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


  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 );


(by Mark-Jason Dominus)

I once read an article written by the authors of 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 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 S to be as long as possible. In this case 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 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 a and 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 a b c z. But actually, the LCS is a x b y c z:

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


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


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 );

LCS may be passed an optional third parameter; this is a CODE reference to a key generation function. See /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.


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

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 hunks; each hunk represents a contiguous section of items which should be added, deleted, or replaced. The return value of 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


   [ [ '-', 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 a at position 0 of the first sequence should be deleted (-). The second hunk says that the d at position 2 of the second sequence should be inserted (+). The third hunk says that the h at position 4 of the first sequence should be removed and replaced with the f from position 4 of the second sequence. The other two hunks similarly.

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

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


traverse_sequences is the most general facility provided by this module; diff and 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. 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 $A[$i] and $B[$j] which are equal and which are part of the LCS, there will be some moment during the execution of traverse_sequences when arrow A is pointing to $A[$i] and arrow B is pointing to $B[$j]. When this happens, traverse_sequences will call the 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. traverse_sequences will advance that arrow and will call the DISCARD_A or the DISCARD_B callback, depending on which arrow it advanced. If both arrows point to elements that are not part of the LCS, then traverse_sequences will advance one of them and call the appropriate callback, but it is not specified which it will call.

The arguments to 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, traverse_sequences will call the A_FINISHED callback when it advances arrow B, if there is such a function; if not it will call DISCARD_B instead. Similarly if arrow B finishes first. 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.

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

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


diff, LCS, and 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.


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


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 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.

rhew.org (c) 2001 James Rhew