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- /*
- * 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;
-
- /**
- * Compares two {@link Sequence}s to create an {@link EditList} of changes.
- * <p>
- * An algorithm's {@code diff} method must be callable from concurrent threads
- * without data collisions. This permits some algorithms to use a singleton
- * pattern, with concurrent invocations using the same singleton. Other
- * algorithms may support parameterization, in which case the caller can create
- * a unique instance per thread.
- */
- public abstract class DiffAlgorithm {
- /**
- * Supported diff algorithm
- */
- public enum SupportedAlgorithm {
- /**
- * Myers diff algorithm
- */
- MYERS,
-
- /**
- * Histogram diff algorithm
- */
- HISTOGRAM
- }
-
- /**
- * @param alg
- * the diff algorithm for which an implementation should be
- * returned
- * @return an implementation of the specified diff algorithm
- */
- public static DiffAlgorithm getAlgorithm(SupportedAlgorithm alg) {
- switch (alg) {
- case MYERS:
- return MyersDiff.INSTANCE;
- case HISTOGRAM:
- return new HistogramDiff();
- default:
- throw new IllegalArgumentException();
- }
- }
-
- /**
- * Compare two sequences and identify a list of edits between them.
- *
- * @param <S>
- * type of sequence being compared.
- * @param cmp
- * the comparator supplying the element equivalence function.
- * @param a
- * the first (also known as old or pre-image) sequence. Edits
- * returned by this algorithm will reference indexes using the
- * 'A' side: {@link Edit#getBeginA()}, {@link Edit#getEndA()}.
- * @param b
- * the second (also known as new or post-image) sequence. Edits
- * returned by this algorithm will reference indexes using the
- * 'B' side: {@link Edit#getBeginB()}, {@link Edit#getEndB()}.
- * @return a modifiable edit list comparing the two sequences. If empty, the
- * sequences are identical according to {@code cmp}'s rules. The
- * result list is never null.
- */
- public <S extends Sequence> EditList diff(
- SequenceComparator<? super S> cmp, S a, S b) {
- Edit region = cmp.reduceCommonStartEnd(a, b, coverEdit(a, b));
-
- switch (region.getType()) {
- case INSERT:
- case DELETE:
- return EditList.singleton(region);
-
- case REPLACE: {
- if (region.getLengthA() == 1 && region.getLengthB() == 1)
- return EditList.singleton(region);
-
- SubsequenceComparator<S> cs = new SubsequenceComparator<>(cmp);
- Subsequence<S> as = Subsequence.a(a, region);
- Subsequence<S> bs = Subsequence.b(b, region);
- EditList e = Subsequence.toBase(diffNonCommon(cs, as, bs), as, bs);
- return normalize(cmp, e, a, b);
- }
-
- case EMPTY:
- return new EditList(0);
-
- default:
- throw new IllegalStateException();
- }
- }
-
- private static <S extends Sequence> Edit coverEdit(S a, S b) {
- return new Edit(0, a.size(), 0, b.size());
- }
-
- /**
- * Reorganize an {@link EditList} for better diff consistency.
- * <p>
- * {@code DiffAlgorithms} may return {@link Edit.Type#INSERT} or
- * {@link Edit.Type#DELETE} edits that can be "shifted". For
- * example, the deleted section
- * <pre>
- * -a
- * -b
- * -c
- * a
- * b
- * c
- * </pre>
- * can be shifted down by 1, 2 or 3 locations.
- * <p>
- * To avoid later merge issues, we shift such edits to a
- * consistent location. {@code normalize} uses a simple strategy of
- * shifting such edits to their latest possible location.
- * <p>
- * This strategy may not always produce an aesthetically pleasing
- * diff. For instance, it works well with
- * <pre>
- * function1 {
- * ...
- * }
- *
- * +function2 {
- * + ...
- * +}
- * +
- * function3 {
- * ...
- * }
- * </pre>
- * but less so for
- * <pre>
- * #
- * # comment1
- * #
- * function1() {
- * }
- *
- * #
- * +# comment3
- * +#
- * +function3() {
- * +}
- * +
- * +#
- * # comment2
- * #
- * function2() {
- * }
- * </pre>
- * <a href="https://github.com/mhagger/diff-slider-tools">More
- * sophisticated strategies</a> are possible, say by calculating a
- * suitable "aesthetic cost" for each possible position and using
- * the lowest cost, but {@code normalize} just shifts edits
- * to the end as much as possible.
- *
- * @param <S>
- * type of sequence being compared.
- * @param cmp
- * the comparator supplying the element equivalence function.
- * @param e
- * a modifiable edit list comparing the provided sequences.
- * @param a
- * the first (also known as old or pre-image) sequence.
- * @param b
- * the second (also known as new or post-image) sequence.
- * @return a modifiable edit list with edit regions shifted to their
- * latest possible location. The result list is never null.
- * @since 4.7
- */
- private static <S extends Sequence> EditList normalize(
- SequenceComparator<? super S> cmp, EditList e, S a, S b) {
- Edit prev = null;
- for (int i = e.size() - 1; i >= 0; i--) {
- Edit cur = e.get(i);
- Edit.Type curType = cur.getType();
-
- int maxA = (prev == null) ? a.size() : prev.beginA;
- int maxB = (prev == null) ? b.size() : prev.beginB;
-
- if (curType == Edit.Type.INSERT) {
- while (cur.endA < maxA && cur.endB < maxB
- && cmp.equals(b, cur.beginB, b, cur.endB)) {
- cur.shift(1);
- }
- } else if (curType == Edit.Type.DELETE) {
- while (cur.endA < maxA && cur.endB < maxB
- && cmp.equals(a, cur.beginA, a, cur.endA)) {
- cur.shift(1);
- }
- }
- prev = cur;
- }
- return e;
- }
-
- /**
- * Compare two sequences and identify a list of edits between them.
- *
- * This method should be invoked only after the two sequences have been
- * proven to have no common starting or ending elements. The expected
- * elimination of common starting and ending elements is automatically
- * performed by the {@link #diff(SequenceComparator, Sequence, Sequence)}
- * method, which invokes this method using {@link Subsequence}s.
- *
- * @param <S>
- * type of sequence being compared.
- * @param cmp
- * the comparator supplying the element equivalence function.
- * @param a
- * the first (also known as old or pre-image) sequence. Edits
- * returned by this algorithm will reference indexes using the
- * 'A' side: {@link Edit#getBeginA()}, {@link Edit#getEndA()}.
- * @param b
- * the second (also known as new or post-image) sequence. Edits
- * returned by this algorithm will reference indexes using the
- * 'B' side: {@link Edit#getBeginB()}, {@link Edit#getEndB()}.
- * @return a modifiable edit list comparing the two sequences.
- */
- public abstract <S extends Sequence> EditList diffNonCommon(
- SequenceComparator<? super S> cmp, S a, S b);
- }
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