helma.util
Class Diff

java.lang.Object
  extended by helma.util.Diff

public class Diff
extends java.lang.Object

A class to compare vectors of objects. The result of comparison is a list of change objects which form an edit script. The objects compared are traditionally lines of text from two files. Comparison options such as "ignore whitespace" are implemented by modifying the equals and hashcode methods for the objects compared.

The basic algorithm is described in:
"An O(ND) Difference Algorithm and its Variations", Eugene Myers, Algorithmica Vol. 1 No. 2, 1986, p 251.

This class outputs different results from GNU diff 1.15 on some inputs. Our results are actually better (smaller change list, smaller total size of changes), but it would be nice to know why. Perhaps there is a memory overwrite bug in GNU diff 1.15.


Nested Class Summary
static class Diff.change
          The result of comparison is an "edit script": a chain of change objects.
 
Field Summary
 boolean heuristic
          When set to true, the comparison uses a heuristic to speed it up.
 boolean no_discards
          When set to true, the algorithm returns a guarranteed minimal set of changes.
 
Constructor Summary
Diff(java.lang.Object[] a, java.lang.Object[] b)
          Prepare to find differences between two arrays.
 
Method Summary
 Diff.change diff_2(boolean reverse)
           
 Diff.change diff()
          Convenience method to get a diff on the two files.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

heuristic

public boolean heuristic
When set to true, the comparison uses a heuristic to speed it up. With this heuristic, for files with a constant small density of changes, the algorithm is linear in the file size.


no_discards

public boolean no_discards
When set to true, the algorithm returns a guarranteed minimal set of changes. This makes things slower, sometimes much slower.

Constructor Detail

Diff

public Diff(java.lang.Object[] a,
            java.lang.Object[] b)
Prepare to find differences between two arrays. Each element of the arrays is translated to an "equivalence number" based on the result of equals. The original Object arrays are no longer needed for computing the differences. They will be needed again later to print the results of the comparison as an edit script, if desired.

Method Detail

diff

public Diff.change diff()
Convenience method to get a diff on the two files.


diff_2

public Diff.change diff_2(boolean reverse)


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