org.eclipse.jgit.diff.DiffAlgorithm Maven / Gradle / Ivy
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package org.eclipse.jgit.diff;
/**
* Compares two {@link Sequence}s to create an {@link EditList} of changes.
*
* 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
}
/**
* @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
* type of sequence being compared.
* @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 Edit#getBeginA()}, {@link 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 Edit#getBeginB()}, {@link 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 EditList diff(
SequenceComparator 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: {
SubsequenceComparator cs = new SubsequenceComparator(cmp);
Subsequence as = Subsequence.a(a, region);
Subsequence bs = Subsequence.b(b, region);
EditList e = Subsequence.toBase(diffNonCommon(cs, as, bs), as, bs);
// The last insertion may need to be shifted later if it
// inserts elements that were previously reduced out as
// common at the end.
//
Edit last = e.get(e.size() - 1);
if (last.getType() == Edit.Type.INSERT) {
while (last.endB < b.size()
&& cmp.equals(b, last.beginB, b, last.endB)) {
last.beginA++;
last.endA++;
last.beginB++;
last.endB++;
}
}
return e;
}
case EMPTY:
return new EditList(0);
default:
throw new IllegalStateException();
}
}
private static Edit coverEdit(S a, S b) {
return new Edit(0, a.size(), 0, b.size());
}
/**
* 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 Subsequence}s.
*
* @param
* type of sequence being compared.
* @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 Edit#getBeginA()}, {@link 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 Edit#getBeginB()}, {@link Edit#getEndB()}.
* @return a modifiable edit list comparing the two sequences.
*/
public abstract EditList diffNonCommon(
SequenceComparator cmp, S a, S b);
}
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