lib/Algorithm/Diff.pm

package Algorithm::Diff;
use strict;
use vars qw($VERSION @EXPORT_OK @ISA @EXPORT);
use integer;            # see below in _replaceNextLargerWith() for mod to make
                                        # if you don't use this
require Exporter;
@ISA = qw(Exporter);
@EXPORT = qw();
@EXPORT_OK = qw(LCS diff traverse_sequences);
$VERSION = sprintf('%d.%02d', (q$Revision: 1.10 $ =~ /\d+/g));

# McIlroy-Hunt diff algorithm
# Adapted from the Smalltalk code of Mario I. Wolczko, <mario@wolczko.com>
# by Ned Konz, perl@bike-nomad.com

=head1 NAME

Algorithm::Diff - Compute `intelligent' differences between two files / lists

=head1 SYNOPSIS

  use Algorithm::Diff qw(diff LCS traverse_sequences);

  @lcs    = LCS( \@seq1, \@seq2 );

  @lcs    = LCS( \@seq1, \@seq2, $key_generation_function );

  $lcsref = LCS( \@seq1, \@seq2 );

  $lcsref = LCS( \@seq1, \@seq2, $key_generation_function );

  @diffs = diff( \@seq1, \@seq2 );

  @diffs = diff( \@seq1, \@seq2, $key_generation_function );
  
  traverse_sequences( \@seq1, \@seq2,
                     { MATCH => $callback,
                       DISCARD_A => $callback,
                       DISCARD_B => $callback,
                     } );

  traverse_sequences( \@seq1, \@seq2,
                     { MATCH => $callback,
                       DISCARD_A => $callback,
                       DISCARD_B => $callback,
                     },
                     $key_generation_function );

=head1 INTRODUCTION

(by Mark-Jason Dominus)

I once read an article written by the authors of C<diff>; they said
that they hard worked very hard on the algorithm until they found the
right one.

I think what they ended up using (and I hope someone will correct me,
because I am not very confident about this) was the `longest common
subsequence' method.  in the LCS problem, you have two sequences of
items:

        a b c d f g h j q z

        a b c d e f g i j k r x y z

and you want to find the longest sequence of items that is present in
both original sequences in the same order.  That is, you want to find
a new sequence I<S> which can be obtained from the first sequence by
deleting some items, and from the secend sequence by deleting other
items.  You also want I<S> to be as long as possible.  In this case
I<S> is

        a b c d f g j z

From there it's only a small step to get diff-like output:

        e   h i   k   q r x y 
        +   - +   +   - + + +

This module solves the LCS problem.  It also includes a canned
function to generate C<diff>-like output.

It might seem from the example above that the LCS of two sequences is
always pretty obvious, but that's not always the case, especially when
the two sequences have many repeated elements.  For example, consider

        a x b y c z p d q
        a b c a x b y c z

A naive approach might start by matching up the C<a> and C<b> that
appear at the beginning of each sequence, like this:

        a x b y c         z p d q
        a   b   c a b y c z

This finds the common subsequence C<a b c z>.  But actually, the LCS
is C<a x b y c z>:

              a x b y c z p d q
        a b c a x b y c z

=head1 USAGE

This module provides three exportable functions, which we'll deal with in
ascending order of difficulty: C<LCS>, C<diff>, and
C<traverse_sequences>.

=head2 C<LCS>

Given references to two lists of items, LCS returns an array containing their
longest common subsequence.  In scalar context, it returns a reference to
such a list.

  @lcs    = LCS( \@seq1, \@seq2 );
  $lcsref = LCS( \@seq1, \@seq2 );

C<LCS> may be passed an optional third parameter; this is a CODE
reference to a key generation function.  See L</KEY GENERATION
FUNCTIONS>.

  @lcs    = LCS( \@seq1, \@seq2, $keyGen );
  $lcsref = LCS( \@seq1, \@seq2, $keyGen );

Additional parameters, if any, will be passed to the key generation
routine.

=head2 C<diff>

  @diffs     = diff( \@seq1, \@seq2 );
  $diffs_ref = diff( \@seq1, \@seq2 );

C<diff> computes the smallest set of additions and deletions necessary
to turn the first sequence into the second, and returns a description
of these changes.  The description is a list of I<hunks>; each hunk
represents a contiguous section of items which should be added,
deleted, or replaced.  The return value of C<diff> is a list of
hunks, or, in scalar context, a reference to such a list.

Here is an example:  The diff of the following two sequences:

  a b c e h j l m n p
  b c d e f j k l m r s t

Result:

 [ 
   [ [ '-', 0, 'a' ] ],       

   [ [ '+', 2, 'd' ] ],

   [ [ '-', 4, 'h' ] , 
     [ '+', 4, 'f' ] ],

   [ [ '+', 6, 'k' ] ],

   [ [ '-', 8, 'n' ], 
     [ '-', 9, 'p' ], 
     [ '+', 9, 'r' ], 
     [ '+', 10, 's' ], 
     [ '+', 11, 't' ],
   ]
 ]

There are five hunks here.  The first hunk says that the C<a> at
position 0 of the first sequence should be deleted (C<->).  The second
hunk says that the C<d> at position 2 of the second sequence should
be inserted (C<+>).  The third hunk says that the C<h> at position 4
of the first sequence should be removed and replaced with the C<f>
from position 4 of the second sequence.  The other two hunks similarly. 

C<diff> may be passed an optional third parameter; this is a CODE
reference to a key generation function.  See L</KEY GENERATION
FUNCTIONS>.

Additional parameters, if any, will be passed to the key generation
routine.

=head2 C<traverse_sequences>

C<traverse_sequences> is the most general facility provided by this
module; C<diff> and C<LCS> are implemented as calls to it.

Imagine that there are two arrows.  Arrow A points to an element of
sequence A, and arrow B points to an element of the sequence B.
Initially, the arrows point to the first elements of the respective
sequences.  C<traverse_sequences> will advance the arrows through the
sequences one element at a time, calling an appropriate user-specified
callback function before each advance.  It willadvance the arrows in
such a way that if there are equal elements C<$A[$i]> and C<$B[$j]>
which are equal and which are part of the LCS, there will be some
moment during the execution of C<traverse_sequences> when arrow A is
pointing to C<$A[$i]> and arrow B is pointing to C<$B[$j]>.  When this
happens, C<traverse_sequences> will call the C<MATCH> callback
function and then it will advance both arrows. 

Otherwise, one of the arrows is pointing to an element of its sequence
that is not part of the LCS.  C<traverse_sequences> will advance that
arrow and will call the C<DISCARD_A> or the C<DISCARD_B> callback,
depending on which arrow it advanced.  If both arrows point to
elements that are not part of the LCS, then C<traverse_sequences> will
advance one of them and call the appropriate callback, but it is not
specified which it will call.

The arguments to C<traverse_sequences> are the two sequences to
traverse, and a callback which specifies the callback functions, like
this:

  traverse_sequences( \@seq1, \@seq2,
                     { MATCH => $callback_1,
                       DISCARD_A => $callback_2,
                       DISCARD_B => $callback_3,
                     } );

Callbacks are invoked with at least the indices of the two arrows as
their arguments.  They are not expected to return any values.  If a
callback is omitted from the table, it is not called.

If arrow A reaches the end of its sequence, before arrow B does,
C<traverse_sequences> will call the C<A_FINISHED> callback when it
advances arrow B, if there is such a function; if not it will call
C<DISCARD_B> instead.  Similarly if arrow B finishes first.
C<traverse_sequences> returns when both arrows are at the ends of
their respective sequences.  It returns true on success and false on
failure.  At present there is no way to fail.

C<traverse_sequences> may be passed an optional fourth parameter; this
is a CODE reference to a key generation function.  See L</KEY
GENERATION FUNCTIONS>.

Additional parameters, if any, will be passed to the key generation
function.

=head1 KEY GENERATION FUNCTIONS

C<diff>, C<LCS>, and C<traverse_sequences> accept an optional last parameter.
This is a CODE reference to a key generating (hashing) function that should
return a string that uniquely identifies a given element.
It should be the case that if two elements are to be considered equal,
their keys should be the same (and the other way around).
If no key generation function is provided, the key will be the
element as a string.

By default, comparisons will use "eq" and elements will be turned into keys
using the default stringizing operator '""'.

Where this is important is when you're comparing something other than
strings. If it is the case that you have multiple different objects 
that should be considered to be equal, you should supply a key
generation function. Otherwise, you have to make sure that your arrays
contain unique references.

For instance, consider this example:

  package Person;

  sub new
  {
    my $package = shift;
    return bless { name => '', ssn => '', @_ }, $package;
  }

  sub clone
  {
    my $old = shift;
    my $new = bless { %$old }, ref($old);
  }

  sub hash
  {
    return shift()->{'ssn'};
  }

  my $person1 = Person->new( name => 'Joe', ssn => '123-45-6789' );
  my $person2 = Person->new( name => 'Mary', ssn => '123-47-0000' );
  my $person3 = Person->new( name => 'Pete', ssn => '999-45-2222' );
  my $person4 = Person->new( name => 'Peggy', ssn => '123-45-9999' );
  my $person5 = Person->new( name => 'Frank', ssn => '000-45-9999' );

If you did this:

  my $array1 = [ $person1, $person2, $person4 ];
  my $array2 = [ $person1, $person3, $person4, $person5 ];
  Algorithm::Diff::diff( $array1, $array2 );

everything would work out OK (each of the objects would be converted
into a string like "Person=HASH(0x82425b0)" for comparison).

But if you did this:

  my $array1 = [ $person1, $person2, $person4 ];
  my $array2 = [ $person1, $person3, $person4->clone(), $person5 ];
  Algorithm::Diff::diff( $array1, $array2 );

$person4 and $person4->clone() (which have the same name and SSN)
would be seen as different objects. If you wanted them to be considered
equivalent, you would have to pass in a key generation function:

  my $array1 = [ $person1, $person2, $person4 ];
  my $array2 = [ $person1, $person3, $person4->clone(), $person5 ];
  Algorithm::Diff::diff( $array1, $array2, \&Person::hash );

This would use the 'ssn' field in each Person as a comparison key, and
so would consider $person4 and $person4->clone() as equal.

You may also pass additional parameters to the key generation function
if you wish.

=head1 AUTHOR

This version by Ned Konz, perl@bike-nomad.com

=head1 CREDITS

Versions through 0.59 (and much of this documentation) were written by:

Mark-Jason Dominus, mjd-perl-diff@plover.com

This version borrows the documentation and names of the routines
from Mark-Jason's, but has all new code in Diff.pm.

This code was adapted from the Smalltalk code of
Mario Wolczko <mario@wolczko.com>, which is available at
ftp://st.cs.uiuc.edu/pub/Smalltalk/MANCHESTER/manchester/4.0/diff.st

The algorithm is that described in 
I<A Fast Algorithm for Computing Longest Common Subsequences>,
CACM, vol.20, no.5, pp.350-353, May 1977, with a few
minor improvements to improve the speed.

=cut

# Create a hash that maps each element of $aCollection to the set of positions
# it occupies in $aCollection, restricted to the elements within the range of
# indexes specified by $start and $end.
# The fourth parameter is a subroutine reference that will be called to
# generate a string to use as a key.
# Additional parameters, if any, will be passed to this subroutine.
#
# my $hashRef = _withPositionsOfInInterval( \@array, $start, $end, $keyGen );

sub _withPositionsOfInInterval
{
        my $aCollection = shift;        # array ref
        my $start = shift;
        my $end = shift;
        my $keyGen = shift;
        my %d;
        my $index;
        for ( $index = $start; $index <= $end; $index++ )
        {
                my $element = $aCollection->[ $index ];
                my $key = &$keyGen( $element, @_ );
                if ( exists( $d{ $key } ) )
                {
                        push( @{ $d{ $key } }, $index );
                }
                else
                {
                        $d{ $key } = [ $index ];
                }
        }
        return wantarray ? %d: \%d;
}

# Find the place at which aValue would normally be inserted into the array. If
# that place is already occupied by aValue, do nothing, and return undef. If
# the place does not exist (i.e., it is off the end of the array), add it to
# the end, otherwise replace the element at that point with aValue.
# It is assumed that the array's values are numeric.
# This is where the bulk (75%) of the time is spent in this module, so try to
# make it fast!

sub _replaceNextLargerWith
{
        my ( $array, $aValue, $high ) = @_;
        $high ||= $#$array;

        # off the end?
        if ( $high == -1 || $aValue > $array->[ -1 ] )
        {
                push( @$array, $aValue );
                return $high + 1;
        }

        # binary search for insertion point...
        my $low = 0;
        my $index;
        my $found;
        while ( $low <= $high )
        {
                $index = ( $high + $low ) / 2;
#               $index = int(( $high + $low ) / 2);             # without 'use integer'
                $found = $array->[ $index ];

                if ( $aValue == $found )
                {
                        return undef;
                }
                elsif ( $aValue > $found )
                {
                        $low = $index + 1;
                }
                else
                {
                        $high = $index - 1;
                }
        }

        # now insertion point is in $low.
        $array->[ $low ] = $aValue;             # overwrite next larger
        return $low;
}

# This method computes the longest common subsequence in $a and $b.

# Result is array or ref, whose contents is such that
#       $a->[ $i ] = $b->[ $result[ $i ] ]
# foreach $i in ( 0..scalar( @result ) if $result[ $i ] is defined.

# An additional argument may be passed; this is a hash or key generating
# function that should return a string that uniquely identifies the given
# element.  It should be the case that if the key is the same, the elements
# will compare the same. If this parameter is undef or missing, the key
# will be the element as a string.

# By default, comparisons will use "eq" and elements will be turned into keys
# using the default stringizing operator '""'.

# Additional parameters, if any, will be passed to the key generation routine.

sub _longestCommonSubsequence
{
        my $a = shift;  # array ref
        my $b = shift;  # array ref
        my $keyGen = shift;     # code ref
        my $compare;    # code ref

        # set up code refs
        # Note that these are optimized.
        if ( !defined( $keyGen ) )      # optimize for strings
        {
                $keyGen = sub { $_[0] };
                $compare = sub { my ($a, $b) = @_; $a eq $b };
        }
        else
        {
                $compare = sub {
                        my $a = shift; my $b = shift;
                        &$keyGen( $a, @_ ) eq &$keyGen( $b, @_ )
                };
        }

        my ($aStart, $aFinish, $bStart, $bFinish, $matchVector) = (0, $#$a, 0, $#$b, []);

        # First we prune off any common elements at the beginning
        while ( $aStart <= $aFinish
                and $bStart <= $bFinish
                and &$compare( $a->[ $aStart ], $b->[ $bStart ], @_ ) )
        {
                $matchVector->[ $aStart++ ] = $bStart++;
        }

        # now the end
        while ( $aStart <= $aFinish
                and $bStart <= $bFinish
                and &$compare( $a->[ $aFinish ], $b->[ $bFinish ], @_ ) )
        {
                $matchVector->[ $aFinish-- ] = $bFinish--;
        }

        # Now compute the equivalence classes of positions of elements
        my $bMatches = _withPositionsOfInInterval( $b, $bStart, $bFinish, $keyGen, @_ );
        my $thresh = [];
        my $links = [];

        my ( $i, $ai, $j, $k );
        for ( $i = $aStart; $i <= $aFinish; $i++ )
        {
                $ai = &$keyGen( $a->[ $i ] );
                if ( exists( $bMatches->{ $ai } ) )
                {
                        $k = 0;
                        for $j ( reverse( @{ $bMatches->{ $ai } } ) )
                        {
                                # optimization: most of the time this will be true
                                if ( $k
                                        and $thresh->[ $k ] > $j
                                        and $thresh->[ $k - 1 ] < $j )
                                {
                                        $thresh->[ $k ] = $j;
                                }
                                else
                                {
                                        $k = _replaceNextLargerWith( $thresh, $j, $k );
                                }

                                # oddly, it's faster to always test this (CPU cache?).
                                if ( defined( $k ) )
                                {
                                        $links->[ $k ] = 
                                                [ ( $k ? $links->[ $k - 1 ] : undef ), $i, $j ];
                                }
                        }
                }
        }

        if ( @$thresh )
        {
                for ( my $link = $links->[ $#$thresh ]; $link; $link = $link->[ 0 ] )
                {
                        $matchVector->[ $link->[ 1 ] ] = $link->[ 2 ];
                }
        }

        return wantarray ? @$matchVector : $matchVector;
}

sub traverse_sequences
{
        my $a = shift;  # array ref
        my $b = shift;  # array ref
        my $callbacks = shift || { };
        my $keyGen = shift;
        my $matchCallback = $callbacks->{'MATCH'} || sub { };
        my $discardACallback = $callbacks->{'DISCARD_A'} || sub { };
        my $discardBCallback = $callbacks->{'DISCARD_B'} || sub { };
        my $matchVector = _longestCommonSubsequence( $a, $b, $keyGen, @_ );
        # Process all the lines in match vector
        my $lastA = $#$a;
        my $lastB = $#$b;
        my $bi = 0;
        my $ai;
        for ( $ai = 0; $ai <= $#$matchVector; $ai++ )
        {
                my $bLine = $matchVector->[ $ai ];
                if ( defined( $bLine ) )
                {
                        &$discardBCallback( $ai, $bi++, @_ ) while $bi < $bLine;
                        &$matchCallback( $ai, $bi++, @_ );
                }
                else
                {
                        &$discardACallback( $ai, $bi, @_ );
                }
        }

        &$discardACallback( $ai++, $bi, @_ ) while ( $ai <= $lastA );
        &$discardBCallback( $ai, $bi++, @_ ) while ( $bi <= $lastB );
        return 1;
}

sub LCS
{
        my $a = shift;  # array ref
        my $matchVector = _longestCommonSubsequence( $a, @_ );
        my @retval;
        my $i;
        for ( $i = 0; $i <= $#$matchVector; $i++ )
        {
                if ( defined( $matchVector->[ $i ] ) )
                {
                        push( @retval, $a->[ $i ] );
                }
        }
        return wantarray ? @retval : \@retval;
}

sub diff
{
        my $a = shift;  # array ref
        my $b = shift;  # array ref
        my $retval = [];
        my $hunk = [];
        my $discard = sub { push( @$hunk, [ '-', $_[ 0 ], $a->[ $_[ 0 ] ] ] ) };
        my $add = sub { push( @$hunk, [ '+', $_[ 1 ], $b->[ $_[ 1 ] ] ] ) };
        my $match = sub { push( @$retval, $hunk ) if scalar(@$hunk); $hunk = [] };
        traverse_sequences( $a, $b,
                { MATCH => $match, DISCARD_A => $discard, DISCARD_B => $add },
                @_ );
        &$match();
        return wantarray ? @$retval : $retval;
}

1;
1	package Algorithm::Diff;
2	use strict;
3	use vars qw($VERSION @EXPORT_OK @ISA @EXPORT);
4	use integer; # see below in _replaceNextLargerWith() for mod to make
5	# if you don't use this
6	require Exporter;
7	@ISA = qw(Exporter);
8	@EXPORT = qw();
9	@EXPORT_OK = qw(LCS diff traverse_sequences);
10	$VERSION = sprintf('%d.%02d', (q$Revision: 1.10 $ =~ /\d+/g));
11
12	# McIlroy-Hunt diff algorithm
13	# Adapted from the Smalltalk code of Mario I. Wolczko, <mario@wolczko.com>
14	# by Ned Konz, perl@bike-nomad.com
15
16	=head1 NAME
17
18	Algorithm::Diff - Compute `intelligent' differences between two files / lists
19
20	=head1 SYNOPSIS
21
22	use Algorithm::Diff qw(diff LCS traverse_sequences);
23
24	@lcs = LCS( \@seq1, \@seq2 );
25
26	@lcs = LCS( \@seq1, \@seq2, $key_generation_function );
27
28	$lcsref = LCS( \@seq1, \@seq2 );
29
30	$lcsref = LCS( \@seq1, \@seq2, $key_generation_function );
31
32	@diffs = diff( \@seq1, \@seq2 );
33
34	@diffs = diff( \@seq1, \@seq2, $key_generation_function );
35
36	traverse_sequences( \@seq1, \@seq2,
37	{ MATCH => $callback,
38	DISCARD_A => $callback,
39	DISCARD_B => $callback,
40	} );
41
42	traverse_sequences( \@seq1, \@seq2,
43	{ MATCH => $callback,
44	DISCARD_A => $callback,
45	DISCARD_B => $callback,
46	},
47	$key_generation_function );
48
49	=head1 INTRODUCTION
50
51	(by Mark-Jason Dominus)
52
53	I once read an article written by the authors of C<diff>; they said
54	that they hard worked very hard on the algorithm until they found the
55	right one.
56
57	I think what they ended up using (and I hope someone will correct me,
58	because I am not very confident about this) was the `longest common
59	subsequence' method. in the LCS problem, you have two sequences of
60	items:
61
62	a b c d f g h j q z
63
64	a b c d e f g i j k r x y z
65
66	and you want to find the longest sequence of items that is present in
67	both original sequences in the same order. That is, you want to find
68	a new sequence I<S> which can be obtained from the first sequence by
69	deleting some items, and from the secend sequence by deleting other
70	items. You also want I<S> to be as long as possible. In this case
71	I<S> is
72
73	a b c d f g j z
74
75	From there it's only a small step to get diff-like output:
76
77	e h i k q r x y
78	+ - + + - + + +
79
80	This module solves the LCS problem. It also includes a canned
81	function to generate C<diff>-like output.
82
83	It might seem from the example above that the LCS of two sequences is
84	always pretty obvious, but that's not always the case, especially when
85	the two sequences have many repeated elements. For example, consider
86
87	a x b y c z p d q
88	a b c a x b y c z
89
90	A naive approach might start by matching up the C<a> and C<b> that
91	appear at the beginning of each sequence, like this:
92
93	a x b y c z p d q
94	a b c a b y c z
95
96	This finds the common subsequence C<a b c z>. But actually, the LCS
97	is C<a x b y c z>:
98
99	a x b y c z p d q
100	a b c a x b y c z
101
102	=head1 USAGE
103
104	This module provides three exportable functions, which we'll deal with in
105	ascending order of difficulty: C<LCS>, C<diff>, and
106	C<traverse_sequences>.
107
108	=head2 C<LCS>
109
110	Given references to two lists of items, LCS returns an array containing their
111	longest common subsequence. In scalar context, it returns a reference to
112	such a list.
113
114	@lcs = LCS( \@seq1, \@seq2 );
115	$lcsref = LCS( \@seq1, \@seq2 );
116
117	C<LCS> may be passed an optional third parameter; this is a CODE
118	reference to a key generation function. See L</KEY GENERATION
119	FUNCTIONS>.
120
121	@lcs = LCS( \@seq1, \@seq2, $keyGen );
122	$lcsref = LCS( \@seq1, \@seq2, $keyGen );
123
124	Additional parameters, if any, will be passed to the key generation
125	routine.
126
127	=head2 C<diff>
128
129	@diffs = diff( \@seq1, \@seq2 );
130	$diffs_ref = diff( \@seq1, \@seq2 );
131
132	C<diff> computes the smallest set of additions and deletions necessary
133	to turn the first sequence into the second, and returns a description
134	of these changes. The description is a list of I<hunks>; each hunk
135	represents a contiguous section of items which should be added,
136	deleted, or replaced. The return value of C<diff> is a list of
137	hunks, or, in scalar context, a reference to such a list.
138
139	Here is an example: The diff of the following two sequences:
140
141	a b c e h j l m n p
142	b c d e f j k l m r s t
143
144	Result:
145
146	[
147	[ [ '-', 0, 'a' ] ],
148
149	[ [ '+', 2, 'd' ] ],
150
151	[ [ '-', 4, 'h' ] ,
152	[ '+', 4, 'f' ] ],
153
154	[ [ '+', 6, 'k' ] ],
155
156	[ [ '-', 8, 'n' ],
157	[ '-', 9, 'p' ],
158	[ '+', 9, 'r' ],
159	[ '+', 10, 's' ],
160	[ '+', 11, 't' ],
161	]
162	]
163
164	There are five hunks here. The first hunk says that the C<a> at
165	position 0 of the first sequence should be deleted (C<->). The second
166	hunk says that the C<d> at position 2 of the second sequence should
167	be inserted (C<+>). The third hunk says that the C<h> at position 4
168	of the first sequence should be removed and replaced with the C<f>
169	from position 4 of the second sequence. The other two hunks similarly.
170
171	C<diff> may be passed an optional third parameter; this is a CODE
172	reference to a key generation function. See L</KEY GENERATION
173	FUNCTIONS>.
174
175	Additional parameters, if any, will be passed to the key generation
176	routine.
177
178	=head2 C<traverse_sequences>
179
180	C<traverse_sequences> is the most general facility provided by this
181	module; C<diff> and C<LCS> are implemented as calls to it.
182
183	Imagine that there are two arrows. Arrow A points to an element of
184	sequence A, and arrow B points to an element of the sequence B.
185	Initially, the arrows point to the first elements of the respective
186	sequences. C<traverse_sequences> will advance the arrows through the
187	sequences one element at a time, calling an appropriate user-specified
188	callback function before each advance. It willadvance the arrows in
189	such a way that if there are equal elements C<$A[$i]> and C<$B[$j]>
190	which are equal and which are part of the LCS, there will be some
191	moment during the execution of C<traverse_sequences> when arrow A is
192	pointing to C<$A[$i]> and arrow B is pointing to C<$B[$j]>. When this
193	happens, C<traverse_sequences> will call the C<MATCH> callback
194	function and then it will advance both arrows.
195
196	Otherwise, one of the arrows is pointing to an element of its sequence
197	that is not part of the LCS. C<traverse_sequences> will advance that
198	arrow and will call the C<DISCARD_A> or the C<DISCARD_B> callback,
199	depending on which arrow it advanced. If both arrows point to
200	elements that are not part of the LCS, then C<traverse_sequences> will
201	advance one of them and call the appropriate callback, but it is not
202	specified which it will call.
203
204	The arguments to C<traverse_sequences> are the two sequences to
205	traverse, and a callback which specifies the callback functions, like
206	this:
207
208	traverse_sequences( \@seq1, \@seq2,
209	{ MATCH => $callback_1,
210	DISCARD_A => $callback_2,
211	DISCARD_B => $callback_3,
212	} );
213
214	Callbacks are invoked with at least the indices of the two arrows as
215	their arguments. They are not expected to return any values. If a
216	callback is omitted from the table, it is not called.
217
218	If arrow A reaches the end of its sequence, before arrow B does,
219	C<traverse_sequences> will call the C<A_FINISHED> callback when it
220	advances arrow B, if there is such a function; if not it will call
221	C<DISCARD_B> instead. Similarly if arrow B finishes first.
222	C<traverse_sequences> returns when both arrows are at the ends of
223	their respective sequences. It returns true on success and false on
224	failure. At present there is no way to fail.
225
226	C<traverse_sequences> may be passed an optional fourth parameter; this
227	is a CODE reference to a key generation function. See L</KEY
228	GENERATION FUNCTIONS>.
229
230	Additional parameters, if any, will be passed to the key generation
231	function.
232
233	=head1 KEY GENERATION FUNCTIONS
234
235	C<diff>, C<LCS>, and C<traverse_sequences> accept an optional last parameter.
236	This is a CODE reference to a key generating (hashing) function that should
237	return a string that uniquely identifies a given element.
238	It should be the case that if two elements are to be considered equal,
239	their keys should be the same (and the other way around).
240	If no key generation function is provided, the key will be the
241	element as a string.
242
243	By default, comparisons will use "eq" and elements will be turned into keys
244	using the default stringizing operator '""'.
245
246	Where this is important is when you're comparing something other than
247	strings. If it is the case that you have multiple different objects
248	that should be considered to be equal, you should supply a key
249	generation function. Otherwise, you have to make sure that your arrays
250	contain unique references.
251
252	For instance, consider this example:
253
254	package Person;
255
256	sub new
257	{
258	my $package = shift;
259	return bless { name => '', ssn => '', @_ }, $package;
260	}
261
262	sub clone
263	{
264	my $old = shift;
265	my $new = bless { %$old }, ref($old);
266	}
267
268	sub hash
269	{
270	return shift()->{'ssn'};
271	}
272
273	my $person1 = Person->new( name => 'Joe', ssn => '123-45-6789' );
274	my $person2 = Person->new( name => 'Mary', ssn => '123-47-0000' );
275	my $person3 = Person->new( name => 'Pete', ssn => '999-45-2222' );
276	my $person4 = Person->new( name => 'Peggy', ssn => '123-45-9999' );
277	my $person5 = Person->new( name => 'Frank', ssn => '000-45-9999' );
278
279	If you did this:
280
281	my $array1 = [ $person1, $person2, $person4 ];
282	my $array2 = [ $person1, $person3, $person4, $person5 ];
283	Algorithm::Diff::diff( $array1, $array2 );
284
285	everything would work out OK (each of the objects would be converted
286	into a string like "Person=HASH(0x82425b0)" for comparison).
287
288	But if you did this:
289
290	my $array1 = [ $person1, $person2, $person4 ];
291	my $array2 = [ $person1, $person3, $person4->clone(), $person5 ];
292	Algorithm::Diff::diff( $array1, $array2 );
293
294	$person4 and $person4->clone() (which have the same name and SSN)
295	would be seen as different objects. If you wanted them to be considered
296	equivalent, you would have to pass in a key generation function:
297
298	my $array1 = [ $person1, $person2, $person4 ];
299	my $array2 = [ $person1, $person3, $person4->clone(), $person5 ];
300	Algorithm::Diff::diff( $array1, $array2, \&Person::hash );
301
302	This would use the 'ssn' field in each Person as a comparison key, and
303	so would consider $person4 and $person4->clone() as equal.
304
305	You may also pass additional parameters to the key generation function
306	if you wish.
307
308	=head1 AUTHOR
309
310	This version by Ned Konz, perl@bike-nomad.com
311
312	=head1 CREDITS
313
314	Versions through 0.59 (and much of this documentation) were written by:
315
316	Mark-Jason Dominus, mjd-perl-diff@plover.com
317
318	This version borrows the documentation and names of the routines
319	from Mark-Jason's, but has all new code in Diff.pm.
320
321	This code was adapted from the Smalltalk code of
322	Mario Wolczko <mario@wolczko.com>, which is available at
323	ftp://st.cs.uiuc.edu/pub/Smalltalk/MANCHESTER/manchester/4.0/diff.st
324
325	The algorithm is that described in
326	I<A Fast Algorithm for Computing Longest Common Subsequences>,
327	CACM, vol.20, no.5, pp.350-353, May 1977, with a few
328	minor improvements to improve the speed.
329
330	=cut
331
332	# Create a hash that maps each element of $aCollection to the set of positions
333	# it occupies in $aCollection, restricted to the elements within the range of
334	# indexes specified by $start and $end.
335	# The fourth parameter is a subroutine reference that will be called to
336	# generate a string to use as a key.
337	# Additional parameters, if any, will be passed to this subroutine.
338	#
339	# my $hashRef = _withPositionsOfInInterval( \@array, $start, $end, $keyGen );
340
341	sub _withPositionsOfInInterval
342	{
343	my $aCollection = shift; # array ref
344	my $start = shift;
345	my $end = shift;
346	my $keyGen = shift;
347	my %d;
348	my $index;
349	for ( $index = $start; $index <= $end; $index++ )
350	{
351	my $element = $aCollection->[ $index ];
352	my $key = &$keyGen( $element, @_ );
353	if ( exists( $d{ $key } ) )
354	{
355	push( @{ $d{ $key } }, $index );
356	}
357	else
358	{
359	$d{ $key } = [ $index ];
360	}
361	}
362	return wantarray ? %d: \%d;
363	}
364
365	# Find the place at which aValue would normally be inserted into the array. If
366	# that place is already occupied by aValue, do nothing, and return undef. If
367	# the place does not exist (i.e., it is off the end of the array), add it to
368	# the end, otherwise replace the element at that point with aValue.
369	# It is assumed that the array's values are numeric.
370	# This is where the bulk (75%) of the time is spent in this module, so try to
371	# make it fast!
372
373	sub _replaceNextLargerWith
374	{
375	my ( $array, $aValue, $high ) = @_;
376	$high \|\|= $#$array;
377
378	# off the end?
379	if ( $high == -1 \|\| $aValue > $array->[ -1 ] )
380	{
381	push( @$array, $aValue );
382	return $high + 1;
383	}
384
385	# binary search for insertion point...
386	my $low = 0;
387	my $index;
388	my $found;
389	while ( $low <= $high )
390	{
391	$index = ( $high + $low ) / 2;
392	# $index = int(( $high + $low ) / 2); # without 'use integer'
393	$found = $array->[ $index ];
394
395	if ( $aValue == $found )
396	{
397	return undef;
398	}
399	elsif ( $aValue > $found )
400	{
401	$low = $index + 1;
402	}
403	else
404	{
405	$high = $index - 1;
406	}
407	}
408
409	# now insertion point is in $low.
410	$array->[ $low ] = $aValue; # overwrite next larger
411	return $low;
412	}
413
414	# This method computes the longest common subsequence in $a and $b.
415
416	# Result is array or ref, whose contents is such that
417	# $a->[ $i ] = $b->[ $result[ $i ] ]
418	# foreach $i in ( 0..scalar( @result ) if $result[ $i ] is defined.
419
420	# An additional argument may be passed; this is a hash or key generating
421	# function that should return a string that uniquely identifies the given
422	# element. It should be the case that if the key is the same, the elements
423	# will compare the same. If this parameter is undef or missing, the key
424	# will be the element as a string.
425
426	# By default, comparisons will use "eq" and elements will be turned into keys
427	# using the default stringizing operator '""'.
428
429	# Additional parameters, if any, will be passed to the key generation routine.
430
431	sub _longestCommonSubsequence
432	{
433	my $a = shift; # array ref
434	my $b = shift; # array ref
435	my $keyGen = shift; # code ref
436	my $compare; # code ref
437
438	# set up code refs
439	# Note that these are optimized.
440	if ( !defined( $keyGen ) ) # optimize for strings
441	{
442	$keyGen = sub { $_[0] };
443	$compare = sub { my ($a, $b) = @_; $a eq $b };
444	}
445	else
446	{
447	$compare = sub {
448	my $a = shift; my $b = shift;
449	&$keyGen( $a, @_ ) eq &$keyGen( $b, @_ )
450	};
451	}
452
453	my ($aStart, $aFinish, $bStart, $bFinish, $matchVector) = (0, $#$a, 0, $#$b, []);
454
455	# First we prune off any common elements at the beginning
456	while ( $aStart <= $aFinish
457	and $bStart <= $bFinish
458	and &$compare( $a->[ $aStart ], $b->[ $bStart ], @_ ) )
459	{
460	$matchVector->[ $aStart++ ] = $bStart++;
461	}
462
463	# now the end
464	while ( $aStart <= $aFinish
465	and $bStart <= $bFinish
466	and &$compare( $a->[ $aFinish ], $b->[ $bFinish ], @_ ) )
467	{
468	$matchVector->[ $aFinish-- ] = $bFinish--;
469	}
470
471	# Now compute the equivalence classes of positions of elements
472	my $bMatches = _withPositionsOfInInterval( $b, $bStart, $bFinish, $keyGen, @_ );
473	my $thresh = [];
474	my $links = [];
475
476	my ( $i, $ai, $j, $k );
477	for ( $i = $aStart; $i <= $aFinish; $i++ )
478	{
479	$ai = &$keyGen( $a->[ $i ] );
480	if ( exists( $bMatches->{ $ai } ) )
481	{
482	$k = 0;
483	for $j ( reverse( @{ $bMatches->{ $ai } } ) )
484	{
485	# optimization: most of the time this will be true
486	if ( $k
487	and $thresh->[ $k ] > $j
488	and $thresh->[ $k - 1 ] < $j )
489	{
490	$thresh->[ $k ] = $j;
491	}
492	else
493	{
494	$k = _replaceNextLargerWith( $thresh, $j, $k );
495	}
496
497	# oddly, it's faster to always test this (CPU cache?).
498	if ( defined( $k ) )
499	{
500	$links->[ $k ] =
501	[ ( $k ? $links->[ $k - 1 ] : undef ), $i, $j ];
502	}
503	}
504	}
505	}
506
507	if ( @$thresh )
508	{
509	for ( my $link = $links->[ $#$thresh ]; $link; $link = $link->[ 0 ] )
510	{
511	$matchVector->[ $link->[ 1 ] ] = $link->[ 2 ];
512	}
513	}
514
515	return wantarray ? @$matchVector : $matchVector;
516	}
517
518	sub traverse_sequences
519	{
520	my $a = shift; # array ref
521	my $b = shift; # array ref
522	my $callbacks = shift \|\| { };
523	my $keyGen = shift;
524	my $matchCallback = $callbacks->{'MATCH'} \|\| sub { };
525	my $discardACallback = $callbacks->{'DISCARD_A'} \|\| sub { };
526	my $discardBCallback = $callbacks->{'DISCARD_B'} \|\| sub { };
527	my $matchVector = _longestCommonSubsequence( $a, $b, $keyGen, @_ );
528	# Process all the lines in match vector
529	my $lastA = $#$a;
530	my $lastB = $#$b;
531	my $bi = 0;
532	my $ai;
533	for ( $ai = 0; $ai <= $#$matchVector; $ai++ )
534	{
535	my $bLine = $matchVector->[ $ai ];
536	if ( defined( $bLine ) )
537	{
538	&$discardBCallback( $ai, $bi++, @_ ) while $bi < $bLine;
539	&$matchCallback( $ai, $bi++, @_ );
540	}
541	else
542	{
543	&$discardACallback( $ai, $bi, @_ );
544	}
545	}
546
547	&$discardACallback( $ai++, $bi, @_ ) while ( $ai <= $lastA );
548	&$discardBCallback( $ai, $bi++, @_ ) while ( $bi <= $lastB );
549	return 1;
550	}
551
552	sub LCS
553	{
554	my $a = shift; # array ref
555	my $matchVector = _longestCommonSubsequence( $a, @_ );
556	my @retval;
557	my $i;
558	for ( $i = 0; $i <= $#$matchVector; $i++ )
559	{
560	if ( defined( $matchVector->[ $i ] ) )
561	{
562	push( @retval, $a->[ $i ] );
563	}
564	}
565	return wantarray ? @retval : \@retval;
566	}
567
568	sub diff
569	{
570	my $a = shift; # array ref
571	my $b = shift; # array ref
572	my $retval = [];
573	my $hunk = [];
574	my $discard = sub { push( @$hunk, [ '-', $_[ 0 ], $a->[ $_[ 0 ] ] ] ) };
575	my $add = sub { push( @$hunk, [ '+', $_[ 1 ], $b->[ $_[ 1 ] ] ] ) };
576	my $match = sub { push( @$retval, $hunk ) if scalar(@$hunk); $hunk = [] };
577	traverse_sequences( $a, $b,
578	{ MATCH => $match, DISCARD_A => $discard, DISCARD_B => $add },
579	@_ );
580	&$match();
581	return wantarray ? @$retval : $retval;
582	}
583
584	1;