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java.lang.Object org.eclipse.jgit.diff.DiffAlgorithm org.eclipse.jgit.diff.LowLevelDiffAlgorithm org.eclipse.jgit.diff.HistogramDiff
public class HistogramDiff
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 lowoccurrence 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 outperforms 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.
Nested Class Summary 

Nested classes/interfaces inherited from class org.eclipse.jgit.diff.DiffAlgorithm 

DiffAlgorithm.SupportedAlgorithm 
Constructor Summary  

HistogramDiff()

Method Summary  


diffNonCommon(EditList edits,
HashedSequenceComparator<S> cmp,
HashedSequence<S> a,
HashedSequence<S> b,
Edit region)
Compare two sequences and identify a list of edits between them. 

void 
setFallbackAlgorithm(DiffAlgorithm alg)
Set the algorithm used when there are too many element occurrences. 

void 
setMaxChainLength(int maxLen)
Maximum number of positions to consider for a given element hash. 
Methods inherited from class org.eclipse.jgit.diff.LowLevelDiffAlgorithm 

diffNonCommon 
Methods inherited from class org.eclipse.jgit.diff.DiffAlgorithm 

diff, getAlgorithm 
Methods inherited from class java.lang.Object 

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait 
Constructor Detail 

public HistogramDiff()
Method Detail 

public void setFallbackAlgorithm(DiffAlgorithm alg)
alg
 the secondary algorithm. If null the region will be denoted as
a single REPLACE block.public void setMaxChainLength(int maxLen)
maxLen
 new maximum length.public <S extends Sequence> void diffNonCommon(EditList edits, HashedSequenceComparator<S> cmp, HashedSequence<S> a, HashedSequence<S> b, Edit region)
LowLevelDiffAlgorithm
DiffAlgorithm.diff(SequenceComparator, Sequence, Sequence)
method, which invokes this method using Subsequence
s.
diffNonCommon
in class LowLevelDiffAlgorithm
S
 type of sequence being compared.edits
 result list to append the region's edits onto.cmp
 the comparator supplying the element equivalence function.a
 the first (also known as old or preimage) sequence. Edits
returned by this algorithm will reference indexes using the
'A' side: Edit.getBeginA()
, Edit.getEndA()
.b
 the second (also known as new or postimage) sequence. Edits
returned by this algorithm will reference indexes using the
'B' side: Edit.getBeginB()
, Edit.getEndB()
.region
 the region being compared within the two sequences.


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