LayoutUnit RenderMultiColumnSet::columnGap() const
{
    // FIXME: Eventually we will cache the column gap when the widths of columns start varying, but for now we just
    // go to the parent block to get the gap.
    RenderMultiColumnBlock* parentBlock = toRenderMultiColumnBlock(parent());
    if (parentBlock->style()->hasNormalColumnGap())
        return parentBlock->style()->fontDescription().computedPixelSize(); // "1em" is recommended as the normal gap setting. Matches <p> margins.
    return parentBlock->style()->columnGap();
}
void RenderMultiColumnSet::prepareForLayout()
{
    RenderMultiColumnBlock* multicolBlock = toRenderMultiColumnBlock(parent());
    RenderStyle* multicolStyle = multicolBlock->style();

    // Set box logical top.
    ASSERT(!previousSiblingBox() || !previousSiblingBox()->isRenderMultiColumnSet()); // FIXME: multiple set not implemented; need to examine previous set to calculate the correct logical top.
    setLogicalTop(multicolBlock->borderBefore() + multicolBlock->paddingBefore());

    // Set box width.
    updateLogicalWidth();

    if (multicolBlock->requiresBalancing()) {
        // Set maximum column height. We will not stretch beyond this.
        m_maxColumnHeight = RenderFlowThread::maxLogicalHeight();
        if (!multicolStyle->logicalHeight().isAuto()) {
            m_maxColumnHeight = multicolBlock->computeContentLogicalHeight(multicolStyle->logicalHeight(), -1);
            if (m_maxColumnHeight == -1)
                m_maxColumnHeight = RenderFlowThread::maxLogicalHeight();
        }
        if (!multicolStyle->logicalMaxHeight().isUndefined()) {
            LayoutUnit logicalMaxHeight = multicolBlock->computeContentLogicalHeight(multicolStyle->logicalMaxHeight(), -1);
            if (logicalMaxHeight != -1 && m_maxColumnHeight > logicalMaxHeight)
                m_maxColumnHeight = logicalMaxHeight;
        }
        m_maxColumnHeight = heightAdjustedForSetOffset(m_maxColumnHeight);
        m_computedColumnHeight = 0; // Restart balancing.
    } else {
        setAndConstrainColumnHeight(heightAdjustedForSetOffset(multicolBlock->columnHeightAvailable()));
    }

    clearForcedBreaks();

    // Nuke previously stored minimum column height. Contents may have changed for all we know.
    m_minimumColumnHeight = 0;
}
void RenderMultiColumnFlowThread::autoGenerateRegionsToBlockOffset(LayoutUnit /*offset*/)
{
    // This function ensures we have the correct column set information at all times.
    // For a simple multi-column layout in continuous media, only one column set child is required.
    // Once a column is nested inside an enclosing pagination context, the number of column sets
    // required becomes 2n-1, where n is the total number of nested pagination contexts. For example:
    //
    // Column layout with no enclosing pagination model = 2 * 1 - 1 = 1 column set.
    // Columns inside pages = 2 * 2 - 1 = 3 column sets (bottom of first page, all the subsequent pages, then the last page).
    // Columns inside columns inside pages = 2 * 3 - 1 = 5 column sets.
    //
    // In addition, column spans will force a column set to "split" into before/after sets around the spanning element.
    //
    // Finally, we will need to deal with columns inside regions. If regions have variable widths, then there will need
    // to be unique column sets created inside any region whose width is different from its surrounding regions. This is
    // actually pretty similar to the spanning case, in that we break up the column sets whenever the width varies.
    //
    // FIXME: For now just make one column set. This matches the old multi-column code.
    // Right now our goal is just feature parity with the old multi-column code so that we can switch over to the
    // new code as soon as possible.
    RenderMultiColumnSet* firstSet = toRenderMultiColumnSet(firstRegion());
    if (firstSet)
        return;
    
    invalidateRegions();

    RenderMultiColumnBlock* parentBlock = toRenderMultiColumnBlock(parent());
    firstSet = new RenderMultiColumnSet(*this, RenderStyle::createAnonymousStyleWithDisplay(&parentBlock->style(), BLOCK));
    firstSet->initializeStyle();
    parentBlock->RenderBlock::addChild(firstSet);

    // Even though we aren't placed yet, we can go ahead and set up our size. At this point we're
    // typically in the middle of laying out the thread, attempting to paginate, and we need to do
    // some rudimentary "layout" of the set now, so that pagination will work.
    firstSet->prepareForLayout();

    validateRegions();
}