/*
 * 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;

import java.util.ArrayList;
import java.util.List;

An extended form of Bram Cohen's patience diff algorithm.

This implementation was derived by using the 4 rules that are outlined in
Bram Cohen's blog,
and then was further extended to support low-occurrence common elements.

The basic idea of the algorithm is to create a histogram of occurrences for
each element of sequence A. Each element of sequence B is then considered in
turn. If the element also exists in sequence A, and has a lower occurrence
count, the positions are considered as a candidate for the longest common
subsequence (LCS). After scanning of B is complete the LCS that has the
lowest number of occurrences is chosen as a split point. The region is split
around the LCS, and the algorithm is recursively applied to the sections
before and after the LCS.

By always selecting a LCS position with the lowest occurrence count, this
algorithm behaves exactly like Bram Cohen's patience diff whenever there is a
unique common element available between the two sequences. When no unique
elements exist, the lowest occurrence element is chosen instead. This offers
more readable diffs than simply falling back on the standard Myers' O(ND)
algorithm would produce.
 To prevent the algorithm from having an O(N^2) running time, an upper limit on the number of unique elements in a histogram bucket is configured by setMaxChainLength(int). If sequence A has more than this many elements that hash into the same hash bucket, the algorithm passes the region to setFallbackAlgorithm(DiffAlgorithm). If no fallback algorithm is configured, the region is emitted as a replace edit. 
 During scanning of sequence B, any element of A that occurs more than setMaxChainLength(int) times is never considered for an LCS match position, even if it is common between the two sequences. This limits the number of locations in sequence A that must be considered to find the LCS, and helps maintain a lower running time bound. 
 So long as setMaxChainLength(int) is a small constant (such as 64), the algorithm runs in O(N * D) time, where N is the sum of the input lengths and D is the number of edits in the resulting EditList. If the supplied SequenceComparator has a good hash function, this implementation typically out-performs MyersDiff, even though its theoretical running time is the same. 

This implementation has an internal limitation that prevents it from handling
sequences with more than 268,435,456 (2^28) elements.
/**
 * An extended form of Bram Cohen's patience diff algorithm.
 * <p>
 * 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>,
 * and then was further extended to support low-occurrence common elements.
 * <p>
 * The basic idea of the algorithm is to create a histogram of occurrences for
 * each element of sequence A. Each element of sequence B is then considered in
 * turn. If the element also exists in sequence A, and has a lower occurrence
 * count, the positions are considered as a candidate for the longest common
 * subsequence (LCS). After scanning of B is complete the LCS that has the
 * lowest number of occurrences is chosen as a split point. The region is split
 * around the LCS, and the algorithm is recursively applied to the sections
 * before and after the LCS.
 * <p>
 * By always selecting a LCS position with the lowest occurrence count, this
 * algorithm behaves exactly like Bram Cohen's patience diff whenever there is a
 * unique common element available between the two sequences. When no unique
 * elements exist, the lowest occurrence element is chosen instead. This offers
 * more readable diffs than simply falling back on the standard Myers' O(ND)
 * algorithm would produce.
 * <p>
 * To prevent the algorithm from having an O(N^2) running time, an upper limit
 * on the number of unique elements in a histogram bucket is configured by
 * {@link #setMaxChainLength(int)}. If sequence A has more than this many
 * elements that hash into the same hash bucket, the algorithm passes the region
 * to {@link #setFallbackAlgorithm(DiffAlgorithm)}. If no fallback algorithm is
 * configured, the region is emitted as a replace edit.
 * <p>
 * During scanning of sequence B, any element of A that occurs more than
 * {@link #setMaxChainLength(int)} times is never considered for an LCS match
 * position, even if it is common between the two sequences. This limits the
 * number of locations in sequence A that must be considered to find the LCS,
 * and helps maintain a lower running time bound.
 * <p>
 * So long as {@link #setMaxChainLength(int)} is a small constant (such as 64),
 * the algorithm runs in O(N * D) time, where N is the sum of the input lengths
 * and D is the number of edits in the resulting EditList. If the supplied
 * {@link org.eclipse.jgit.diff.SequenceComparator} has a good hash function,
 * this implementation typically out-performs
 * {@link org.eclipse.jgit.diff.MyersDiff}, even though its theoretical running
 * time is the same.
 * <p>
 * This implementation has an internal limitation that prevents it from handling
 * sequences with more than 268,435,456 (2^28) elements.
 */
public class HistogramDiff extends LowLevelDiffAlgorithm {
	
Algorithm to use when there are too many element occurrences. /** Algorithm to use when there are too many element occurrences. */
	DiffAlgorithm fallback = MyersDiff.INSTANCE;

	
Maximum number of positions to consider for a given element hash.
All elements with the same hash are stored into a single chain. The chain
size is capped to ensure search is linear time at O(len_A + len_B) rather
than quadratic at O(len_A * len_B).
/**
	 * Maximum number of positions to consider for a given element hash.
	 *
	 * All elements with the same hash are stored into a single chain. The chain
	 * size is capped to ensure search is linear time at O(len_A + len_B) rather
	 * than quadratic at O(len_A * len_B).
	 */
	int maxChainLength = 64;

	
Set the algorithm used when there are too many element occurrences.
Params:
alg – 
           the secondary algorithm. If null the region will be denoted as
           a single REPLACE block./**
	 * Set the algorithm used when there are too many element occurrences.
	 *
	 * @param alg
	 *            the secondary algorithm. If null the region will be denoted as
	 *            a single REPLACE block.
	 */
	public void setFallbackAlgorithm(DiffAlgorithm alg) {
		fallback = alg;
	}

	
Maximum number of positions to consider for a given element hash.
All elements with the same hash are stored into a single chain. The chain
size is capped to ensure search is linear time at O(len_A + len_B) rather
than quadratic at O(len_A * len_B).
Params:
maxLen – 
           new maximum length./**
	 * Maximum number of positions to consider for a given element hash.
	 *
	 * All elements with the same hash are stored into a single chain. The chain
	 * size is capped to ensure search is linear time at O(len_A + len_B) rather
	 * than quadratic at O(len_A * len_B).
	 *
	 * @param maxLen
	 *            new maximum length.
	 */
	public void setMaxChainLength(int maxLen) {
		maxChainLength = maxLen;
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public <S extends Sequence> void diffNonCommon(EditList edits,
			HashedSequenceComparator<S> cmp, HashedSequence<S> a,
			HashedSequence<S> b, Edit region) {
		new State<>(edits, cmp, a, b).diffRegion(region);
	}

	private class State<S extends Sequence> {
		private final HashedSequenceComparator<S> cmp;
		private final HashedSequence<S> a;
		private final HashedSequence<S> b;
		private final List<Edit> queue = new ArrayList<>();

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

		State(EditList edits, HashedSequenceComparator<S> cmp,
				HashedSequence<S> a, HashedSequence<S> b) {
			this.cmp = cmp;
			this.a = a;
			this.b = b;
			this.edits = edits;
		}

		void diffRegion(Edit r) {
			diffReplace(r);
			while (!queue.isEmpty())
				diff(queue.remove(queue.size() - 1));
		}

		private void diffReplace(Edit r) {
			Edit lcs = new HistogramDiffIndex<>(maxChainLength, cmp, a, b, r)
					.findLongestCommonSequence();
			if (lcs != null) {
				// If we were given an edit, we can prove a result here.
				//
				if (lcs.isEmpty()) {
					// An empty edit indicates there is nothing in common.
					// Replace the entire region.
					//
					edits.add(r);
				} else {
					queue.add(r.after(lcs));
					queue.add(r.before(lcs));
				}

			} else if (fallback instanceof LowLevelDiffAlgorithm) {
				LowLevelDiffAlgorithm fb = (LowLevelDiffAlgorithm) fallback;
				fb.diffNonCommon(edits, cmp, a, b, r);

			} else if (fallback != null) {
				SubsequenceComparator<HashedSequence<S>> cs = subcmp();
				Subsequence<HashedSequence<S>> as = Subsequence.a(a, r);
				Subsequence<HashedSequence<S>> bs = Subsequence.b(b, r);

				EditList res = fallback.diffNonCommon(cs, as, bs);
				edits.addAll(Subsequence.toBase(res, as, bs));

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

		private void diff(Edit r) {
			switch (r.getType()) {
			case INSERT:
			case DELETE:
				edits.add(r);
				break;

			case REPLACE:
				if (r.getLengthA() == 1 && r.getLengthB() == 1)
					edits.add(r);
				else
					diffReplace(r);
				break;

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

		private SubsequenceComparator<HashedSequence<S>> subcmp() {
			return new SubsequenceComparator<>(cmp);
		}
	}
}