| /* |
| * Copyright (C) 2010, Google Inc. |
| * 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 java.util.ArrayList; |
| import java.util.List; |
| |
| /** |
| * An extended form of Bram Cohen's patience diff algorithm. |
| * <p> |
| * This implementation was derived by using the 4 rules that are outlined in |
| * Bram Cohen's <a href="http://bramcohen.livejournal.com/73318.html">blog</a>, |
| * and then was further extended to support low-occurrence common elements. |
| * <p> |
| * The basic idea of the algorithm is to create a histogram of occurrences for |
| * each element of sequence A. Each element of sequence B is then considered in |
| * turn. If the element also exists in sequence A, and has a lower occurrence |
| * count, the positions are considered as a candidate for the longest common |
| * subsequence (LCS). After scanning of B is complete the LCS that has the |
| * lowest number of occurrences is chosen as a split point. The region is split |
| * around the LCS, and the algorithm is recursively applied to the sections |
| * before and after the LCS. |
| * <p> |
| * By always selecting a LCS position with the lowest occurrence count, this |
| * algorithm behaves exactly like Bram Cohen's patience diff whenever there is a |
| * unique common element available between the two sequences. When no unique |
| * elements exist, the lowest occurrence element is chosen instead. This offers |
| * more readable diffs than simply falling back on the standard Myers' O(ND) |
| * algorithm would produce. |
| * <p> |
| * To prevent the algorithm from having an O(N^2) running time, an upper limit |
| * on the number of unique elements in a histogram bucket is configured by |
| * {@link #setMaxChainLength(int)}. If sequence A has more than this many |
| * elements that hash into the same hash bucket, the algorithm passes the region |
| * to {@link #setFallbackAlgorithm(DiffAlgorithm)}. If no fallback algorithm is |
| * configured, the region is emitted as a replace edit. |
| * <p> |
| * During scanning of sequence B, any element of A that occurs more than |
| * {@link #setMaxChainLength(int)} times is never considered for an LCS match |
| * position, even if it is common between the two sequences. This limits the |
| * number of locations in sequence A that must be considered to find the LCS, |
| * and helps maintain a lower running time bound. |
| * <p> |
| * So long as {@link #setMaxChainLength(int)} is a small constant (such as 64), |
| * the algorithm runs in O(N * D) time, where N is the sum of the input lengths |
| * and D is the number of edits in the resulting EditList. If the supplied |
| * {@link org.eclipse.jgit.diff.SequenceComparator} has a good hash function, |
| * this implementation typically out-performs |
| * {@link org.eclipse.jgit.diff.MyersDiff}, even though its theoretical running |
| * time is the same. |
| * <p> |
| * This implementation has an internal limitation that prevents it from handling |
| * sequences with more than 268,435,456 (2^28) elements. |
| */ |
| public class HistogramDiff extends LowLevelDiffAlgorithm { |
| /** Algorithm to use when there are too many element occurrences. */ |
| DiffAlgorithm fallback = MyersDiff.INSTANCE; |
| |
| /** |
| * Maximum number of positions to consider for a given element hash. |
| * |
| * All elements with the same hash are stored into a single chain. The chain |
| * size is capped to ensure search is linear time at O(len_A + len_B) rather |
| * than quadratic at O(len_A * len_B). |
| */ |
| int maxChainLength = 64; |
| |
| /** |
| * Set the algorithm used when there are too many element occurrences. |
| * |
| * @param alg |
| * the secondary algorithm. If null the region will be denoted as |
| * a single REPLACE block. |
| */ |
| public void setFallbackAlgorithm(DiffAlgorithm alg) { |
| fallback = alg; |
| } |
| |
| /** |
| * Maximum number of positions to consider for a given element hash. |
| * |
| * All elements with the same hash are stored into a single chain. The chain |
| * size is capped to ensure search is linear time at O(len_A + len_B) rather |
| * than quadratic at O(len_A * len_B). |
| * |
| * @param maxLen |
| * new maximum length. |
| */ |
| public void setMaxChainLength(int maxLen) { |
| maxChainLength = maxLen; |
| } |
| |
| /** {@inheritDoc} */ |
| @Override |
| public <S extends Sequence> void diffNonCommon(EditList edits, |
| HashedSequenceComparator<S> cmp, HashedSequence<S> a, |
| HashedSequence<S> b, Edit region) { |
| new State<>(edits, cmp, a, b).diffRegion(region); |
| } |
| |
| private class State<S extends Sequence> { |
| private final HashedSequenceComparator<S> cmp; |
| private final HashedSequence<S> a; |
| private final HashedSequence<S> b; |
| private final List<Edit> queue = new ArrayList<>(); |
| |
| /** Result edits we have determined that must be made to convert a to b. */ |
| final EditList edits; |
| |
| State(EditList edits, HashedSequenceComparator<S> cmp, |
| HashedSequence<S> a, HashedSequence<S> b) { |
| this.cmp = cmp; |
| this.a = a; |
| this.b = b; |
| this.edits = edits; |
| } |
| |
| void diffRegion(Edit r) { |
| diffReplace(r); |
| while (!queue.isEmpty()) |
| diff(queue.remove(queue.size() - 1)); |
| } |
| |
| private void diffReplace(Edit r) { |
| Edit lcs = new HistogramDiffIndex<>(maxChainLength, cmp, a, b, r) |
| .findLongestCommonSequence(); |
| if (lcs != null) { |
| // If we were given an edit, we can prove a result here. |
| // |
| if (lcs.isEmpty()) { |
| // An empty edit indicates there is nothing in common. |
| // Replace the entire region. |
| // |
| edits.add(r); |
| } else { |
| queue.add(r.after(lcs)); |
| queue.add(r.before(lcs)); |
| } |
| |
| } else if (fallback instanceof LowLevelDiffAlgorithm) { |
| LowLevelDiffAlgorithm fb = (LowLevelDiffAlgorithm) fallback; |
| fb.diffNonCommon(edits, cmp, a, b, r); |
| |
| } else if (fallback != null) { |
| SubsequenceComparator<HashedSequence<S>> cs = subcmp(); |
| Subsequence<HashedSequence<S>> as = Subsequence.a(a, r); |
| Subsequence<HashedSequence<S>> bs = Subsequence.b(b, r); |
| |
| EditList res = fallback.diffNonCommon(cs, as, bs); |
| edits.addAll(Subsequence.toBase(res, as, bs)); |
| |
| } else { |
| edits.add(r); |
| } |
| } |
| |
| private void diff(Edit r) { |
| switch (r.getType()) { |
| case INSERT: |
| case DELETE: |
| edits.add(r); |
| break; |
| |
| case REPLACE: |
| if (r.getLengthA() == 1 && r.getLengthB() == 1) |
| edits.add(r); |
| else |
| diffReplace(r); |
| break; |
| |
| case EMPTY: |
| default: |
| throw new IllegalStateException(); |
| } |
| } |
| |
| private SubsequenceComparator<HashedSequence<S>> subcmp() { |
| return new SubsequenceComparator<>(cmp); |
| } |
| } |
| } |