org.eclipse.jgit/src/org/eclipse/jgit/revplot/PlotCommitList.java - jgit - Git at Google

 /*
  * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org>,
  * Copyright (C) 2010, Christian Halstrick <christian.halstrick@sap.com>
  * Copyright (C) 2014, Konrad Kügler and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
  * https://www.eclipse.org/org/documents/edl-v10.php.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */

 package org.eclipse.jgit.revplot;

 import java.text.MessageFormat;
 import java.util.BitSet;
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.TreeSet;

 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.revwalk.RevCommitList;
 import org.eclipse.jgit.revwalk.RevWalk;

 /**
  * An ordered list of {@link org.eclipse.jgit.revplot.PlotCommit} subclasses.
  * <p>
  * Commits are allocated into lanes as they enter the list, based upon their
  * connections between descendant (child) commits and ancestor (parent) commits.
  * <p>
  * The source of the list must be a {@link org.eclipse.jgit.revplot.PlotWalk}
  * and {@link #fillTo(int)} must be used to populate the list.
  *
  * @param <L>
  *            type of lane used by the application.
  */
 public class PlotCommitList<L extends PlotLane> extends
 		RevCommitList<PlotCommit<L>> {
 	static final int MAX_LENGTH = 25;

 	private int positionsAllocated;

 	private final TreeSet<Integer> freePositions = new TreeSet<>();

 	private final HashSet<PlotLane> activeLanes = new HashSet<>(32);

 	/** number of (child) commits on a lane */
 	private final HashMap<PlotLane, Integer> laneLength = new HashMap<>(
 			32);

 	@Override
 	public void clear() {
 		super.clear();
 		positionsAllocated = 0;
 		freePositions.clear();
 		activeLanes.clear();
 		laneLength.clear();
 	}

 	@Override
 	public void source(RevWalk w) {
 		if (!(w instanceof PlotWalk))
 			throw new ClassCastException(MessageFormat.format(JGitText.get().classCastNotA, PlotWalk.class.getName()));
 		super.source(w);
 	}

 	/**
 	 * Find the set of lanes passing through a commit's row.
 	 * <p>
 	 * Lanes passing through a commit are lanes that the commit is not directly
 	 * on, but that need to travel through this commit to connect a descendant
 	 * (child) commit to an ancestor (parent) commit. Typically these lanes will
 	 * be drawn as lines in the passed commit's box, and the passed commit won't
 	 * appear to be connected to those lines.
 	 * <p>
 	 * This method modifies the passed collection by adding the lanes in any
 	 * order.
 	 *
 	 * @param currCommit
 	 *            the commit the caller needs to get the lanes from.
 	 * @param result
 	 *            collection to add the passing lanes into.
 	 */
 	@SuppressWarnings("unchecked")
 	public void findPassingThrough(final PlotCommit<L> currCommit,
 			final Collection<L> result) {
 		for (PlotLane p : currCommit.passingLanes)
 			result.add((L) p);
 	}

 	@SuppressWarnings("ReferenceEquality")
 	@Override
 	protected void enter(int index, PlotCommit<L> currCommit) {
 		setupChildren(currCommit);

 		final int nChildren = currCommit.getChildCount();
 		if (nChildren == 0) {
 			currCommit.lane = nextFreeLane();
 		} else if (nChildren == 1
 				&& currCommit.children[0].getParentCount() < 2) {
 			// Only one child, child has only us as their parent.
 			// Stay in the same lane as the child.

 			@SuppressWarnings("unchecked")
 			final PlotCommit<L> c = currCommit.children[0];
 			currCommit.lane = c.lane;
 			Integer len = laneLength.get(currCommit.lane);
 			len = len != null ? Integer.valueOf(len.intValue() + 1)
 					: Integer.valueOf(0);
 			laneLength.put(currCommit.lane, len);
 		} else {
 			// More than one child, or our child is a merge.

 			// We look for the child lane the current commit should continue.
 			// Candidate lanes for this are those with children, that have the
 			// current commit as their first parent.
 			// There can be multiple candidate lanes. In that case the longest
 			// lane is chosen, as this is usually the lane representing the
 			// branch the commit actually was made on.

 			// When there are no candidate lanes (i.e. the current commit has
 			// only children whose non-first parent it is) we place the current
 			// commit on a new lane.

 			// The lane the current commit will be placed on:
 			PlotLane reservedLane = null;
 			PlotCommit childOnReservedLane = null;
 			int lengthOfReservedLane = -1;

 			for (int i = 0; i < nChildren; i++) {
 				@SuppressWarnings("unchecked")
 				final PlotCommit<L> c = currCommit.children[i];
 				if (c.getParent(0) == currCommit) {
 					Integer len = laneLength.get(c.lane);
 					// we may be the first parent for multiple lines of
 					// development, try to continue the longest one
 					if (len.intValue() > lengthOfReservedLane) {
 						reservedLane = c.lane;
 						childOnReservedLane = c;
 						lengthOfReservedLane = len.intValue();
 					}
 				}
 			}

 			if (reservedLane != null) {
 				currCommit.lane = reservedLane;
 				laneLength.put(reservedLane,
 						Integer.valueOf(lengthOfReservedLane + 1));
 				handleBlockedLanes(index, currCommit, childOnReservedLane);
 			} else {
 				currCommit.lane = nextFreeLane();
 				handleBlockedLanes(index, currCommit, null);
 			}

 			// close lanes of children, if there are no first parents that might
 			// want to continue the child lanes
 			for (int i = 0; i < nChildren; i++) {
 				final PlotCommit c = currCommit.children[i];
 				PlotCommit firstParent = (PlotCommit) c.getParent(0);
 				if (firstParent.lane != null && firstParent.lane != c.lane)
 					closeLane(c.lane);
 			}
 		}

 		continueActiveLanes(currCommit);
 		if (currCommit.getParentCount() == 0)
 			closeLane(currCommit.lane);
 	}

 	private void continueActiveLanes(PlotCommit currCommit) {
 		for (PlotLane lane : activeLanes)
 			if (lane != currCommit.lane)
 				currCommit.addPassingLane(lane);
 	}

 	/**
 	 * Sets up fork and merge information in the involved PlotCommits.
 	 * Recognizes and handles blockades that involve forking or merging arcs.
 	 *
 	 * @param index
 	 *            the index of <code>currCommit</code> in the list
 	 * @param currCommit
 	 *            the current commit
 	 * @param childOnLane
 	 *            the direct child on the same lane as <code>currCommit</code>,
 	 *            may be null if <code>currCommit</code> is the first commit on
 	 *            the lane
 	 */
 	@SuppressWarnings("ReferenceEquality")
 	private void handleBlockedLanes(final int index, final PlotCommit currCommit,
 			final PlotCommit childOnLane) {
 		for (PlotCommit child : currCommit.children) {
 			if (child == childOnLane)
 				continue; // simple continuations of lanes are handled by
 							// continueActiveLanes() calls in enter()

 			// Is the child a merge or is it forking off?
 			boolean childIsMerge = child.getParent(0) != currCommit;
 			if (childIsMerge) {
 				PlotLane laneToUse = currCommit.lane;
 				laneToUse = handleMerge(index, currCommit, childOnLane, child,
 						laneToUse);
 				child.addMergingLane(laneToUse);
 			} else {
 				// We want to draw a forking arc in the child's lane.
 				// As an active lane, the child lane already continues
 				// (unblocked) up to this commit, we only need to mark it as
 				// forking off from the current commit.
 				PlotLane laneToUse = child.lane;
 				currCommit.addForkingOffLane(laneToUse);
 			}
 		}
 	}

 	// Handles the case where currCommit is a non-first parent of the child
 	@SuppressWarnings("ReferenceEquality")
 	private PlotLane handleMerge(final int index, final PlotCommit currCommit,
 			final PlotCommit childOnLane, PlotCommit child, PlotLane laneToUse) {

 		// find all blocked positions between currCommit and this child

 		int childIndex = index; // useless initialization, should
 								// always be set in the loop below
 		BitSet blockedPositions = new BitSet();
 		for (int r = index - 1; r >= 0; r--) {
 			final PlotCommit rObj = get(r);
 			if (rObj == child) {
 				childIndex = r;
 				break;
 			}
 			addBlockedPosition(blockedPositions, rObj);
 		}

 		// handle blockades

 		if (blockedPositions.get(laneToUse.getPosition())) {
 			// We want to draw a merging arc in our lane to the child,
 			// which is on another lane, but our lane is blocked.

 			// Check if childOnLane is beetween commit and the child we
 			// are currently processing
 			boolean needDetour = false;
 			if (childOnLane != null) {
 				for (int r = index - 1; r > childIndex; r--) {
 					final PlotCommit rObj = get(r);
 					if (rObj == childOnLane) {
 						needDetour = true;
 						break;
 					}
 				}
 			}

 			if (needDetour) {
 				// It is childOnLane which is blocking us. Repositioning
 				// our lane would not help, because this repositions the
 				// child too, keeping the blockade.
 				// Instead, we create a "detour lane" which gets us
 				// around the blockade. That lane has no commits on it.
 				laneToUse = nextFreeLane(blockedPositions);
 				currCommit.addForkingOffLane(laneToUse);
 				closeLane(laneToUse);
 			} else {
 				// The blockade is (only) due to other (already closed)
 				// lanes at the current lane's position. In this case we
 				// reposition the current lane.
 				// We are the first commit on this lane, because
 				// otherwise the child commit on this lane would have
 				// kept other lanes from blocking us. Since we are the
 				// first commit, we can freely reposition.
 				int newPos = getFreePosition(blockedPositions);
 				freePositions.add(Integer.valueOf(laneToUse
 						.getPosition()));
 				laneToUse.position = newPos;
 			}
 		}

 		// Actually connect currCommit to the merge child
 		drawLaneToChild(index, child, laneToUse);
 		return laneToUse;
 	}

 	/**
 	 * Connects the commit at commitIndex to the child, using the given lane.
 	 * All blockades on the lane must be resolved before calling this method.
 	 *
 	 * @param commitIndex
 	 *            commit index
 	 * @param child
 	 *            child to connect
 	 * @param laneToContinue
 	 *            lane to continue
 	 */
 	@SuppressWarnings("ReferenceEquality")
 	private void drawLaneToChild(final int commitIndex, PlotCommit child,
 			PlotLane laneToContinue) {
 		for (int r = commitIndex - 1; r >= 0; r--) {
 			final PlotCommit rObj = get(r);
 			if (rObj == child)
 				break;
 			if (rObj != null)
 				rObj.addPassingLane(laneToContinue);
 		}
 	}

 	private static void addBlockedPosition(BitSet blockedPositions,
 			final PlotCommit rObj) {
 		if (rObj != null) {
 			PlotLane lane = rObj.getLane();
 			// Positions may be blocked by a commit on a lane.
 			if (lane != null)
 				blockedPositions.set(lane.getPosition());
 			// Positions may also be blocked by forking off and merging lanes.
 			// We don't consider passing lanes, because every passing lane forks
 			// off and merges at it ends.
 			for (PlotLane l : rObj.forkingOffLanes)
 				blockedPositions.set(l.getPosition());
 			for (PlotLane l : rObj.mergingLanes)
 				blockedPositions.set(l.getPosition());
 		}
 	}

 	@SuppressWarnings("unchecked")
 	private void closeLane(PlotLane lane) {
 		if (activeLanes.remove(lane)) {
 			recycleLane((L) lane);
 			laneLength.remove(lane);
 			freePositions.add(Integer.valueOf(lane.getPosition()));
 		}
 	}

 	private void setupChildren(PlotCommit<L> currCommit) {
 		final int nParents = currCommit.getParentCount();
 		for (int i = 0; i < nParents; i++)
 			((PlotCommit) currCommit.getParent(i)).addChild(currCommit);
 	}

 	private PlotLane nextFreeLane() {
 		return nextFreeLane(null);
 	}

 	private PlotLane nextFreeLane(BitSet blockedPositions) {
 		final PlotLane p = createLane();
 		p.position = getFreePosition(blockedPositions);
 		activeLanes.add(p);
 		laneLength.put(p, Integer.valueOf(1));
 		return p;
 	}

 	/**
 	 * @param blockedPositions
 	 *            may be null
 	 * @return a free lane position
 	 */
 	private int getFreePosition(BitSet blockedPositions) {
 		if (freePositions.isEmpty())
 			return positionsAllocated++;

 		if (blockedPositions != null) {
 			for (Integer pos : freePositions)
 				if (!blockedPositions.get(pos.intValue())) {
 					freePositions.remove(pos);
 					return pos.intValue();
 				}
 			return positionsAllocated++;
 		}
 		final Integer min = freePositions.first();
 		freePositions.remove(min);
 		return min.intValue();
 	}

 	/**
 	 * Create a new {@link PlotLane} appropriate for this particular
 	 * {@link PlotCommitList}.
 	 *
 	 * @return a new {@link PlotLane} appropriate for this particular
 	 *         {@link PlotCommitList}.
 	 */
 	@SuppressWarnings("unchecked")
 	protected L createLane() {
 		return (L) new PlotLane();
 	}

 	/**
 	 * Return colors and other reusable information to the plotter when a lane
 	 * is no longer needed.
 	 *
 	 * @param lane
 	 *            a lane
 	 */
 	protected void recycleLane(L lane) {
 		// Nothing.
 	}
 }
	/*
	* Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org>,
	* Copyright (C) 2010, Christian Halstrick <christian.halstrick@sap.com>
	* Copyright (C) 2014, Konrad Kügler and others
	*
	* This program and the accompanying materials are made available under the
	* terms of the Eclipse Distribution License v. 1.0 which is available at
	* https://www.eclipse.org/org/documents/edl-v10.php.
	*
	* SPDX-License-Identifier: BSD-3-Clause
	*/

	package org.eclipse.jgit.revplot;

	import java.text.MessageFormat;
	import java.util.BitSet;
	import java.util.Collection;
	import java.util.HashMap;
	import java.util.HashSet;
	import java.util.TreeSet;

	import org.eclipse.jgit.internal.JGitText;
	import org.eclipse.jgit.revwalk.RevCommitList;
	import org.eclipse.jgit.revwalk.RevWalk;

	/**
	* An ordered list of {@link org.eclipse.jgit.revplot.PlotCommit} subclasses.
	* <p>
	* Commits are allocated into lanes as they enter the list, based upon their
	* connections between descendant (child) commits and ancestor (parent) commits.
	* <p>
	* The source of the list must be a {@link org.eclipse.jgit.revplot.PlotWalk}
	* and {@link #fillTo(int)} must be used to populate the list.
	*
	* @param <L>
	* type of lane used by the application.
	*/
	public class PlotCommitList<L extends PlotLane> extends
	RevCommitList<PlotCommit<L>> {
	static final int MAX_LENGTH = 25;

	private int positionsAllocated;

	private final TreeSet<Integer> freePositions = new TreeSet<>();

	private final HashSet<PlotLane> activeLanes = new HashSet<>(32);

	/** number of (child) commits on a lane */
	private final HashMap<PlotLane, Integer> laneLength = new HashMap<>(
	32);

	@Override
	public void clear() {
	super.clear();
	positionsAllocated = 0;
	freePositions.clear();
	activeLanes.clear();
	laneLength.clear();
	}

	@Override
	public void source(RevWalk w) {
	if (!(w instanceof PlotWalk))
	throw new ClassCastException(MessageFormat.format(JGitText.get().classCastNotA, PlotWalk.class.getName()));
	super.source(w);
	}

	/**
	* Find the set of lanes passing through a commit's row.
	* <p>
	* Lanes passing through a commit are lanes that the commit is not directly
	* on, but that need to travel through this commit to connect a descendant
	* (child) commit to an ancestor (parent) commit. Typically these lanes will
	* be drawn as lines in the passed commit's box, and the passed commit won't
	* appear to be connected to those lines.
	* <p>
	* This method modifies the passed collection by adding the lanes in any
	* order.
	*
	* @param currCommit
	* the commit the caller needs to get the lanes from.
	* @param result
	* collection to add the passing lanes into.
	*/
	@SuppressWarnings("unchecked")
	public void findPassingThrough(final PlotCommit<L> currCommit,
	final Collection<L> result) {
	for (PlotLane p : currCommit.passingLanes)
	result.add((L) p);
	}

	@SuppressWarnings("ReferenceEquality")
	@Override
	protected void enter(int index, PlotCommit<L> currCommit) {
	setupChildren(currCommit);

	final int nChildren = currCommit.getChildCount();
	if (nChildren == 0) {
	currCommit.lane = nextFreeLane();
	} else if (nChildren == 1
	&& currCommit.children[0].getParentCount() < 2) {
	// Only one child, child has only us as their parent.
	// Stay in the same lane as the child.

	@SuppressWarnings("unchecked")
	final PlotCommit<L> c = currCommit.children[0];
	currCommit.lane = c.lane;
	Integer len = laneLength.get(currCommit.lane);
	len = len != null ? Integer.valueOf(len.intValue() + 1)
	: Integer.valueOf(0);
	laneLength.put(currCommit.lane, len);
	} else {
	// More than one child, or our child is a merge.

	// We look for the child lane the current commit should continue.
	// Candidate lanes for this are those with children, that have the
	// current commit as their first parent.
	// There can be multiple candidate lanes. In that case the longest
	// lane is chosen, as this is usually the lane representing the
	// branch the commit actually was made on.

	// When there are no candidate lanes (i.e. the current commit has
	// only children whose non-first parent it is) we place the current
	// commit on a new lane.

	// The lane the current commit will be placed on:
	PlotLane reservedLane = null;
	PlotCommit childOnReservedLane = null;
	int lengthOfReservedLane = -1;

	for (int i = 0; i < nChildren; i++) {
	@SuppressWarnings("unchecked")
	final PlotCommit<L> c = currCommit.children[i];
	if (c.getParent(0) == currCommit) {
	Integer len = laneLength.get(c.lane);
	// we may be the first parent for multiple lines of
	// development, try to continue the longest one
	if (len.intValue() > lengthOfReservedLane) {
	reservedLane = c.lane;
	childOnReservedLane = c;
	lengthOfReservedLane = len.intValue();
	}
	}
	}

	if (reservedLane != null) {
	currCommit.lane = reservedLane;
	laneLength.put(reservedLane,
	Integer.valueOf(lengthOfReservedLane + 1));
	handleBlockedLanes(index, currCommit, childOnReservedLane);
	} else {
	currCommit.lane = nextFreeLane();
	handleBlockedLanes(index, currCommit, null);
	}

	// close lanes of children, if there are no first parents that might
	// want to continue the child lanes
	for (int i = 0; i < nChildren; i++) {
	final PlotCommit c = currCommit.children[i];
	PlotCommit firstParent = (PlotCommit) c.getParent(0);
	if (firstParent.lane != null && firstParent.lane != c.lane)
	closeLane(c.lane);
	}
	}

	continueActiveLanes(currCommit);
	if (currCommit.getParentCount() == 0)
	closeLane(currCommit.lane);
	}

	private void continueActiveLanes(PlotCommit currCommit) {
	for (PlotLane lane : activeLanes)
	if (lane != currCommit.lane)
	currCommit.addPassingLane(lane);
	}

	/**
	* Sets up fork and merge information in the involved PlotCommits.
	* Recognizes and handles blockades that involve forking or merging arcs.
	*
	* @param index
	* the index of <code>currCommit</code> in the list
	* @param currCommit
	* the current commit
	* @param childOnLane
	* the direct child on the same lane as <code>currCommit</code>,
	* may be null if <code>currCommit</code> is the first commit on
	* the lane
	*/
	@SuppressWarnings("ReferenceEquality")
	private void handleBlockedLanes(final int index, final PlotCommit currCommit,
	final PlotCommit childOnLane) {
	for (PlotCommit child : currCommit.children) {
	if (child == childOnLane)
	continue; // simple continuations of lanes are handled by
	// continueActiveLanes() calls in enter()

	// Is the child a merge or is it forking off?
	boolean childIsMerge = child.getParent(0) != currCommit;
	if (childIsMerge) {
	PlotLane laneToUse = currCommit.lane;
	laneToUse = handleMerge(index, currCommit, childOnLane, child,
	laneToUse);
	child.addMergingLane(laneToUse);
	} else {
	// We want to draw a forking arc in the child's lane.
	// As an active lane, the child lane already continues
	// (unblocked) up to this commit, we only need to mark it as
	// forking off from the current commit.
	PlotLane laneToUse = child.lane;
	currCommit.addForkingOffLane(laneToUse);
	}
	}
	}

	// Handles the case where currCommit is a non-first parent of the child
	@SuppressWarnings("ReferenceEquality")
	private PlotLane handleMerge(final int index, final PlotCommit currCommit,
	final PlotCommit childOnLane, PlotCommit child, PlotLane laneToUse) {

	// find all blocked positions between currCommit and this child

	int childIndex = index; // useless initialization, should
	// always be set in the loop below
	BitSet blockedPositions = new BitSet();
	for (int r = index - 1; r >= 0; r--) {
	final PlotCommit rObj = get(r);
	if (rObj == child) {
	childIndex = r;
	break;
	}
	addBlockedPosition(blockedPositions, rObj);
	}

	// handle blockades

	if (blockedPositions.get(laneToUse.getPosition())) {
	// We want to draw a merging arc in our lane to the child,
	// which is on another lane, but our lane is blocked.

	// Check if childOnLane is beetween commit and the child we
	// are currently processing
	boolean needDetour = false;
	if (childOnLane != null) {
	for (int r = index - 1; r > childIndex; r--) {
	final PlotCommit rObj = get(r);
	if (rObj == childOnLane) {
	needDetour = true;
	break;
	}
	}
	}

	if (needDetour) {
	// It is childOnLane which is blocking us. Repositioning
	// our lane would not help, because this repositions the
	// child too, keeping the blockade.
	// Instead, we create a "detour lane" which gets us
	// around the blockade. That lane has no commits on it.
	laneToUse = nextFreeLane(blockedPositions);
	currCommit.addForkingOffLane(laneToUse);
	closeLane(laneToUse);
	} else {
	// The blockade is (only) due to other (already closed)
	// lanes at the current lane's position. In this case we
	// reposition the current lane.
	// We are the first commit on this lane, because
	// otherwise the child commit on this lane would have
	// kept other lanes from blocking us. Since we are the
	// first commit, we can freely reposition.
	int newPos = getFreePosition(blockedPositions);
	freePositions.add(Integer.valueOf(laneToUse
	.getPosition()));
	laneToUse.position = newPos;
	}
	}

	// Actually connect currCommit to the merge child
	drawLaneToChild(index, child, laneToUse);
	return laneToUse;
	}

	/**
	* Connects the commit at commitIndex to the child, using the given lane.
	* All blockades on the lane must be resolved before calling this method.
	*
	* @param commitIndex
	* commit index
	* @param child
	* child to connect
	* @param laneToContinue
	* lane to continue
	*/
	@SuppressWarnings("ReferenceEquality")
	private void drawLaneToChild(final int commitIndex, PlotCommit child,
	PlotLane laneToContinue) {
	for (int r = commitIndex - 1; r >= 0; r--) {
	final PlotCommit rObj = get(r);
	if (rObj == child)
	break;
	if (rObj != null)
	rObj.addPassingLane(laneToContinue);
	}
	}

	private static void addBlockedPosition(BitSet blockedPositions,
	final PlotCommit rObj) {
	if (rObj != null) {
	PlotLane lane = rObj.getLane();
	// Positions may be blocked by a commit on a lane.
	if (lane != null)
	blockedPositions.set(lane.getPosition());
	// Positions may also be blocked by forking off and merging lanes.
	// We don't consider passing lanes, because every passing lane forks
	// off and merges at it ends.
	for (PlotLane l : rObj.forkingOffLanes)
	blockedPositions.set(l.getPosition());
	for (PlotLane l : rObj.mergingLanes)
	blockedPositions.set(l.getPosition());
	}
	}

	@SuppressWarnings("unchecked")
	private void closeLane(PlotLane lane) {
	if (activeLanes.remove(lane)) {
	recycleLane((L) lane);
	laneLength.remove(lane);
	freePositions.add(Integer.valueOf(lane.getPosition()));
	}
	}

	private void setupChildren(PlotCommit<L> currCommit) {
	final int nParents = currCommit.getParentCount();
	for (int i = 0; i < nParents; i++)
	((PlotCommit) currCommit.getParent(i)).addChild(currCommit);
	}

	private PlotLane nextFreeLane() {
	return nextFreeLane(null);
	}

	private PlotLane nextFreeLane(BitSet blockedPositions) {
	final PlotLane p = createLane();
	p.position = getFreePosition(blockedPositions);
	activeLanes.add(p);
	laneLength.put(p, Integer.valueOf(1));
	return p;
	}

	/**
	* @param blockedPositions
	* may be null
	* @return a free lane position
	*/
	private int getFreePosition(BitSet blockedPositions) {
	if (freePositions.isEmpty())
	return positionsAllocated++;

	if (blockedPositions != null) {
	for (Integer pos : freePositions)
	if (!blockedPositions.get(pos.intValue())) {
	freePositions.remove(pos);
	return pos.intValue();
	}
	return positionsAllocated++;
	}
	final Integer min = freePositions.first();
	freePositions.remove(min);
	return min.intValue();
	}

	/**
	* Create a new {@link PlotLane} appropriate for this particular
	* {@link PlotCommitList}.
	*
	* @return a new {@link PlotLane} appropriate for this particular
	* {@link PlotCommitList}.
	*/
	@SuppressWarnings("unchecked")
	protected L createLane() {
	return (L) new PlotLane();
	}

	/**
	* Return colors and other reusable information to the plotter when a lane
	* is no longer needed.
	*
	* @param lane
	* a lane
	*/
	protected void recycleLane(L lane) {
	// Nothing.
	}
	}