Java tutorial
/* * 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 org.eclipse.jgit.diff.Sequence}s to create an * {@link org.eclipse.jgit.diff.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 } /** * Get diff algorithm * * @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 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 org.eclipse.jgit.diff.Edit#getBeginA()}, * {@link org.eclipse.jgit.diff.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 org.eclipse.jgit.diff.Edit#getBeginB()}, * {@link org.eclipse.jgit.diff.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 org.eclipse.jgit.diff.Subsequence}s. * * @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 org.eclipse.jgit.diff.Edit#getBeginA()}, * {@link org.eclipse.jgit.diff.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 org.eclipse.jgit.diff.Edit#getBeginB()}, * {@link org.eclipse.jgit.diff.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); }