index > 0 ? m_contentRuns[index - 1].breakOffset() : group().logicalTopInFlowThread();
+    return m_contentRuns[index].columnLogicalHeight(startOffset);
+}
+
+void InitialColumnHeightFinder::examineBoxAfterEntering(const LayoutBox& box)
+{
+    ASSERT(isFirstAfterBreak(flowThreadOffset()) || !box.paginationStrut());
+    if (box.hasForcedBreakBefore())
+        addContentRun(flowThreadOffset());
+    if (box.hasForcedBreakAfter())
+        addContentRun(flowThreadOffset() + box.logicalHeight());
+}
+
+void InitialColumnHeightFinder::examineBoxBeforeLeaving(const LayoutBox& box)
+{
+}
+
+void InitialColumnHeightFinder::examineLine(const RootInlineBox& line)
+{
+}
+
+void InitialColumnHeightFinder::addContentRun(LayoutUnit endOffsetInFlowThread)
+{
+    if (!m_contentRuns.isEmpty() && endOffsetInFlowThread <= m_contentRuns.last().breakOffset())
+        return;
+    // Append another item as long as we haven't exceeded used column count. What ends up in the
+    // overflow area shouldn't affect column balancing.
+    if (m_contentRuns.size() < group().columnSet().usedColumnCount())
+        m_contentRuns.append(ContentRun(endOffsetInFlowThread));
+}
+
+unsigned InitialColumnHeightFinder::contentRunIndexWithTallestColumns() const
+{
+    unsigned indexWithLargestHeight = 0;
+    LayoutUnit largestHeight;
+    LayoutUnit previousOffset = group().logicalTopInFlowThread();
+    size_t runCount = m_contentRuns.size();
+    ASSERT(runCount);
+    for (size_t i = 0; i < runCount; i++) {
+        const ContentRun& run = m_contentRuns[i];
+        LayoutUnit height = run.columnLogicalHeight(previousOffset);
+        if (largestHeight < height) {
+            largestHeight = height;
+            indexWithLargestHeight = i;
+        }
+        previousOffset = run.breakOffset();
+    }
+    return indexWithLargestHeight;
+}
+
+void InitialColumnHeightFinder::distributeImplicitBreaks()
+{
+    // Insert a final content run to encompass all content. This will include overflow if this is
+    // the last group in the multicol container.
+    addContentRun(group().logicalBottomInFlowThread());
+    unsigned columnCount = m_contentRuns.size();
+
+    // If there is room for more breaks (to reach the used value of column-count), imagine that we
+    // insert implicit breaks at suitable locations. At any given time, the content run with the
+    // currently tallest columns will get another implicit break "inserted", which will increase its
+    // column count by one and shrink its columns' height. Repeat until we have the desired total
+    // number of breaks. The largest column height among the runs will then be the initial column
+    // height for the balancer to use.
+    while (columnCount < group().columnSet().usedColumnCount()) {
+        unsigned index = contentRunIndexWithTallestColumns();
+        m_contentRuns[index].assumeAnotherImplicitBreak();
+        columnCount++;
+    }
+}
+
+MinimumSpaceShortageFinder::MinimumSpaceShortageFinder(const MultiColumnFragmentainerGroup& group)
+    : ColumnBalancer(group)
+    , m_minimumSpaceShortage(LayoutUnit::max())
+    , m_pendingStrut(LayoutUnit::min())
+    , m_forcedBreaksCount(0)
+{
+    traverse();
+}
+
+void MinimumSpaceShortageFinder::examineBoxAfterEntering(const LayoutBox& box)
+{
+    if (box.hasForcedBreakBefore())
+        m_forcedBreaksCount++;
+    if (box.hasForcedBreakAfter())
+        m_forcedBreaksCount++;
+
+    // Look for breaks before the child box.
+    bool isFirstAfterBreak = this->isFirstAfterBreak(flowThreadOffset());
+    ASSERT(isFirstAfterBreak || !box.paginationStrut());
+    if (isFirstAfterBreak && !box.hasForcedBreakBefore()) {
+        // This box is first after a soft break.
+        LayoutUnit strut = box.paginationStrut();
+        if (box.isUnsplittableForPagination()) {
+            // Since we cannot break inside the box, just figure out how much more space we would
+            // need to prevent it from being pushed to the next column.
+            recordSpaceShortage(box.logicalHeight() - strut);
+        } else if (m_pendingStrut == LayoutUnit::min()) {
+            // We now want to look for the first piece of unbreakable content (e.g. a line or a
+            // block-displayed image) inside this block. That ought to be a good candidate for
+            // minimum space shortage; a much better one than reporting space shortage for the
+            // entire block (which we'll also do (further down), in case we couldn't find anything
+            // more suitable).
+            m_pendingStrut = strut;
+        }
+    }
+
+    if (!box.isUnsplittableForPagination()) {
+        // See if this breakable box crosses column boundaries.
+        LayoutUnit bottomInFlowThread = flowThreadOffset() + box.logicalHeight();
+        if (isFirstAfterBreak || group().columnLogicalTopForOffset(flowThreadOffset()) != group().columnLogicalTopForOffset(bottomInFlowThread)) {
+            // If the child crosses a column boundary, record space shortage, in case nothing
+            // inside it has already done so. The column balancer needs to know by how much it
+            // has to stretch the columns to make more content fit. If no breaks are reported
+            // (but do occur), the balancer will have no clue. Only measure the space after the
+            // last column boundary, in case it crosses more than one.
+            LayoutUnit spaceUsedInLastColumn = bottomInFlowThread - group().columnLogicalTopForOffset(bottomInFlowThread);
+            recordSpaceShortage(spaceUsedInLastColumn);
+        }
+    }
+}
+
+void MinimumSpaceShortageFinder::examineBoxBeforeLeaving(const LayoutBox& box)
+{
+    if (m_pendingStrut == LayoutUnit::min() || !box.isUnsplittableForPagination())
+        return;
+
+    // The previous break was before a breakable block. Here's the first piece of unbreakable
+    // content after / inside that block. We want to record the distance from the top of the column
+    // to the bottom of this box as space shortage.
+    LayoutUnit logicalOffsetFromCurrentColumn = flowThreadOffset() - group().columnLogicalTopForOffset(flowThreadOffset());
+    recordSpaceShortage(logicalOffsetFromCurrentColumn + box.logicalHeight() - m_pendingStrut);
+    m_pendingStrut = LayoutUnit::min();
+}
+
+void MinimumSpaceShortageFinder::examineLine(const RootInlineBox& line)
+{
+    LayoutUnit lineTop = line.lineTopWithLeading();
+    LayoutUnit lineTopInFlowThread = flowThreadOffset() + lineTop;
+    LayoutUnit lineHeight = line.lineBottomWithLeading() - lineTop;
+    if (m_pendingStrut != LayoutUnit::min()) {
+        // The previous break was before a breakable block. Here's the first line after / inside
+        // that block. We want to record the distance from the top of the column to the bottom of
+        // this box as space shortage.
+        LayoutUnit logicalOffsetFromCurrentColumn = lineTopInFlowThread - group().columnLogicalTopForOffset(lineTopInFlowThread);
+        recordSpaceShortage(logicalOffsetFromCurrentColumn + lineHeight - m_pendingStrut);
+        m_pendingStrut = LayoutUnit::min();
+        return;
+    }
+    ASSERT(isFirstAfterBreak(lineTopInFlowThread) || !line.paginationStrut());
+    if (isFirstAfterBreak(lineTopInFlowThread))
+        recordSpaceShortage(lineHeight - line.paginationStrut());
+}
+
+} // namespace blink