jgit/org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffAlgorithm.java

/*
 * 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;

/**
 * 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);
}