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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;
/**
* 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);
}
}
}
}
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