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/*
* Copyright (C) 2010, Google Inc. and others
*
* This program and the accompanying materials are made available under the
* terms of the Eclipse Distribution License v. 1.0 which is available at
* https://www.eclipse.org/org/documents/edl-v10.php.
*
* SPDX-License-Identifier: BSD-3-Clause
*/
package org.eclipse.jgit.diff;
/**
* Equivalence function for a {@link org.eclipse.jgit.diff.Sequence} compared by
* difference algorithm.
* <p>
* Difference algorithms can use a comparator to compare portions of two
* sequences and discover the minimal edits required to transform from one
* sequence to the other sequence.
* <p>
* Indexes within a sequence are zero-based.
*
* @param <S>
* type of sequence the comparator supports.
*/
public abstract class SequenceComparator<S extends Sequence> {
/**
* Compare two items to determine if they are equivalent.
*
* It is permissible to compare sequence {@code a} with itself (by passing
* {@code a} again in position {@code b}).
*
* @param a
* the first sequence.
* @param ai
* item of {@code ai} to compare.
* @param b
* the second sequence.
* @param bi
* item of {@code bi} to compare.
* @return true if the two items are identical according to this function's
* equivalence rule.
*/
public abstract boolean equals(S a, int ai, S b, int bi);
/**
* Get a hash value for an item in a sequence.
*
* If two items are equal according to this comparator's
* {@link #equals(Sequence, int, Sequence, int)} method, then this hash
* method must produce the same integer result for both items.
*
* It is not required for two items to have different hash values if they
* are unequal according to the {@code equals()} method.
*
* @param seq
* the sequence.
* @param ptr
* the item to obtain the hash for.
* @return hash the hash value.
*/
public abstract int hash(S seq, int ptr);
/**
* Modify the edit to remove common leading and trailing items.
*
* The supplied edit {@code e} is reduced in size by moving the beginning A
* and B points so the edit does not cover any items that are in common
* between the two sequences. The ending A and B points are also shifted to
* remove common items from the end of the region.
*
* @param a
* the first sequence.
* @param b
* the second sequence.
* @param e
* the edit to start with and update.
* @return {@code e} if it was updated in-place, otherwise a new edit
* containing the reduced region.
*/
public Edit reduceCommonStartEnd(S a, S b, Edit e) {
// Skip over items that are common at the start.
//
while (e.beginA < e.endA && e.beginB < e.endB
&& equals(a, e.beginA, b, e.beginB)) {
e.beginA++;
e.beginB++;
}
// Skip over items that are common at the end.
//
while (e.beginA < e.endA && e.beginB < e.endB
&& equals(a, e.endA - 1, b, e.endB - 1)) {
e.endA--;
e.endB--;
}
return e;
}
}
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