Package org.openstreetmap.josm.tools

Class Diff

java.lang.Object

org.openstreetmap.josm.tools.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

Nested Classes

Modifier and Type	Class	Description
static class	Diff.Change	The result of comparison is an "edit script": a chain of change objects.
(package private) class	Diff.FileData	Data on one input file being compared.
(package private) static class	Diff.ForwardScript
(package private) static class	Diff.ReverseScript	Scan the tables of which lines are inserted and deleted, producing an edit script in reverse order.
static interface	Diff.ScriptBuilder	Script builder.

Field Summary

Fields

Modifier and Type	Field	Description
private int[]	bdiag
private int	bdiagoff
private int	cost
private int	equivMax	1 more than the maximum equivalence value used for this or its sibling file.
private int[]	fdiag
private int	fdiagoff
private Diff.FileData[]	filevec
static Diff.ScriptBuilder	forwardScript	Standard Forward ScriptBuilder.
static Diff.ScriptBuilder	reverseScript	Standard Reverse ScriptBuilder.
private int[]	xvec
private int[]	yvec

Constructor Summary

Constructors

Constructor	Description
Diff(java.lang.Object[] a, java.lang.Object[] b)	Prepare to find differences between two arrays.

Method Summary

Modifier and Type	Method	Description
private void	compareseq(int xoff, int xlim, int yoff, int ylim)	Compare in detail contiguous subsequences of the two files which are known, as a whole, to match each other.
private int	diag(int xoff, int xlim, int yoff, int ylim)	Find the midpoint of the shortest edit script for a specified portion of the two files.
Diff.Change	diff(Diff.ScriptBuilder bld)	Get the results of comparison as an edit script.
Diff.Change	diff2(boolean reverse)	Report the differences of two files.
private void	discardConfusingLines()	Discard lines from one file that have no matches in the other file.
private void	shiftBoundaries()	Adjust inserts/deletes of blank lines to join changes as much as possible.

Methods inherited from class java.lang.Object

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

Field Detail

equivMax

private int equivMax

1 more than the maximum equivalence value used for this or its sibling file.

xvec

private int[] xvec

yvec

private int[] yvec

fdiag

private int[] fdiag

bdiag

private int[] bdiag

fdiagoff

private int fdiagoff

bdiagoff

private int bdiagoff

filevec

private final Diff.FileData[] filevec

cost

private int cost

forwardScript

public static final Diff.ScriptBuilder forwardScript

Standard Forward ScriptBuilder.

reverseScript

public static final Diff.ScriptBuilder reverseScript

Standard Reverse ScriptBuilder.

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.

Parameters:

a - first array

b - second array

Method Detail

diag

private int diag(int xoff,
                 int xlim,
                 int yoff,
                 int ylim)

Find the midpoint of the shortest edit script for a specified portion of the two files.

We scan from the beginnings of the files, and simultaneously from the ends, doing a breadth-first search through the space of edit-sequence. When the two searches meet, we have found the midpoint of the shortest edit sequence.

The value returned is the number of the diagonal on which the midpoint lies. The diagonal number equals the number of inserted lines minus the number of deleted lines (counting only lines before the midpoint). The edit cost is stored into COST; this is the total number of lines inserted or deleted (counting only lines before the midpoint).

This function assumes that the first lines of the specified portions of the two files do not match, and likewise that the last lines do not match. The caller must trim matching lines from the beginning and end of the portions it is going to specify.

Note that if we return the "wrong" diagonal value, or if the value of bdiag at that diagonal is "wrong", the worst this can do is cause suboptimal diff output. It cannot cause incorrect diff output.

Parameters:

xoff - xoff

xlim - xlim

yoff - yoff

ylim - ylim

Returns:

midpoint of the shortest edit script

compareseq

private void compareseq(int xoff,
                        int xlim,
                        int yoff,
                        int ylim)

Compare in detail contiguous subsequences of the two files which are known, as a whole, to match each other.

The results are recorded in the vectors filevec[N].changed_flag, by storing a 1 in the element for each line that is an insertion or deletion.

The subsequence of file 0 is [XOFF, XLIM) and likewise for file 1.

Note that XLIM, YLIM are exclusive bounds. All line numbers are origin-0 and discarded lines are not counted.

Parameters:

xoff - xoff

xlim - xlim

yoff - yoff

ylim - ylim

discardConfusingLines

private void discardConfusingLines()

Discard lines from one file that have no matches in the other file.

shiftBoundaries

private void shiftBoundaries()

Adjust inserts/deletes of blank lines to join changes as much as possible.

diff2

public final Diff.Change diff2(boolean reverse)

Report the differences of two files. DEPTH is the current directory depth.

Parameters:

reverse - if true use reverseScript else use forwardScript

Returns:

the differences of two files

diff

public Diff.Change diff(Diff.ScriptBuilder bld)

Get the results of comparison as an edit script. The script is described by a list of changes. The standard ScriptBuilder implementations provide for forward and reverse edit scripts. Alternate implementations could, for instance, list common elements instead of differences.

Parameters:

bld - an object to build the script from change flags

Returns:

the head of a list of changes