1 /*
2 * Copyright (C) 2010, Google Inc. and others
3 *
4 * This program and the accompanying materials are made available under the
5 * terms of the Eclipse Distribution License v. 1.0 which is available at
6 * https://www.eclipse.org/org/documents/edl-v10.php.
7 *
8 * SPDX-License-Identifier: BSD-3-Clause
9 */
10
11 package org.eclipse.jgit.diff;
12
13 /**
14 * Equivalence function for a {@link org.eclipse.jgit.diff.Sequence} compared by
15 * difference algorithm.
16 * <p>
17 * Difference algorithms can use a comparator to compare portions of two
18 * sequences and discover the minimal edits required to transform from one
19 * sequence to the other sequence.
20 * <p>
21 * Indexes within a sequence are zero-based.
22 *
23 * @param <S>
24 * type of sequence the comparator supports.
25 */
26 public abstract class SequenceComparator<S extends Sequence> {
27 /**
28 * Compare two items to determine if they are equivalent.
29 *
30 * It is permissible to compare sequence {@code a} with itself (by passing
31 * {@code a} again in position {@code b}).
32 *
33 * @param a
34 * the first sequence.
35 * @param ai
36 * item of {@code ai} to compare.
37 * @param b
38 * the second sequence.
39 * @param bi
40 * item of {@code bi} to compare.
41 * @return true if the two items are identical according to this function's
42 * equivalence rule.
43 */
44 public abstract boolean equals(S a, int ai, S b, int bi);
45
46 /**
47 * Get a hash value for an item in a sequence.
48 *
49 * If two items are equal according to this comparator's
50 * {@link #equals(Sequence, int, Sequence, int)} method, then this hash
51 * method must produce the same integer result for both items.
52 *
53 * It is not required for two items to have different hash values if they
54 * are unequal according to the {@code equals()} method.
55 *
56 * @param seq
57 * the sequence.
58 * @param ptr
59 * the item to obtain the hash for.
60 * @return hash the hash value.
61 */
62 public abstract int hash(S seq, int ptr);
63
64 /**
65 * Modify the edit to remove common leading and trailing items.
66 *
67 * The supplied edit {@code e} is reduced in size by moving the beginning A
68 * and B points so the edit does not cover any items that are in common
69 * between the two sequences. The ending A and B points are also shifted to
70 * remove common items from the end of the region.
71 *
72 * @param a
73 * the first sequence.
74 * @param b
75 * the second sequence.
76 * @param e
77 * the edit to start with and update.
78 * @return {@code e} if it was updated in-place, otherwise a new edit
79 * containing the reduced region.
80 */
81 public Edit reduceCommonStartEnd(S a, S b, Edit e) {
82 // Skip over items that are common at the start.
83 //
84 while (e.beginA < e.endA && e.beginB < e.endB
85 && equals(a, e.beginA, b, e.beginB)) {
86 e.beginA++;
87 e.beginB++;
88 }
89
90 // Skip over items that are common at the end.
91 //
92 while (e.beginA < e.endA && e.beginB < e.endB
93 && equals(a, e.endA - 1, b, e.endB - 1)) {
94 e.endA--;
95 e.endB--;
96 }
97
98 return e;
99 }
100 }