jgit/org.eclipse.jgit/src/org/eclipse/jgit/diff/MyersDiff.java

/*
 * Copyright (C) 2008-2009, Johannes E. Schindelin <johannes.schindelin@gmx.de>
 * Copyright (C) 2009, Johannes Schindelin <johannes.schindelin@gmx.de>
 * and other copyright owners as documented in the project's IP log.
 *
 * This program and the accompanying materials are made available
 * under the terms of the Eclipse Distribution License v1.0 which
 * accompanies this distribution, is reproduced below, and is
 * available at http://www.eclipse.org/org/documents/edl-v10.php
 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or
 * without modification, are permitted provided that the following
 * conditions are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 *
 * - Redistributions in binary form must reproduce the above
 *   copyright notice, this list of conditions and the following
 *   disclaimer in the documentation and/or other materials provided
 *   with the distribution.
 *
 * - Neither the name of the Eclipse Foundation, Inc. nor the
 *   names of its contributors may be used to endorse or promote
 *   products derived from this software without specific prior
 *   written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
 * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

package org.eclipse.jgit.diff;

import org.eclipse.jgit.util.IntList;
import org.eclipse.jgit.util.LongList;

/**
 * Diff algorithm, based on "An O(ND) Difference Algorithm and its
 * Variations", by Eugene Myers.
 *
 * The basic idea is to put the line numbers of text A as columns ("x") and the
 * lines of text B as rows ("y").  Now you try to find the shortest "edit path"
 * from the upper left corner to the lower right corner, where you can
 * always go horizontally or vertically, but diagonally from (x,y) to
 * (x+1,y+1) only if line x in text A is identical to line y in text B.
 *
 * Myers' fundamental concept is the "furthest reaching D-path on diagonal k":
 * a D-path is an edit path starting at the upper left corner and containing
 * exactly D non-diagonal elements ("differences").  The furthest reaching
 * D-path on diagonal k is the one that contains the most (diagonal) elements
 * which ends on diagonal k (where k = y - x).
 *
 * Example:
 *
 *    H E L L O   W O R L D
 *    ____
 *  L     \___
 *  O         \___
 *  W             \________
 *
 * Since every D-path has exactly D horizontal or vertical elements, it can
 * only end on the diagonals -D, -D+2, ..., D-2, D.
 *
 * Since every furthest reaching D-path contains at least one furthest
 * reaching (D-1)-path (except for D=0), we can construct them recursively.
 *
 * Since we are really interested in the shortest edit path, we can start
 * looking for a 0-path, then a 1-path, and so on, until we find a path that
 * ends in the lower right corner.
 *
 * To save space, we do not need to store all paths (which has quadratic space
 * requirements), but generate the D-paths simultaneously from both sides.
 * When the ends meet, we will have found "the middle" of the path.  From the
 * end points of that diagonal part, we can generate the rest recursively.
 *
 * This only requires linear space.
 *
 * The overall (runtime) complexity is
 *
 *	O(N * D^2 + 2 * N/2 * (D/2)^2 + 4 * N/4 * (D/4)^2 + ...)
 *	= O(N * D^2 * 5 / 4) = O(N * D^2),
 *
 * (With each step, we have to find the middle parts of twice as many regions
 * as before, but the regions (as well as the D) are halved.)
 *
 * So the overall runtime complexity stays the same with linear space,
 * albeit with a larger constant factor.
 */
public class MyersDiff {
	/**
	 * The list of edits found during the last call to {@link #calculateEdits()}
	 */
	protected EditList edits;

	/**
	 * The first text to be compared. Referred to as "Text A" in the comments
	 */
	protected Sequence a;

	/**
	 * The second text to be compared. Referred to as "Text B" in the comments
	 */
	protected Sequence b;

	/**
	 * The only constructor
	 *
	 * @param a   the text A which should be compared
	 * @param b   the text B which should be compared
	 */
	public MyersDiff(Sequence a, Sequence b) {
		this.a = a;
		this.b = b;
		calculateEdits();
	}

	/**
	 * @return the list of edits found during the last call to {@link #calculateEdits()}
	 */
	public EditList getEdits() {
		return edits;
	}

	// TODO: use ThreadLocal for future multi-threaded operations
	MiddleEdit middle = new MiddleEdit();

	/**
	 * Entrypoint into the algorithm this class is all about. This method triggers that the
	 * differences between A and B are calculated in form of a list of edits.
	 */
	protected void calculateEdits() {
		edits = new EditList();

		middle.initialize(0, a.size(), 0, b.size());
		if (middle.beginA >= middle.endA &&
				middle.beginB >= middle.endB)
			return;

		calculateEdits(middle.beginA, middle.endA,
				middle.beginB, middle.endB);
	}

	/**
	 * Calculates the differences between a given part of A against another given part of B
	 * @param beginA start of the part of A which should be compared (0<=beginA<sizeof(A))
	 * @param endA end of the part of A which should be compared (beginA<=endA<sizeof(A))
	 * @param beginB start of the part of B which should be compared (0<=beginB<sizeof(B))
	 * @param endB end of the part of B which should be compared (beginB<=endB<sizeof(B))
	 */
	protected void calculateEdits(int beginA, int endA,
			int beginB, int endB) {
		Edit edit = middle.calculate(beginA, endA, beginB, endB);

		if (beginA < edit.beginA || beginB < edit.beginB) {
			int k = edit.beginB - edit.beginA;
			int x = middle.backward.snake(k, edit.beginA);
			calculateEdits(beginA, x, beginB, k + x);
		}

		if (edit.getType() != Edit.Type.EMPTY)
			edits.add(edits.size(), edit);

		// after middle
		if (endA > edit.endA || endB > edit.endB) {
			int k = edit.endB - edit.endA;
			int x = middle.forward.snake(k, edit.endA);
			calculateEdits(x, endA, k + x, endB);
		}
	}

	/**
	 * A class to help bisecting the sequences a and b to find minimal
	 * edit paths.
	 *
	 * As the arrays are reused for space efficiency, you will need one
	 * instance per thread.
	 *
	 * The entry function is the calculate() method.
	 */
	class MiddleEdit {
		void initialize(int beginA, int endA, int beginB, int endB) {
			this.beginA = beginA; this.endA = endA;
			this.beginB = beginB; this.endB = endB;

			// strip common parts on either end
			int k = beginB - beginA;
			this.beginA = forward.snake(k, beginA);
			this.beginB = k + this.beginA;

			k = endB - endA;
			this.endA = backward.snake(k, endA);
			this.endB = k + this.endA;
		}

		/*
		 * This function calculates the "middle" Edit of the shortest
		 * edit path between the given subsequences of a and b.
		 *
		 * Once a forward path and a backward path meet, we found the
		 * middle part.  From the last snake end point on both of them,
		 * we construct the Edit.
		 *
		 * It is assumed that there is at least one edit in the range.
		 */
		// TODO: measure speed impact when this is synchronized
		Edit calculate(int beginA, int endA, int beginB, int endB) {
			if (beginA == endA || beginB == endB)
				return new Edit(beginA, endA, beginB, endB);
			this.beginA = beginA; this.endA = endA;
			this.beginB = beginB; this.endB = endB;

			/*
			 * Following the conventions in Myers' paper, "k" is
			 * the difference between the index into "b" and the
			 * index into "a".
			 */
			int minK = beginB - endA;
			int maxK = endB - beginA;

			forward.initialize(beginB - beginA, beginA, minK, maxK);
			backward.initialize(endB - endA, endA, minK, maxK);

			for (int d = 1; ; d++)
				if (forward.calculate(d) ||
						backward.calculate(d))
					return edit;
		}

		/*
		 * For each d, we need to hold the d-paths for the diagonals
		 * k = -d, -d + 2, ..., d - 2, d.  These are stored in the
		 * forward (and backward) array.
		 *
		 * As we allow subsequences, too, this needs some refinement:
		 * the forward paths start on the diagonal forwardK =
		 * beginB - beginA, and backward paths start on the diagonal
		 * backwardK = endB - endA.
		 *
		 * So, we need to hold the forward d-paths for the diagonals
		 * k = forwardK - d, forwardK - d + 2, ..., forwardK + d and
		 * the analogue for the backward d-paths.  This means that
		 * we can turn (k, d) into the forward array index using this
		 * formula:
		 *
		 *	i = (d + k - forwardK) / 2
		 *
		 * There is a further complication: the edit paths should not
		 * leave the specified subsequences, so k is bounded by
		 * minK = beginB - endA and maxK = endB - beginA.  However,
		 * (k - forwardK) _must_ be odd whenever d is odd, and it
		 * _must_ be even when d is even.
		 *
		 * The values in the "forward" and "backward" arrays are
		 * positions ("x") in the sequence a, to get the corresponding
		 * positions ("y") in the sequence b, you have to calculate
		 * the appropriate k and then y:
		 *
		 *	k = forwardK - d + i * 2
		 *	y = k + x
		 *
		 * (substitute backwardK for forwardK if you want to get the
		 * y position for an entry in the "backward" array.
		 */
		EditPaths forward = new ForwardEditPaths();
		EditPaths backward = new BackwardEditPaths();

		/* Some variables which are shared between methods */
		protected int beginA, endA, beginB, endB;
		protected Edit edit;

		abstract class EditPaths {
			private IntList x = new IntList();
			private LongList snake = new LongList();
			int beginK, endK, middleK;
			int prevBeginK, prevEndK;
			/* if we hit one end early, no need to look further */
			int minK, maxK; // TODO: better explanation

			final int getIndex(int d, int k) {
// TODO: remove
if (((d + k - middleK) % 2) == 1)
	throw new RuntimeException("odd: " + d + " + " + k + " - " + middleK);
				return (d + k - middleK) / 2;
			}

			final int getX(int d, int k) {
// TODO: remove
if (k < beginK || k > endK)
	throw new RuntimeException("k " + k + " not in " + beginK + " - " + endK);
				return x.get(getIndex(d, k));
			}

			final long getSnake(int d, int k) {
// TODO: remove
if (k < beginK || k > endK)
	throw new RuntimeException("k " + k + " not in " + beginK + " - " + endK);
				return snake.get(getIndex(d, k));
			}

			private int forceKIntoRange(int k) {
				/* if k is odd, so must be the result */
				if (k < minK)
					return minK + ((k ^ minK) & 1);
				else if (k > maxK)
					return maxK - ((k ^ maxK) & 1);
				return k;
			}

			void initialize(int k, int x, int minK, int maxK) {
				this.minK = minK;
				this.maxK = maxK;
				beginK = endK = middleK = k;
				this.x.clear();
				this.x.add(x);
				snake.clear();
				snake.add(newSnake(k, x));
			}

			abstract int snake(int k, int x);
			abstract int getLeft(int x);
			abstract int getRight(int x);
			abstract boolean isBetter(int left, int right);
			abstract void adjustMinMaxK(final int k, final int x);
			abstract boolean meets(int d, int k, int x, long snake);

			final long newSnake(int k, int x) {
				long y = k + x;
				long ret = ((long) x) << 32;
				return ret | y;
			}

			final int snake2x(long snake) {
				return (int) (snake >>> 32);
			}

			final int snake2y(long snake) {
				return (int) snake;
			}

			final boolean makeEdit(long snake1, long snake2) {
				int x1 = snake2x(snake1), x2 = snake2x(snake2);
				int y1 = snake2y(snake1), y2 = snake2y(snake2);
				/*
				 * Check for incompatible partial edit paths:
				 * when there are ambiguities, we might have
				 * hit incompatible (i.e. non-overlapping)
				 * forward/backward paths.
				 *
				 * In that case, just pretend that we have
				 * an empty edit at the end of one snake; this
				 * will force a decision which path to take
				 * in the next recursion step.
				 */
				if (x1 > x2 || y1 > y2) {
					x1 = x2;
					y1 = y2;
				}
				edit = new Edit(x1, x2, y1, y2);
				return true;
			}

			boolean calculate(int d) {
				prevBeginK = beginK;
				prevEndK = endK;
				beginK = forceKIntoRange(middleK - d);
				endK = forceKIntoRange(middleK + d);
				// TODO: handle i more efficiently
				// TODO: walk snake(k, getX(d, k)) only once per (d, k)
				// TODO: move end points out of the loop to avoid conditionals inside the loop
				// go backwards so that we can avoid temp vars
				for (int k = endK; k >= beginK; k -= 2) {
					int left = -1, right = -1;
					long leftSnake = -1L, rightSnake = -1L;
					// TODO: refactor into its own function
					if (k > prevBeginK) {
						int i = getIndex(d - 1, k - 1);
						left = x.get(i);
						int end = snake(k - 1, left);
						leftSnake = left != end ?
							newSnake(k - 1, end) :
							snake.get(i);
						if (meets(d, k - 1, end, leftSnake))
							return true;
						left = getLeft(end);
					}
					if (k < prevEndK) {
						int i = getIndex(d - 1, k + 1);
						right = x.get(i);
						int end = snake(k + 1, right);
						rightSnake = right != end ?
							newSnake(k + 1, end) :
							snake.get(i);
						if (meets(d, k + 1, end, rightSnake))
							return true;
						right = getRight(end);
					}
					int newX;
					long newSnake;
					if (k >= prevEndK ||
							(k > prevBeginK &&
							 isBetter(left, right))) {
						newX = left;
						newSnake = leftSnake;
					}
					else {
						newX = right;
						newSnake = rightSnake;
					}
					if (meets(d, k, newX, newSnake))
						return true;
					adjustMinMaxK(k, newX);
					int i = getIndex(d, k);
					x.set(i, newX);
					snake.set(i, newSnake);
				}
				return false;
			}
		}

		class ForwardEditPaths extends EditPaths {
			final int snake(int k, int x) {
				for (; x < endA && k + x < endB; x++)
					if (!a.equals(x, b, k + x))
						break;
				return x;
			}

			final int getLeft(final int x) {
				return x;
			}

			final int getRight(final int x) {
				return x + 1;
			}

			final boolean isBetter(final int left, final int right) {
				return left > right;
			}

			final void adjustMinMaxK(final int k, final int x) {
				if (x >= endA || k + x >= endB) {
					if (k > backward.middleK)
						maxK = k;
					else
						minK = k;
				}
			}

			final boolean meets(int d, int k, int x, long snake) {
				if (k < backward.beginK || k > backward.endK)
					return false;
				// TODO: move out of loop
				if (((d - 1 + k - backward.middleK) % 2) == 1)
					return false;
				if (x < backward.getX(d - 1, k))
					return false;
				makeEdit(snake, backward.getSnake(d - 1, k));
				return true;
			}
		}

		class BackwardEditPaths extends EditPaths {
			final int snake(int k, int x) {
				for (; x > beginA && k + x > beginB; x--)
					if (!a.equals(x - 1, b, k + x - 1))
						break;
				return x;
			}

			final int getLeft(final int x) {
				return x - 1;
			}

			final int getRight(final int x) {
				return x;
			}

			final boolean isBetter(final int left, final int right) {
				return left < right;
			}

			final void adjustMinMaxK(final int k, final int x) {
				if (x <= beginA || k + x <= beginB) {
					if (k > forward.middleK)
						maxK = k;
					else
						minK = k;
				}
			}

			final boolean meets(int d, int k, int x, long snake) {
				if (k < forward.beginK || k > forward.endK)
					return false;
				// TODO: move out of loop
				if (((d + k - forward.middleK) % 2) == 1)
					return false;
				if (x > forward.getX(d, k))
					return false;
				makeEdit(forward.getSnake(d, k), snake);
				return true;
			}
		}
	}

	/**
	 * @param args two filenames specifying the contents to be diffed
	 */
	public static void main(String[] args) {
		if (args.length != 2) {
			System.err.println("Need 2 arguments");
			System.exit(1);
		}
		try {
			RawText a = new RawText(new java.io.File(args[0]));
			RawText b = new RawText(new java.io.File(args[1]));
			MyersDiff diff = new MyersDiff(a, b);
			System.out.println(diff.getEdits().toString());
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}