1 /*
2 * Copyright (C) 2010, Google Inc.
3 * and other copyright owners as documented in the project's IP log.
4 *
5 * This program and the accompanying materials are made available
6 * under the terms of the Eclipse Distribution License v1.0 which
7 * accompanies this distribution, is reproduced below, and is
8 * available at http://www.eclipse.org/org/documents/edl-v10.php
9 *
10 * All rights reserved.
11 *
12 * Redistribution and use in source and binary forms, with or
13 * without modification, are permitted provided that the following
14 * conditions are met:
15 *
16 * - Redistributions of source code must retain the above copyright
17 * notice, this list of conditions and the following disclaimer.
18 *
19 * - Redistributions in binary form must reproduce the above
20 * copyright notice, this list of conditions and the following
21 * disclaimer in the documentation and/or other materials provided
22 * with the distribution.
23 *
24 * - Neither the name of the Eclipse Foundation, Inc. nor the
25 * names of its contributors may be used to endorse or promote
26 * products derived from this software without specific prior
27 * written permission.
28 *
29 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
30 * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
31 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
32 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
33 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
34 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
35 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
36 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
37 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
38 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
39 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
40 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
41 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
42 */
43
44 package org.eclipse.jgit.diff;
45
46 import java.util.ArrayList;
47 import java.util.List;
48
49 /**
50 * An extended form of Bram Cohen's patience diff algorithm.
51 * <p>
52 * This implementation was derived by using the 4 rules that are outlined in
53 * Bram Cohen's <a href="http://bramcohen.livejournal.com/73318.html">blog</a>,
54 * and then was further extended to support low-occurrence common elements.
55 * <p>
56 * The basic idea of the algorithm is to create a histogram of occurrences for
57 * each element of sequence A. Each element of sequence B is then considered in
58 * turn. If the element also exists in sequence A, and has a lower occurrence
59 * count, the positions are considered as a candidate for the longest common
60 * subsequence (LCS). After scanning of B is complete the LCS that has the
61 * lowest number of occurrences is chosen as a split point. The region is split
62 * around the LCS, and the algorithm is recursively applied to the sections
63 * before and after the LCS.
64 * <p>
65 * By always selecting a LCS position with the lowest occurrence count, this
66 * algorithm behaves exactly like Bram Cohen's patience diff whenever there is a
67 * unique common element available between the two sequences. When no unique
68 * elements exist, the lowest occurrence element is chosen instead. This offers
69 * more readable diffs than simply falling back on the standard Myers' O(ND)
70 * algorithm would produce.
71 * <p>
72 * To prevent the algorithm from having an O(N^2) running time, an upper limit
73 * on the number of unique elements in a histogram bucket is configured by
74 * {@link #setMaxChainLength(int)}. If sequence A has more than this many
75 * elements that hash into the same hash bucket, the algorithm passes the region
76 * to {@link #setFallbackAlgorithm(DiffAlgorithm)}. If no fallback algorithm is
77 * configured, the region is emitted as a replace edit.
78 * <p>
79 * During scanning of sequence B, any element of A that occurs more than
80 * {@link #setMaxChainLength(int)} times is never considered for an LCS match
81 * position, even if it is common between the two sequences. This limits the
82 * number of locations in sequence A that must be considered to find the LCS,
83 * and helps maintain a lower running time bound.
84 * <p>
85 * So long as {@link #setMaxChainLength(int)} is a small constant (such as 64),
86 * the algorithm runs in O(N * D) time, where N is the sum of the input lengths
87 * and D is the number of edits in the resulting EditList. If the supplied
88 * {@link SequenceComparator} has a good hash function, this implementation
89 * typically out-performs {@link MyersDiff}, even though its theoretical running
90 * time is the same.
91 * <p>
92 * This implementation has an internal limitation that prevents it from handling
93 * sequences with more than 268,435,456 (2^28) elements.
94 */
95 public class HistogramDiff extends LowLevelDiffAlgorithm {
96 /** Algorithm to use when there are too many element occurrences. */
97 DiffAlgorithm fallback = MyersDiff.INSTANCE;
98
99 /**
100 * Maximum number of positions to consider for a given element hash.
101 *
102 * All elements with the same hash are stored into a single chain. The chain
103 * size is capped to ensure search is linear time at O(len_A + len_B) rather
104 * than quadratic at O(len_A * len_B).
105 */
106 int maxChainLength = 64;
107
108 /**
109 * Set the algorithm used when there are too many element occurrences.
110 *
111 * @param alg
112 * the secondary algorithm. If null the region will be denoted as
113 * a single REPLACE block.
114 */
115 public void setFallbackAlgorithm(DiffAlgorithm alg) {
116 fallback = alg;
117 }
118
119 /**
120 * Maximum number of positions to consider for a given element hash.
121 *
122 * All elements with the same hash are stored into a single chain. The chain
123 * size is capped to ensure search is linear time at O(len_A + len_B) rather
124 * than quadratic at O(len_A * len_B).
125 *
126 * @param maxLen
127 * new maximum length.
128 */
129 public void setMaxChainLength(int maxLen) {
130 maxChainLength = maxLen;
131 }
132
133 public <S extends Sequence> void diffNonCommon(EditList edits,
134 HashedSequenceComparator<S> cmp, HashedSequence<S> a,
135 HashedSequence<S> b, Edit region) {
136 new State<S>(edits, cmp, a, b).diffRegion(region);
137 }
138
139 private class State<S extends Sequence> {
140 private final HashedSequenceComparator<S> cmp;
141 private final HashedSequence<S> a;
142 private final HashedSequence<S> b;
143 private final List<Edit> queue = new ArrayList<Edit>();
144
145 /** Result edits we have determined that must be made to convert a to b. */
146 final EditList edits;
147
148 State(EditList edits, HashedSequenceComparator<S> cmp,
149 HashedSequence<S> a, HashedSequence<S> b) {
150 this.cmp = cmp;
151 this.a = a;
152 this.b = b;
153 this.edits = edits;
154 }
155
156 void diffRegion(Edit r) {
157 diffReplace(r);
158 while (!queue.isEmpty())
159 diff(queue.remove(queue.size() - 1));
160 }
161
162 private void diffReplace(Edit r) {
163 Edit lcs = new HistogramDiffIndex<S>(maxChainLength, cmp, a, b, r)
164 .findLongestCommonSequence();
165 if (lcs != null) {
166 // If we were given an edit, we can prove a result here.
167 //
168 if (lcs.isEmpty()) {
169 // An empty edit indicates there is nothing in common.
170 // Replace the entire region.
171 //
172 edits.add(r);
173 } else {
174 queue.add(r.after(lcs));
175 queue.add(r.before(lcs));
176 }
177
178 } else if (fallback instanceof LowLevelDiffAlgorithm) {
179 LowLevelDiffAlgorithm fb = (LowLevelDiffAlgorithm) fallback;
180 fb.diffNonCommon(edits, cmp, a, b, r);
181
182 } else if (fallback != null) {
183 SubsequenceComparator<HashedSequence<S>> cs = subcmp();
184 Subsequence<HashedSequence<S>> as = Subsequence.a(a, r);
185 Subsequence<HashedSequence<S>> bs = Subsequence.b(b, r);
186
187 EditList res = fallback.diffNonCommon(cs, as, bs);
188 edits.addAll(Subsequence.toBase(res, as, bs));
189
190 } else {
191 edits.add(r);
192 }
193 }
194
195 private void diff(Edit r) {
196 switch (r.getType()) {
197 case INSERT:
198 case DELETE:
199 edits.add(r);
200 break;
201
202 case REPLACE:
203 if (r.getLengthA() == 1 && r.getLengthB() == 1)
204 edits.add(r);
205 else
206 diffReplace(r);
207 break;
208
209 case EMPTY:
210 default:
211 throw new IllegalStateException();
212 }
213 }
214
215 private SubsequenceComparator<HashedSequence<S>> subcmp() {
216 return new SubsequenceComparator<HashedSequence<S>>(cmp);
217 }
218 }
219 }