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 /**
47 * Equivalence function for a {@link Sequence} compared by difference algorithm.
48 * <p>
49 * Difference algorithms can use a comparator to compare portions of two
50 * sequences and discover the minimal edits required to transform from one
51 * sequence to the other sequence.
52 * <p>
53 * Indexes within a sequence are zero-based.
54 *
55 * @param <S>
56 * type of sequence the comparator supports.
57 */
58 public abstract class SequenceComparator<S extends Sequence> {
59 /**
60 * Compare two items to determine if they are equivalent.
61 *
62 * It is permissible to compare sequence {@code a} with itself (by passing
63 * {@code a} again in position {@code b}).
64 *
65 * @param a
66 * the first sequence.
67 * @param ai
68 * item of {@code ai} to compare.
69 * @param b
70 * the second sequence.
71 * @param bi
72 * item of {@code bi} to compare.
73 * @return true if the two items are identical according to this function's
74 * equivalence rule.
75 */
76 public abstract boolean equals(S a, int ai, S b, int bi);
77
78 /**
79 * Get a hash value for an item in a sequence.
80 *
81 * If two items are equal according to this comparator's
82 * {@link #equals(Sequence, int, Sequence, int)} method, then this hash
83 * method must produce the same integer result for both items.
84 *
85 * It is not required for two items to have different hash values if they
86 * are are unequal according to the {@code equals()} method.
87 *
88 * @param seq
89 * the sequence.
90 * @param ptr
91 * the item to obtain the hash for.
92 * @return hash the hash value.
93 */
94 public abstract int hash(S seq, int ptr);
95
96 /**
97 * Modify the edit to remove common leading and trailing items.
98 *
99 * The supplied edit {@code e} is reduced in size by moving the beginning A
100 * and B points so the edit does not cover any items that are in common
101 * between the two sequences. The ending A and B points are also shifted to
102 * remove common items from the end of the region.
103 *
104 * @param a
105 * the first sequence.
106 * @param b
107 * the second sequence.
108 * @param e
109 * the edit to start with and update.
110 * @return {@code e} if it was updated in-place, otherwise a new edit
111 * containing the reduced region.
112 */
113 public Edit reduceCommonStartEnd(S a, S b, Edit e) {
114 // Skip over items that are common at the start.
115 //
116 while (e.beginA < e.endA && e.beginB < e.endB
117 && equals(a, e.beginA, b, e.beginB)) {
118 e.beginA++;
119 e.beginB++;
120 }
121
122 // Skip over items that are common at the end.
123 //
124 while (e.beginA < e.endA && e.beginB < e.endB
125 && equals(a, e.endA - 1, b, e.endB - 1)) {
126 e.endA--;
127 e.endB--;
128 }
129
130 return e;
131 }
132 }