fop/src/java/org/apache/fop/layoutmgr/BlockStackingLayoutManager.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/* $Id$ */

package org.apache.fop.layoutmgr;

import java.util.LinkedList;
import java.util.List;
import java.util.ListIterator;
import java.util.Stack;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.apache.fop.area.Area;
import org.apache.fop.area.Block;
import org.apache.fop.area.BlockParent;
import org.apache.fop.fo.Constants;
import org.apache.fop.fo.FObj;
import org.apache.fop.fo.properties.BreakPropertySet;
import org.apache.fop.fo.properties.CommonBorderPaddingBackground;
import org.apache.fop.fo.properties.KeepProperty;
import org.apache.fop.fo.properties.SpaceProperty;
import org.apache.fop.layoutmgr.inline.InlineLayoutManager;
import org.apache.fop.traits.MinOptMax;
import org.apache.fop.util.ListUtil;

/**
 * Base LayoutManager class for all areas which stack their child
 * areas in the block-progression direction, such as Flow, Block, ListBlock.
 */
public abstract class BlockStackingLayoutManager extends AbstractLayoutManager
                                                 implements BlockLevelLayoutManager {

    /** logging instance */
    private static Log log = LogFactory.getLog(BlockStackingLayoutManager.class);

    /** parent area */
    protected BlockParent parentArea;

    /** Value of the block-progression-unit (non-standard property) */
    protected int bpUnit;
    /** space-before value adjusted for block-progression-unit handling */
    protected int adjustedSpaceBefore;
    /** space-after value adjusted for block-progression-unit handling */
    protected int adjustedSpaceAfter;
    /** Only used to store the original list when createUnitElements is called */
    protected List<KnuthElement> storedList;
    /** Indicates whether break before has been served or not */
    protected boolean breakBeforeServed;
    /** Indicates whether the first visible mark has been returned by this LM, yet */
    protected boolean firstVisibleMarkServed;
    /** Reference IPD available */
    protected int referenceIPD;
    /** the effective start-indent value */
    protected int startIndent;
    /** the effective end-indent value */
    protected int endIndent;
    /**
     * Holds the (one-time use) fo:block space-before
     * and -after properties.  Large fo:blocks are split
     * into multiple Area. Blocks to accomodate the subsequent
     * regions (pages) they are placed on.  space-before
     * is applied at the beginning of the first
     * Block and space-after at the end of the last Block
     * used in rendering the fo:block.
     */
    protected MinOptMax foSpaceBefore;
    /** see foSpaceBefore */
    protected MinOptMax foSpaceAfter;

    private Position auxiliaryPosition;

    private int contentAreaIPD;

    /**
     * @param node the fo this LM deals with
     */
    public BlockStackingLayoutManager(FObj node) {
        super(node);
        setGeneratesBlockArea(true);
    }

    /**
     * @return current area being filled
     */
    protected BlockParent getCurrentArea() {
        return this.parentArea;
    }


    /**
     * Set the current area being filled.
     * @param parentArea the current area to be filled
     */
    protected void setCurrentArea(BlockParent parentArea) {
        this.parentArea = parentArea;
    }

    /**
     * Add a block spacer for space before and space after a block.
     * This adds an empty Block area that acts as a block space.
     *
     * @param adjust the adjustment value
     * @param minoptmax the min/opt/max value of the spacing
     */
    public void addBlockSpacing(double adjust, MinOptMax minoptmax) {
        int sp = TraitSetter.getEffectiveSpace(adjust, minoptmax);
        if (sp != 0) {
            Block spacer = new Block();
            spacer.setBPD(sp);
            parentLayoutManager.addChildArea(spacer);
        }
    }

    /**
     * Add the childArea to the passed area.
     * Called by child LayoutManager when it has filled one of its areas.
     * The LM should already have an Area in which to put the child.
     * See if the area will fit in the current area.
     * If so, add it. Otherwise initiate breaking.
     * @param childArea the area to add: will be some block-stacked Area.
     * @param parentArea the area in which to add the childArea
     */
    protected void addChildToArea(Area childArea,
                                     BlockParent parentArea) {
        // This should be a block-level Area (Block in the generic sense)
        if (!(childArea instanceof Block)) {
            //log.error("Child not a Block in BlockStackingLM!");
        }

        parentArea.addBlock((Block) childArea);
        flush(); // hand off current area to parent
    }


    /**
     * Add the childArea to the current area.
     * Called by child LayoutManager when it has filled one of its areas.
     * The LM should already have an Area in which to put the child.
     * See if the area will fit in the current area.
     * If so, add it. Otherwise initiate breaking.
     * @param childArea the area to add: will be some block-stacked Area.
     */
    @Override
    public void addChildArea(Area childArea) {
        addChildToArea(childArea, getCurrentArea());
    }

    /**
     * Force current area to be added to parent area.
     */
    protected void flush() {
        if (getCurrentArea() != null) {
            parentLayoutManager.addChildArea(getCurrentArea());
        }
    }

    /** @return a cached auxiliary Position instance used for things like spaces. */
    protected Position getAuxiliaryPosition() {
        if (this.auxiliaryPosition == null) {
            this.auxiliaryPosition = new NonLeafPosition(this, null);
        }
        return this.auxiliaryPosition;
    }

    /**
     * @param len length in millipoints to span with bp units
     * @return the minimum integer n such that n * bpUnit >= len
     */
    protected int neededUnits(int len) {
        return (int) Math.ceil((float)len / bpUnit);
    }

    /**
     * Determines and sets the content area IPD based on available reference area IPD, start- and
     * end-indent properties.
     * end-indent is adjusted based on overconstrained geometry rules, if necessary.
     *
     * @return the resulting content area IPD
     */
    protected int updateContentAreaIPDwithOverconstrainedAdjust() {
        int ipd = referenceIPD - (startIndent + endIndent);
        if (ipd < 0) {
            //5.3.4, XSL 1.0, Overconstrained Geometry
            log.debug("Adjusting end-indent based on overconstrained geometry rules for " + fobj);
            BlockLevelEventProducer eventProducer = BlockLevelEventProducer.Provider.get(
                    getFObj().getUserAgent().getEventBroadcaster());
            eventProducer.overconstrainedAdjustEndIndent(this,
                    getFObj().getName(), ipd, getFObj().getLocator());
            endIndent += ipd;
            ipd = 0;
            //TODO Should we skip layout for a block that has ipd=0?
        }
        setContentAreaIPD(ipd);
        return ipd;
    }

    /**
     * Sets the content area IPD by directly supplying the value.
     * end-indent is adjusted based on overconstrained geometry rules, if necessary.
     * @param contentIPD the IPD of the content
     * @return the resulting content area IPD
     */
    protected int updateContentAreaIPDwithOverconstrainedAdjust(int contentIPD) {
        int ipd = referenceIPD - (contentIPD + (startIndent + endIndent));
        if (ipd < 0) {
            //5.3.4, XSL 1.0, Overconstrained Geometry
            log.debug("Adjusting end-indent based on overconstrained geometry rules for " + fobj);
            BlockLevelEventProducer eventProducer = BlockLevelEventProducer.Provider.get(
                    getFObj().getUserAgent().getEventBroadcaster());
            eventProducer.overconstrainedAdjustEndIndent(this,
                    getFObj().getName(), ipd, getFObj().getLocator());
            endIndent += ipd;
        }
        setContentAreaIPD(contentIPD);
        return contentIPD;
    }

    /** {@inheritDoc} */
    @Override
    public List getNextKnuthElements(LayoutContext context, int alignment) {
        return getNextKnuthElements(context, alignment, null, null, null);
    }

    /** {@inheritDoc} */
    @Override
    public List getNextKnuthElements(LayoutContext context, int alignment,
            Stack lmStack, Position restartPosition, LayoutManager restartAtLM) {
        referenceIPD = context.getRefIPD();
        updateContentAreaIPDwithOverconstrainedAdjust();

        boolean isRestart = (lmStack != null);
        boolean emptyStack = (!isRestart || lmStack.isEmpty());
        List<ListElement> contentList = new LinkedList<ListElement>();
        List<ListElement> elements = new LinkedList<ListElement>();

        if (!breakBeforeServed(context, elements)) {
            // if this FO has break-before specified, and it
            // has not yet been processed, return now
            return elements;
        }

        addFirstVisibleMarks(elements, context, alignment);

        //Used to indicate a special break-after case when all content has already been generated.
        BreakElement forcedBreakAfterLast = null;

        LayoutContext childLC;
        List<ListElement> childElements;
        LayoutManager currentChildLM;
        if (isRestart) {
            if (emptyStack) {
                assert restartAtLM != null && restartAtLM.getParent() == this;
                currentChildLM = restartAtLM;
            } else {
                currentChildLM = (LayoutManager) lmStack.pop();
            }
            setCurrentChildLM(currentChildLM);
        } else {
            currentChildLM = getChildLM();
        }

        while (currentChildLM != null) {

            childLC = makeChildLayoutContext(context);

            if (!isRestart || emptyStack) {
                if (isRestart) {
                    currentChildLM.reset(); // TODO won't work with forced breaks
                }

                childElements = getNextChildElements(currentChildLM, context, childLC, alignment,
                        null, null, null);
            } else {
                // restart && non-empty LM stack
                childElements = getNextChildElements(currentChildLM, context, childLC, alignment,
                        lmStack, restartPosition, restartAtLM);
                // once encountered, irrelevant for following child LMs
                emptyStack = true;
            }

            if (contentList.isEmpty()) {
                // propagate keep-with-previous up from the first child
                context.updateKeepWithPreviousPending(childLC.getKeepWithPreviousPending());
            }

            // handle non-empty child
            if (childElements != null && !childElements.isEmpty()) {
                if (!contentList.isEmpty()
                        && !ElementListUtils.startsWithForcedBreak(childElements)) {
                    // there is a block handled by prevLM before the one
                    // handled by curLM, and the one handled
                    // by the current LM does not begin with a break
                    addInBetweenBreak(contentList, context, childLC);
                }
                if (childElements.size() == 1
                        && ElementListUtils.startsWithForcedBreak(childElements)) {
                    // a descendant of this block has break-before
                    if (currentChildLM.isFinished() && !hasNextChildLM()) {
                        // if there is no more content, make sure pending
                        // marks are cleared
                        forcedBreakAfterLast = (BreakElement) childElements.get(0);
                        context.clearPendingMarks();
                        // break without adding the child elements
                        break;
                    }
                    if (contentList.isEmpty()) {
                        // empty fo:block: zero-length box makes sure the IDs and/or markers
                        // are registered and borders/padding are painted.
                        elements.add(makeAuxiliaryZeroWidthBox());
                    }
                    // add the forced break
                    contentList.addAll(childElements);
                    // wrap position and return
                    wrapPositionElements(contentList, elements);
                    return elements;
                } else {
                    // add all accumulated child elements
                    contentList.addAll(childElements);
                    if (ElementListUtils.endsWithForcedBreak(childElements)) {
                        // a descendant of this block has break-after
                        if (currentChildLM.isFinished() && !hasNextChildLM()) {
                            // if there is no more content, make sure any
                            // pending marks are cleared
                            forcedBreakAfterLast = (BreakElement) ListUtil.removeLast(contentList);
                            context.clearPendingMarks();
                            break;
                        }
                        //wrap positions and return
                        wrapPositionElements(contentList, elements);
                        return elements;
                    }
                }
                context.updateKeepWithNextPending(childLC.getKeepWithNextPending());
            }
            currentChildLM = getChildLM();
        }

        if (contentList.isEmpty()) {
            if (forcedBreakAfterLast == null) {
                // empty fo:block: zero-length box makes sure the IDs and/or markers
                // are registered.
                elements.add(makeAuxiliaryZeroWidthBox());
            }
        } else {
            // wrap child positions
            wrapPositionElements(contentList, elements);
        }

        addLastVisibleMarks(elements, context, alignment);

        if (forcedBreakAfterLast == null) {
            addKnuthElementsForBreakAfter(elements, context);
        } else {
            forcedBreakAfterLast.clearPendingMarks();
            elements.add(forcedBreakAfterLast);
        }

        context.updateKeepWithNextPending(getKeepWithNext());
        setFinished(true);
        return elements;
    }

    /**
     * Creates and initializes a {@link LayoutContext} to pass to the child LM
     * @param context   the parent {@link LayoutContext}
     * @return a new child layout context
     */
    protected LayoutContext makeChildLayoutContext(LayoutContext context) {
        LayoutContext childLC = new LayoutContext(0);
        childLC.copyPendingMarksFrom(context);
        childLC.setStackLimitBP(context.getStackLimitBP());
        childLC.setRefIPD(referenceIPD);
        return childLC;
    }

    /**
     * Checks if this LM's first "visible marks" (= borders, padding, spaces) have
     * already been processed, and if necessary, adds corresponding elements to
     * the specified list, and updates the given layout context accordingly.
     * @param elements  the element list
     * @param context   the layout context
     * @param alignment the vertical alignment
     */
    protected void addFirstVisibleMarks(List<ListElement> elements,
            LayoutContext context, int alignment) {
        if (!firstVisibleMarkServed) {
            addKnuthElementsForSpaceBefore(elements, alignment);
            context.updateKeepWithPreviousPending(getKeepWithPrevious());
        }
        addKnuthElementsForBorderPaddingBefore(elements, !firstVisibleMarkServed);
        firstVisibleMarkServed = true;

        //Spaces, border and padding to be repeated at each break
        addPendingMarks(context);
    }

    /**
     * Adds elements the LM's last/closing marks to the specified list, and
     * updates the layout context accordingly.
     * @param elements  the element list
     * @param context   the layout context
     * @param alignment the vertical alignment
     */
    protected void addLastVisibleMarks(List<ListElement> elements,
            LayoutContext context, int alignment) {
        addKnuthElementsForBorderPaddingAfter(elements, true);
        addKnuthElementsForSpaceAfter(elements, alignment);

        // All child content processed. Only break-after can occur now, so...
        context.clearPendingMarks();
    }

    /**
     * Check whether there is a break-before condition. If so, and
     * the specified {@code context} allows it, add the necessary elements
     * to the given {@code elements} list.
     * @param context   the layout context
     * @param elements  the element list
     * @return {@code false} if there is a break-before condition, and it has not been served;
     * {@code true} otherwise
     */
    protected boolean breakBeforeServed(LayoutContext context, List<ListElement> elements) {
        if (!breakBeforeServed) {
            breakBeforeServed = true;
            if (!context.suppressBreakBefore()) {
                if (addKnuthElementsForBreakBefore(elements, context)) {
                    return false;
                }
            }
        }
        return breakBeforeServed;
    }

    private KnuthBox makeZeroWidthBox() {
        return new KnuthBox(0, new NonLeafPosition(this, null), false);
    }

    private KnuthBox makeAuxiliaryZeroWidthBox() {
        return new KnuthBox(0, notifyPos(new Position(this)), true);
    }

    private KnuthPenalty makeZeroWidthPenalty(int penaltyValue) {
        return new KnuthPenalty(0, penaltyValue, false, new NonLeafPosition(this, null), false);
    }

    private KnuthGlue makeSpaceAdjustmentGlue(int width, Adjustment adjustmentClass,
                                              boolean isAuxiliary) {
        return new KnuthGlue(width, 0, 0,
                             adjustmentClass,
                             new NonLeafPosition(this, null),
                             isAuxiliary);
    }

    /**
     * Gets the next set of child elements for the given childLM.
     * The default implementation basically copies the pending marks to the child layout context,
     * and subsequently calls the appropriate variant of {@code childLM.getNextKnuthElements()},
     * passing it all relevant parameters.
     * @param childLM   the current child LM
     * @param context   the layout context
     * @param childLC   the child layout context
     * @param alignment the vertical alignment
     * @param lmStack   the stack of currently active LMs (if any)
     * @param restartPosition   the position to restart from (if any)
     * @param restartAtLM   the LM to restart from (if any)
     * @return  list of elements corresponding to the content generated by childLM
     */
    protected List<ListElement> getNextChildElements(LayoutManager childLM, LayoutContext context,
            LayoutContext childLC, int alignment, Stack<LayoutManager> lmStack,
            Position restartPosition, LayoutManager restartAtLM) {

        if (childLM == this.childLMs.get(0)) {
            childLC.setFlags(LayoutContext.SUPPRESS_BREAK_BEFORE);
            //Handled already by the parent (break collapsing, see above)
        }

        if (lmStack == null) {
            // route to default implementation, in case childLM does not provide
            // an override similar to this class
            return childLM.getNextKnuthElements(childLC, alignment);
        } else {
            return childLM.getNextKnuthElements(childLC, alignment, lmStack,
                    restartPosition, restartAtLM);
        }
    }

    /**
     * Adds a break element to the content list between individual child elements.
     * @param contentList the content list
     * @param parentLC the parent layout context
     * @param childLC the currently active child layout context
     */
    protected void addInBetweenBreak(List<ListElement> contentList, LayoutContext parentLC,
                                     LayoutContext childLC) {

        if (mustKeepTogether()
                || parentLC.isKeepWithNextPending()
                || childLC.isKeepWithPreviousPending()) {

            Keep keep = getKeepTogether();

            //Handle pending keep-with-next
            keep = keep.compare(parentLC.getKeepWithNextPending());
            parentLC.clearKeepWithNextPending();

            //Handle pending keep-with-previous from child LM
            keep = keep.compare(childLC.getKeepWithPreviousPending());
            childLC.clearKeepWithPreviousPending();

            // add a penalty to forbid or discourage a break between blocks
            contentList.add(new BreakElement(
                    new Position(this), keep.getPenalty(),
                    keep.getContext(), parentLC));
            return;
        }

        ListElement last = ListUtil.getLast(contentList);
        if (last.isGlue()) {
            // the last element in contentList is a glue;
            // it is a feasible breakpoint, there is no need to add
            // a penalty
            log.warn("glue-type break possibility not handled properly, yet");
            //TODO Does this happen? If yes, need to deal with border and padding
            //at the break possibility
        } else if (!ElementListUtils.endsWithNonInfinitePenalty(contentList)) {

            // TODO vh: this is hacky
            // The getNextKnuthElements method of TableCellLM must not be called
            // twice, otherwise some settings like indents or borders will be
            // counted several times and lead to a wrong output. Anyway the
            // getNextKnuthElements methods should be called only once eventually
            // (i.e., when multi-threading the code), even when there are forced
            // breaks.
            // If we add a break possibility after a forced break the
            // AreaAdditionUtil.addAreas method will act on a sequence starting
            // with a SpaceResolver.SpaceHandlingBreakPosition element, having no
            // LM associated to it. Thus it will stop early instead of adding
            // areas for following Positions. The above test aims at preventing
            // such a situation from occurring. add a null penalty to allow a break
            // between blocks

            // add a null penalty to allow a break between blocks
            contentList.add(new BreakElement(
                    new Position(this), 0, Constants.EN_AUTO, parentLC));
        }
    }

    /** {@inheritDoc} */
    public int negotiateBPDAdjustment(int adj, KnuthElement lastElement) {
        assert (lastElement != null && lastElement.getPosition() != null);
        Position innerPosition = lastElement.getPosition().getPosition();

        if (innerPosition == null && lastElement.isGlue()) {
            // this adjustment applies to space-before or space-after of this block
            if (((KnuthGlue) lastElement).getAdjustmentClass()
                    == Adjustment.SPACE_BEFORE_ADJUSTMENT) {
                // this adjustment applies to space-before
                adjustedSpaceBefore += adj;
            } else {
                // this adjustment applies to space-after
                adjustedSpaceAfter += adj;
            }
            return adj;
        } else if (innerPosition instanceof MappingPosition) {
            // this block has block-progression-unit > 0: the adjustment can concern
            // - the space-before or space-after of this block,
            // - the line number of a descendant of this block
            MappingPosition mappingPos = (MappingPosition)innerPosition;
            if (lastElement.isGlue()) {
                // lastElement is a glue
                ListIterator storedListIterator = storedList.listIterator(
                        mappingPos.getFirstIndex());
                int newAdjustment = 0;
                while (storedListIterator.nextIndex() <= mappingPos.getLastIndex()) {
                    KnuthElement storedElement = (KnuthElement)storedListIterator.next();
                    if (storedElement.isGlue()) {
                        newAdjustment += ((BlockLevelLayoutManager)storedElement
                                .getLayoutManager()).negotiateBPDAdjustment(
                                        adj - newAdjustment, storedElement);
                    }
                }
                newAdjustment = (newAdjustment > 0 ? bpUnit * neededUnits(newAdjustment)
                                                   : -bpUnit * neededUnits(-newAdjustment));
                return newAdjustment;
            } else {
                // lastElement is a penalty: this means that the paragraph
                // has been split between consecutive pages:
                // this may involve a change in the number of lines
                KnuthPenalty storedPenalty = (KnuthPenalty)
                                             storedList.get(mappingPos.getLastIndex());
                if (storedPenalty.getWidth() > 0) {
                    // the original penalty has width > 0
                    return ((BlockLevelLayoutManager)storedPenalty.getLayoutManager())
                           .negotiateBPDAdjustment(storedPenalty.getWidth(),
                                   storedPenalty);
                } else {
                    // the original penalty has width = 0
                    // the adjustment involves only the spaces before and after
                    return adj;
                }
            }
        } else if (innerPosition != null && innerPosition.getLM() != this) {
            // this adjustment concerns another LM
            NonLeafPosition savedPos = (NonLeafPosition) lastElement.getPosition();
            lastElement.setPosition(innerPosition);
            int returnValue = ((BlockLevelLayoutManager)lastElement.getLayoutManager())
                    .negotiateBPDAdjustment(adj, lastElement);
            lastElement.setPosition(savedPos);
            return returnValue;
        } else {
            // this should never happen
            log.error("BlockLayoutManager.negotiateBPDAdjustment(): unexpected Position");
            return 0;
        }
    }

    /** {@inheritDoc} */
    public void discardSpace(KnuthGlue spaceGlue) {
        assert (spaceGlue != null && spaceGlue.getPosition() != null);
        Position innerPosition = spaceGlue.getPosition().getPosition();

        if (innerPosition == null || innerPosition.getLM() == this) {
            // if this block has block-progression-unit > 0, innerPosition can be
            // a MappingPosition
            // spaceGlue represents space before or space after of this block
            if (spaceGlue.getAdjustmentClass() == Adjustment.SPACE_BEFORE_ADJUSTMENT) {
                // space-before must be discarded
                adjustedSpaceBefore = 0;
                foSpaceBefore = MinOptMax.ZERO;
            } else {
                // space-after must be discarded
                adjustedSpaceAfter = 0;
                foSpaceAfter = MinOptMax.ZERO;
                //TODO Why are both cases handled in the same way?
            }
        } else {
            // this element was not created by this BlockLM
            NonLeafPosition savedPos = (NonLeafPosition)spaceGlue.getPosition();
            spaceGlue.setPosition(innerPosition);
            ((BlockLevelLayoutManager) spaceGlue.getLayoutManager()).discardSpace(spaceGlue);
            spaceGlue.setPosition(savedPos);
        }
    }

    /** {@inheritDoc} */
    @Override
    public List getChangedKnuthElements(List oldList, int alignment) {
        ListIterator<KnuthElement> oldListIterator = oldList.listIterator();
        KnuthElement currElement = null;
        KnuthElement prevElement = null;
        List<KnuthElement> returnedList = new LinkedList<KnuthElement>();
        List<KnuthElement> returnList = new LinkedList<KnuthElement>();
        int fromIndex = 0;

        // "unwrap" the Positions stored in the elements
        KnuthElement oldElement;
        while (oldListIterator.hasNext()) {
            oldElement = oldListIterator.next();
            assert oldElement.getPosition() != null;
            Position innerPosition = oldElement.getPosition().getPosition();
            if (innerPosition != null) {
                // oldElement was created by a descendant
                oldElement.setPosition(innerPosition);
            } else {
                // oldElement was created by this LM:
                // modify its position in order to recognize it was not created
                // by a child
                oldElement.setPosition(new Position(this));
            }
        }

        // create the iterator
        ListIterator<KnuthElement> workListIterator = oldList.listIterator();
        while (workListIterator.hasNext()) {
            currElement = workListIterator.next();
            if (prevElement != null
                && prevElement.getLayoutManager() != currElement.getLayoutManager()) {
                // prevElement is the last element generated by the same LM
                BlockLevelLayoutManager prevLM
                        = (BlockLevelLayoutManager)prevElement.getLayoutManager();
                BlockLevelLayoutManager currLM
                        = (BlockLevelLayoutManager)currElement.getLayoutManager();
                boolean somethingAdded = false;
                if (prevLM != this) {
                    returnedList.addAll(
                            prevLM.getChangedKnuthElements(
                                    oldList.subList(fromIndex, workListIterator.previousIndex()),
                                    alignment));
                    somethingAdded = true;
                } else {
                    // do nothing
                }
                fromIndex = workListIterator.previousIndex();

                /*
                 * TODO: why are KnuthPenalties added here,
                 *       while in getNextKE they were changed to BreakElements?
                 */
                // there is another block after this one
                if (somethingAdded
                    && (this.mustKeepTogether()
                        || prevLM.mustKeepWithNext()
                        || currLM.mustKeepWithPrevious())) {
                    // add an infinite penalty to forbid a break between blocks
                    returnedList.add(makeZeroWidthPenalty(KnuthPenalty.INFINITE));
                } else if (somethingAdded
                        && !ListUtil.getLast(returnedList).isGlue()) {
                    // add a null penalty to allow a break between blocks
                    returnedList.add(makeZeroWidthPenalty(KnuthPenalty.INFINITE));
                }
            }
            prevElement = currElement;
        }
        if (currElement != null) {
            LayoutManager currLM = currElement.getLayoutManager();
            if (currLM != this) {
                returnedList.addAll(currLM.getChangedKnuthElements(
                        oldList.subList(fromIndex, oldList.size()), alignment));
            } else {
                // there are no more elements to add
                // remove the last penalty added to returnedList
                if (!returnedList.isEmpty()) {
                    ListUtil.removeLast(returnedList);
                }
            }
        }

        // append elements representing space-before
        boolean spaceBeforeIsConditional = true;
        if (fobj instanceof org.apache.fop.fo.flow.Block) {
            spaceBeforeIsConditional = getSpaceBeforeProperty().isDiscard();
        }
        if (adjustedSpaceBefore != 0) {
            if (!spaceBeforeIsConditional) {
                // add elements to prevent the glue to be discarded
                returnList.add(makeZeroWidthBox());
                returnList.add(makeZeroWidthPenalty(KnuthPenalty.INFINITE));
            }

            returnList.add(makeSpaceAdjustmentGlue(adjustedSpaceBefore,
                    Adjustment.SPACE_BEFORE_ADJUSTMENT,
                    false));
        }

        // "wrap" the Position stored in each element of returnedList
        // and add elements to returnList
        for (KnuthElement el : returnedList) {
            el.setPosition(new NonLeafPosition(this, el.getPosition()));
            returnList.add(el);
        }

        // append elements representing space-after
        boolean spaceAfterIsConditional = true;
        if (fobj instanceof org.apache.fop.fo.flow.Block) {
            spaceAfterIsConditional = getSpaceAfterProperty().isDiscard();
        }
        if (adjustedSpaceAfter != 0) {
            if (!spaceAfterIsConditional) {
                returnList.add(makeZeroWidthPenalty(KnuthPenalty.INFINITE));
            }

            returnList.add(makeSpaceAdjustmentGlue(adjustedSpaceAfter,
                    Adjustment.SPACE_AFTER_ADJUSTMENT,
                    spaceAfterIsConditional));

            if (!spaceAfterIsConditional) {
                returnList.add(makeZeroWidthBox());
            }
        }

        return returnList;
    }

    /**
     * Retrieves and returns the keep-together strength from the parent element.
     * @return the keep-together strength
     */
    protected Keep getParentKeepTogether() {
        Keep keep = Keep.KEEP_AUTO;
        if (getParent() instanceof BlockLevelLayoutManager) {
            keep = ((BlockLevelLayoutManager)getParent()).getKeepTogether();
        } else if (getParent() instanceof InlineLayoutManager) {
            if (((InlineLayoutManager) getParent()).mustKeepTogether()) {
                keep = Keep.KEEP_ALWAYS;
            }
            //TODO Fix me
            //strength = ((InlineLayoutManager) getParent()).getKeepTogetherStrength();
        }
        return keep;
    }

    /** {@inheritDoc} */
    public boolean mustKeepTogether() {
        return !getKeepTogether().isAuto();
    }

    /** {@inheritDoc} */
    public boolean mustKeepWithPrevious() {
        return !getKeepWithPrevious().isAuto();
    }

    /** {@inheritDoc} */
    public boolean mustKeepWithNext() {
        return !getKeepWithNext().isAuto();
    }

    /** {@inheritDoc} */
    public Keep getKeepTogether() {
        Keep keep = Keep.getKeep(getKeepTogetherProperty());
        keep = keep.compare(getParentKeepTogether());
        return keep;
    }

    /** {@inheritDoc} */
    public Keep getKeepWithPrevious() {
        return Keep.getKeep(getKeepWithPreviousProperty());
    }

    /** {@inheritDoc} */
    public Keep getKeepWithNext() {
        return Keep.getKeep(getKeepWithNextProperty());
    }

    /**
     * {@inheritDoc}
     * Default implementation throws a {@link IllegalStateException}.
     * Must be implemented by the subclass, if applicable.
     */
    public KeepProperty getKeepTogetherProperty() {
        throw new IllegalStateException();
    }

    /**
     * {@inheritDoc}
     * Default implementation throws a {@link IllegalStateException}.
     * Must be implemented by the subclass, if applicable.
     */
    public KeepProperty getKeepWithPreviousProperty() {
        throw new IllegalStateException();
    }

    /**
     * {@inheritDoc}
     * Default implementation throws a {@link IllegalStateException}.
     * Must be implemented by the subclass, if applicable.
     */
    public KeepProperty getKeepWithNextProperty() {
        throw new IllegalStateException();
    }

    /**
     * Adds the unresolved elements for border and padding to a layout context so break
     * possibilities can be properly constructed.
     * @param context the layout context
     */
    protected void addPendingMarks(LayoutContext context) {
        CommonBorderPaddingBackground borderAndPadding = getBorderPaddingBackground();
        if (borderAndPadding != null) {
            if (borderAndPadding.getBorderBeforeWidth(false) > 0) {
                context.addPendingBeforeMark(new BorderElement(
                        getAuxiliaryPosition(),
                        borderAndPadding.getBorderInfo(
                                CommonBorderPaddingBackground.BEFORE).getWidth(),
                                RelSide.BEFORE,
                                false, false, this));
            }
            if (borderAndPadding.getPaddingBefore(false, this) > 0) {
                context.addPendingBeforeMark(new PaddingElement(
                        getAuxiliaryPosition(),
                        borderAndPadding.getPaddingLengthProperty(
                                CommonBorderPaddingBackground.BEFORE),
                                RelSide.BEFORE,
                                false, false, this));
            }
            if (borderAndPadding.getBorderAfterWidth(false) > 0) {
                context.addPendingAfterMark(new BorderElement(
                        getAuxiliaryPosition(),
                        borderAndPadding.getBorderInfo(
                                CommonBorderPaddingBackground.AFTER).getWidth(),
                                RelSide.AFTER,
                                false, false, this));
            }
            if (borderAndPadding.getPaddingAfter(false, this) > 0) {
                context.addPendingAfterMark(new PaddingElement(
                        getAuxiliaryPosition(),
                        borderAndPadding.getPaddingLengthProperty(
                                CommonBorderPaddingBackground.AFTER),
                                RelSide.AFTER,
                                false, false, this));
            }
        }
    }

    /** @return the border, padding and background info structure */
    private CommonBorderPaddingBackground getBorderPaddingBackground() {
        if (fobj instanceof org.apache.fop.fo.flow.Block) {
            return ((org.apache.fop.fo.flow.Block)fobj)
                .getCommonBorderPaddingBackground();
        } else if (fobj instanceof org.apache.fop.fo.flow.BlockContainer) {
            return ((org.apache.fop.fo.flow.BlockContainer)fobj)
                .getCommonBorderPaddingBackground();
        } else if (fobj instanceof org.apache.fop.fo.flow.ListBlock) {
            return ((org.apache.fop.fo.flow.ListBlock)fobj)
                .getCommonBorderPaddingBackground();
        } else if (fobj instanceof org.apache.fop.fo.flow.ListItem) {
            return ((org.apache.fop.fo.flow.ListItem)fobj)
                .getCommonBorderPaddingBackground();
        } else if (fobj instanceof org.apache.fop.fo.flow.table.Table) {
            return ((org.apache.fop.fo.flow.table.Table)fobj)
                .getCommonBorderPaddingBackground();
        } else {
            return null;
        }
    }

    /** @return the space-before property */
    protected SpaceProperty getSpaceBeforeProperty() {
        if (fobj instanceof org.apache.fop.fo.flow.Block) {
            return ((org.apache.fop.fo.flow.Block)fobj)
                .getCommonMarginBlock().spaceBefore;
        } else if (fobj instanceof org.apache.fop.fo.flow.BlockContainer) {
            return ((org.apache.fop.fo.flow.BlockContainer)fobj)
                .getCommonMarginBlock().spaceBefore;
        } else if (fobj instanceof org.apache.fop.fo.flow.ListBlock) {
            return ((org.apache.fop.fo.flow.ListBlock)fobj)
                .getCommonMarginBlock().spaceBefore;
        } else if (fobj instanceof org.apache.fop.fo.flow.ListItem) {
            return ((org.apache.fop.fo.flow.ListItem)fobj)
                .getCommonMarginBlock().spaceBefore;
        } else if (fobj instanceof org.apache.fop.fo.flow.table.Table) {
            return ((org.apache.fop.fo.flow.table.Table)fobj)
                .getCommonMarginBlock().spaceBefore;
        } else {
            return null;
        }
    }

    /** @return the space-after property */
    protected SpaceProperty getSpaceAfterProperty() {
        if (fobj instanceof org.apache.fop.fo.flow.Block) {
            return ((org.apache.fop.fo.flow.Block)fobj)
                .getCommonMarginBlock().spaceAfter;
        } else if (fobj instanceof org.apache.fop.fo.flow.BlockContainer) {
            return ((org.apache.fop.fo.flow.BlockContainer)fobj)
                .getCommonMarginBlock().spaceAfter;
        } else if (fobj instanceof org.apache.fop.fo.flow.ListBlock) {
            return ((org.apache.fop.fo.flow.ListBlock)fobj)
                .getCommonMarginBlock().spaceAfter;
        } else if (fobj instanceof org.apache.fop.fo.flow.ListItem) {
            return ((org.apache.fop.fo.flow.ListItem)fobj)
                .getCommonMarginBlock().spaceAfter;
        } else if (fobj instanceof org.apache.fop.fo.flow.table.Table) {
            return ((org.apache.fop.fo.flow.table.Table)fobj)
                .getCommonMarginBlock().spaceAfter;
        } else {
            return null;
        }
    }

    /**
     * Creates Knuth elements for before border padding and adds them to the return list.
     * @param returnList return list to add the additional elements to
     * @param isFirst true if this is the first time a layout manager instance needs to generate
     *                border and padding
     */
    protected void addKnuthElementsForBorderPaddingBefore(List returnList, boolean isFirst) {
        //Border and Padding (before)
        CommonBorderPaddingBackground borderAndPadding = getBorderPaddingBackground();
        if (borderAndPadding != null) {
            if (borderAndPadding.getBorderBeforeWidth(false) > 0) {
                returnList.add(new BorderElement(
                        getAuxiliaryPosition(),
                        borderAndPadding.getBorderInfo(CommonBorderPaddingBackground.BEFORE)
                                .getWidth(),
                        RelSide.BEFORE, isFirst, false, this));
            }
            if (borderAndPadding.getPaddingBefore(false, this) > 0) {
                returnList.add(new PaddingElement(
                        getAuxiliaryPosition(),
                        borderAndPadding.getPaddingLengthProperty(
                                CommonBorderPaddingBackground.BEFORE),
                        RelSide.BEFORE, isFirst, false, this));
            }
        }
    }

    /**
     * Creates Knuth elements for after border padding and adds them to the return list.
     * @param returnList return list to add the additional elements to
     * @param isLast true if this is the last time a layout manager instance needs to generate
     *               border and padding
     */
    protected void addKnuthElementsForBorderPaddingAfter(List returnList, boolean isLast) {
        //Border and Padding (after)
        CommonBorderPaddingBackground borderAndPadding = getBorderPaddingBackground();
        if (borderAndPadding != null) {
            if (borderAndPadding.getPaddingAfter(false, this) > 0) {
                returnList.add(new PaddingElement(
                        getAuxiliaryPosition(),
                        borderAndPadding.getPaddingLengthProperty(
                                CommonBorderPaddingBackground.AFTER),
                        RelSide.AFTER, false, isLast, this));
            }
            if (borderAndPadding.getBorderAfterWidth(false) > 0) {
                returnList.add(new BorderElement(
                        getAuxiliaryPosition(),
                        borderAndPadding.getBorderInfo(CommonBorderPaddingBackground.AFTER)
                                .getWidth(),
                        RelSide.AFTER, false, isLast, this));
            }
        }
    }

    /**
     * Creates Knuth elements for break-before and adds them to the return list.
     * @param returnList return list to add the additional elements to
     * @param context the layout context
     * @return true if an element has been added due to a break-before.
     */
    protected boolean addKnuthElementsForBreakBefore(List returnList, LayoutContext context) {
        int breakBefore = BreakOpportunityHelper.getBreakBefore(this);
        if (breakBefore == EN_PAGE
                || breakBefore == EN_COLUMN
                || breakBefore == EN_EVEN_PAGE
                || breakBefore == EN_ODD_PAGE) {
            // return a penalty element, representing a forced page break
            returnList.add(new BreakElement(getAuxiliaryPosition(),
                    0, -KnuthElement.INFINITE, breakBefore, context));
            return true;
        } else {
            return false;
        }
    }

    /**
     * Creates Knuth elements for break-after and adds them to the return list.
     * @param returnList return list to add the additional elements to
     * @param context the layout context
     * @return true if an element has been added due to a break-after.
     */
    protected boolean addKnuthElementsForBreakAfter(List returnList, LayoutContext context) {
        int breakAfter = -1;
        if (fobj instanceof BreakPropertySet) {
            breakAfter = ((BreakPropertySet)fobj).getBreakAfter();
        }
        if (breakAfter == EN_PAGE
                || breakAfter == EN_COLUMN
                || breakAfter == EN_EVEN_PAGE
                || breakAfter == EN_ODD_PAGE) {
            // add a penalty element, representing a forced page break
            returnList.add(new BreakElement(getAuxiliaryPosition(),
                    0, -KnuthElement.INFINITE, breakAfter, context));
            return true;
        } else {
            return false;
        }
    }

    /**
     * Creates Knuth elements for space-before and adds them to the return list.
     * @param returnList return list to add the additional elements to
     * @param alignment vertical alignment
     */
    protected void addKnuthElementsForSpaceBefore(List returnList, int alignment) {
        SpaceProperty spaceBefore = getSpaceBeforeProperty();
        // append elements representing space-before
        if (spaceBefore != null
                   && !(spaceBefore.getMinimum(this).getLength().getValue(this) == 0
                        && spaceBefore.getMaximum(this).getLength().getValue(this) == 0)) {
            returnList.add(new SpaceElement(getAuxiliaryPosition(), spaceBefore,
                    RelSide.BEFORE,
                    true, false, this));
        }
    }

    /**
     * Creates Knuth elements for space-after and adds them to the return list.
     * @param returnList return list to add the additional elements to
     * @param alignment vertical alignment
     */
    protected void addKnuthElementsForSpaceAfter(List returnList, int alignment) {
        SpaceProperty spaceAfter = getSpaceAfterProperty();
        // append elements representing space-after
        if (spaceAfter != null
                && !(spaceAfter.getMinimum(this).getLength().getValue(this) == 0
                     && spaceAfter.getMaximum(this).getLength().getValue(this) == 0)) {
            returnList.add(new SpaceElement(getAuxiliaryPosition(), spaceAfter,
                    RelSide.AFTER,
                    false, true, this));
        }
    }

    /** A mapping position. */
    protected static class MappingPosition extends Position {

        private int firstIndex;
        private int lastIndex;

        /**
         * Construct mapping position.
         * @param lm layout manager
         * @param first position
         * @param last position
         */
        public MappingPosition(LayoutManager lm, int first, int last) {
            super(lm);
            firstIndex = first;
            lastIndex = last;
        }

        /** @return first index */
        public int getFirstIndex() {
            return firstIndex;
        }

        /** @return last index */
        public int getLastIndex() {
            return lastIndex;
        }
    }

    /**
     * "wrap" the Position inside each element moving the elements from
     * SourceList to targetList
     * @param sourceList source list
     * @param targetList target list receiving the wrapped position elements
     */
    protected void wrapPositionElements(List sourceList, List targetList) {
        wrapPositionElements(sourceList, targetList, false);
    }

    /**
     * "wrap" the Position inside each element moving the elements from
     * SourceList to targetList
     * @param sourceList source list
     * @param targetList target list receiving the wrapped position elements
     * @param force if true, every Position is wrapped regardless of its LM of origin
     */
    protected void wrapPositionElements(List sourceList, List targetList, boolean force) {

        ListIterator listIter = sourceList.listIterator();
        Object tempElement;
        while (listIter.hasNext()) {
            tempElement = listIter.next();
            if (tempElement instanceof ListElement) {
                wrapPositionElement(
                        (ListElement) tempElement,
                        targetList,
                        force);
            } else if (tempElement instanceof List) {
                wrapPositionElements(
                        (List) tempElement,
                        targetList,
                        force);
            }
        }
    }

    /**
     * "wrap" the Position inside the given element and add it to the target list.
     * @param el the list element
     * @param targetList target list receiving the wrapped position elements
     * @param force if true, every Position is wrapped regardless of its LM of origin
     */
    protected void wrapPositionElement(ListElement el, List targetList, boolean force) {
        if (force || el.getLayoutManager() != this) {
            el.setPosition(notifyPos(new NonLeafPosition(this, el.getPosition())));
        }
        targetList.add(el);
    }


    /** @return the sum of start-indent and end-indent */
    protected int getIPIndents() {
        return startIndent + endIndent;
    }

    /**
     * Returns the IPD of the content area
     * @return the IPD of the content area
     */
    @Override
    public int getContentAreaIPD() {
        return contentAreaIPD;
    }

    /**
     * Sets the IPD of the content area
     * @param contentAreaIPD the IPD of the content area
     */
    protected void setContentAreaIPD(int contentAreaIPD) {
        this.contentAreaIPD = contentAreaIPD;
    }

    /**
     * Returns the BPD of the content area
     * @return the BPD of the content area
     */
    @Override
    public int getContentAreaBPD() {
        return -1;
    }

    /** {@inheritDoc} */
    @Override
    public void reset() {
        super.reset();
        breakBeforeServed = false;
        firstVisibleMarkServed = false;
        // TODO startIndent, endIndent
    }

    /**
     * Whether this LM can handle horizontal overflow error messages (only a BlockContainerLayoutManager can).
     * @param milliPoints horizontal overflow
     * @return true if handled by a BlockContainerLayoutManager
     */
    public boolean handleOverflow(int milliPoints) {
        if (getParent() instanceof BlockStackingLayoutManager) {
            return ((BlockStackingLayoutManager) getParent()).handleOverflow(milliPoints);
        }
        return false;
    }
}