Algorithm::DiffOld(3) User Contributed Perl Documentation Algorithm::DiffOld(3)

# [NAME](#NAME)

Algorithm::DiffOld - Compute \`intelligent' differences between two files / lists but use the old (<=0.59) interface.

# [NOTE](#NOTE)

This has been provided as part of the Algorithm::Diff package by Ned Konz. This particular module is **ONLY** for people who **HAVE** to have the old interface, which uses a comparison function rather than a key generating function.

Because each of the lines in one array have to be compared with each of the lines in the other array, this does M\*N comparisons. This can be very slow. I clocked it at taking 18 times as long as the stock version of Algorithm::Diff for a 4000-line file. It will get worse quadratically as array sizes increase.

# [SYNOPSIS](#SYNOPSIS)

```
  use Algorithm::DiffOld qw(diff LCS traverse_sequences);
  @lcs    = LCS( \@seq1, \@seq2, $comparison_function );
  $lcsref = LCS( \@seq1, \@seq2, $comparison_function );
  @diffs = diff( \@seq1, \@seq2, $comparison_function );
  
  traverse_sequences( \@seq1, \@seq2,
                     { MATCH => $callback,
                       DISCARD_A => $callback,
                       DISCARD_B => $callback,
                     },
                     $comparison_function );
```

# [COMPARISON FUNCTIONS](#COMPARISON_FUNCTIONS)

Each of the main routines should be passed a comparison function. If you aren't passing one in, **use Algorithm::Diff instead**.

These functions should return a true value when two items should compare as equal.

For instance,

```
  @lcs    = LCS( \@seq1, \@seq2, sub { my ($a, $b) = @_; $a eq $b } );
```

but if that is all you're doing with your comparison function, just use Algorithm::Diff and let it do this (this is its default).

Or:

```
  sub someFunkyComparisonFunction
  {
        my ($a, $b) = @_;
        $a =~ m{$b};
  }
  @diffs = diff( \@lines, \@patterns, \&someFunkyComparisonFunction );
```

which would allow you to diff an array @lines which consists of text lines with an array @patterns which consists of regular expressions.

This is actually the reason I wrote this version -- there is no way to do this with a key generation function as in the stock Algorithm::Diff.

2026-10-24 perl v5.44.0

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Document: Algorithm::DiffOld(3pm) from perl-algorithm-diff 1.201-r7 (Chainguard OS)
Source: https://cpan.metacpan.org/authors/id/R/RJ/RJBS/Algorithm-Diff-1.201.tar.gz