/*
 * 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
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 * without modification, are permitted provided that the following
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 * - Redistributions of source code must retain the above copyright
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 *   disclaimer in the documentation and/or other materials provided
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package org.eclipse.jgit.diff;

/**
 * An implementation of the patience difference algorithm.
 *
 * This implementation was derived by using the 4 rules that are outlined in
 * Bram Cohen's <a href="http://bramcohen.livejournal.com/73318.html">blog</a>.
 *
 * Because this algorithm requires finding a unique common point to center the
 * longest common subsequence around, input sequences which have no unique
 * elements create a degenerate Edit that simply replaces all of one sequence
 * with all of the other sequence. For many source code files and other human
 * maintained text, this isn't likely to occur. When it does occur, it can be
 * easier to read the resulting large-scale replace than to navigate through a
 * lot of slices of common-but-not-unique lines, like curly braces on lone
 * lines, or XML close tags. Consequently this algorithm is willing to create a
 * degenerate Edit in the worst case, in exchange for what may still be
 * perceived to be an easier to read patch script.
 *
 * In a nutshell, the implementation defines an Edit that replaces all of
 * sequence {@code a} with all of {@code b}. This Edit is reduced and/or split
 * to remove common elements, until only Edits spanning non-common elements
 * remain. Those {@link Edit}s are the differences.
 *
 * A slightly more detailed description of the implementation is:
 *
 * <ol>
 * <li>Define an Edit that spans the entire two sequences. This edit replaces
 * all of {@code a} with all of {@code b}.</li>
 *
 * <li>Shrink the Edit by shifting the starting points later in the sequence to
 * skip over any elements that are common between {@code a} and {@code b}.
 * Likewise shift the ending points earlier in the sequence to skip any trailing
 * elements that are common. The first and last element of the edit are now not
 * common, however there may be common content within the interior of the Edit
 * that hasn't been discovered yet.</li>
 *
 * <li>Find unique elements within the Edit region that are in both sequences.
 * This is currently accomplished by hashing the elements and merging them
 * through a custom hash table in {@link PatienceDiffIndex}.</li>
 *
 * <li>Order the common unique elements by their position within {@code b}.</li>
 *
 * <li>For each unique element, stretch an Edit around it in both directions,
 * consuming neighboring elements that are common to both sequences. Select the
 * longest such Edit out of the unique element list. During this stretching,
 * some subsequent unique elements may be consumed into an earlier's common
 * Edit. This means not all unique elements are evaluated.</li>
 *
 * <li>Split the Edit region at the longest common edit. Because step 2 shrank
 * the initial region, there must be at least one element before, and at least
 * one element after the split.</li>
 *
 * <li>Recurse on the before and after split points, starting from step 3. Step
 * 2 doesn't need to be done again because any common part was already removed
 * by the prior step 2 or 5.</li>
 * </ol>
 */
public class PatienceDiff implements DiffAlgorithm {
	/** Algorithm we use when there are no common unique lines in a region. */
	private DiffAlgorithm fallback;

	/**
	 * Set the algorithm used when there are no common unique lines remaining.
	 *
	 * @param alg
	 *            the secondary algorithm. If null the region will be denoted as
	 *            a single REPLACE block.
	 */
	public void setFallbackAlgorithm(DiffAlgorithm alg) {
		fallback = alg;
	}

	public <S extends Sequence, C extends SequenceComparator<? super S>> EditList diff(
			C cmp, S a, S b) {
		Edit region = new Edit(0, a.size(), 0, b.size());
		region = cmp.reduceCommonStartEnd(a, b, region);

		switch (region.getType()) {
		case INSERT:
		case DELETE: {
			EditList r = new EditList();
			r.add(region);
			return r;
		}

		case REPLACE: {
			SubsequenceComparator<S> cs = new SubsequenceComparator<S>(cmp);
			Subsequence<S> as = Subsequence.a(a, region);
			Subsequence<S> bs = Subsequence.b(b, region);
			return Subsequence.toBase(diffImpl(cs, as, bs), as, bs);
		}

		case EMPTY:
			return new EditList();

		default:
			throw new IllegalStateException();
		}
	}

	private <S extends Sequence, C extends SequenceComparator<? super S>> EditList diffImpl(
			C cmp, S a, S b) {
		State<S> s = new State<S>(new HashedSequencePair<S>(cmp, a, b));
		s.diff(new Edit(0, s.a.size(), 0, s.b.size()), null, 0, 0);
		return s.edits;
	}

	private class State<S extends Sequence> {
		private final HashedSequenceComparator<S> cmp;

		private final HashedSequence<S> a;

		private final HashedSequence<S> b;

		/** Result edits we have determined that must be made to convert a to b. */
		final EditList edits;

		State(HashedSequencePair<S> p) {
			this.cmp = p.getComparator();
			this.a = p.getA();
			this.b = p.getB();
			this.edits = new EditList();
		}

		private void diff(Edit r, long[] pCommon, int pIdx, int pEnd) {
			switch (r.getType()) {
			case INSERT:
			case DELETE:
				edits.add(r);
				return;

			case REPLACE:
				break;

			case EMPTY:
			default:
				throw new IllegalStateException();
			}

			PatienceDiffIndex<S> p;

			p = new PatienceDiffIndex<S>(cmp, a, b, r, pCommon, pIdx, pEnd);
			Edit lcs = p.findLongestCommonSequence();

			if (lcs != null) {
				pCommon = p.nCommon;
				pIdx = p.cIdx;
				pEnd = p.nCnt;
				p = null;

				diff(r.before(lcs), pCommon, 0, pIdx);
				diff(r.after(lcs), pCommon, pIdx + 1, pEnd);

			} else if (fallback != null) {
				p = null;
				pCommon = null;

				SubsequenceComparator<HashedSequence<S>> cs;
				cs = new SubsequenceComparator<HashedSequence<S>>(cmp);

				Subsequence<HashedSequence<S>> as = Subsequence.a(a, r);
				Subsequence<HashedSequence<S>> bs = Subsequence.b(b, r);
				EditList res = fallback.diff(cs, as, bs);
				edits.addAll(Subsequence.toBase(res, as, bs));

			} else {
				edits.add(r);
			}
		}
	}
}