Java tutorial
/* * 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); } } }