NAME
    Algorithm::Merge - Three-way merge and diff

SYNOPSIS
     use Algorithm::Merge qw(merge diff3 traverse_sequences3);

     @merged = merge(\@ancestor, \@a, \@b, { 
                   CONFLICT => sub { } 
               });

     @merged = merge(\@ancestor, \@a, \@b, { 
                   CONFLICT => sub { } 
               }, $key_generation_function);

     $merged = merge(\@ancestor, \@a, \@b, { 
                   CONFLICT => sub { } 
               });

     $merged = merge(\@ancestor, \@a, \@b, { 
                   CONFLICT => sub { } 
               }, $key_generation_function);

     @diff   = diff3(\@ancestor, \@a, \@b);

     @diff   = diff3(\@ancestor, \@a, \@b, $key_generation_function);

     $diff   = diff3(\@ancestor, \@a, \@b);

     $diff   = diff3(\@ancestor, \@a, \@b, $key_generation_function);

     @trav   = traverse_sequences3(\@ancestor, \@a, \@b, { 
                   # callbacks
               });

     @trav   = traverse_sequences3(\@ancestor, \@a, \@b, { 
                   # callbacks
               }, $key_generation_function);

     $trav   = traverse_sequences3(\@ancestor, \@a, \@b, { 
                   # callbacks
               });

     $trav   = traverse_sequences3(\@ancestor, \@a, \@b, { 
                   # callbacks
               }, $key_generation_function);

USAGE
    This module complements Algorithm::Diff by providing three-way merge and
    diff functions.

    In this documentation, the first list to "diff3", "merge", and
    "traverse_sequences3" is called the `original' list. The second list is
    the `left' list. The third list is the `right' list.

    The optional key generation arguments are the same as in
    Algorithm::Diff. See the Algorithm::Diff manpage for more information.

  diff3

    Given references to three lists of items, "diff3" performs a three-way
    difference.

    This function returns an array of operations describing how the left and
    right lists differ from the original list. In scalar context, this
    function returns a reference to such an array.

    Perhaps an example would be useful.

    Given the following three lists,

      original: a b c   e f   h i   k
          left: a b   d e f g   i j k
         right: a b c d e     h i j k

         merge: a b   d e   g   i j k

    we have the following result from diff3:

     [ 'u', 'a',   'a',   'a' ],
     [ 'u', 'b',   'b',   'b' ],
     [ 'l', 'c',   undef, 'c' ],
     [ 'o', undef, 'd',   'd' ],
     [ 'u', 'e',   'e',   'e' ],
     [ 'r', 'f',   'f',   undef ], 
     [ 'o', 'h',   'g',   'h' ],
     [ 'u', 'i',   'i',   'i' ],
     [ 'o', undef, 'j',   'j' ],
     [ 'u', 'k',   'k',   'k' ]

    The first element in each row is the array with the difference:

     c - conflict (no two are the same)
     l - left is different 
     o - original is different
     r - right is different
     u - unchanged

    The next three elements are the lists from the original, left, and right
    arrays respectively that the row refers to (in the synopsis, these are
    "@ancestor", "@a", and "@b", respectively).

  merge

    Given references to three lists of items, "merge" performs a three-way
    merge. The "merge" function uses the "diff3" function to do most of the
    work.

    The only callback currently used is "CONFLICT" which should be a
    reference to a subroutine that accepts two array references. The first
    array reference is to a list of elements from the left list. The second
    array reference is to a list of elements from the right list. This
    callback should return a list of elements to place in the merged list in
    place of the conflict.

    The default "CONFLICT" callback returns the following:

     q{<!-- ------ START CONFLICT ------ -->},
     (@left),
     q{<!-- ---------------------------- -->},
     (@right),
     q{<!-- ------  END  CONFLICT ------ -->},

  traverse_sequences3

    This is the workhorse function that goes through the three sequences and
    calls the callback functions.

    The following callbacks are supported.

    NO_CHANGE
        This is called if all three sequences have the same element at the
        current position. The arguments are the current positions within
        each sequence, the first argument being the current position within
        the first sequence.

    A_DIFF
        This is called if the first sequence is different than the other two
        sequences at the current position. This callback will be called with
        one, two, or three arguments.

        If one argument, then only the element at the given position from
        the first sequence is not in either of the other two sequences.

        If two arguments, then there is no element in the first sequence
        that corresponds to the elements at the given positions in the
        second and third sequences.

        If three arguments, then the element at the given position in the
        first sequence is different than the corresponding element in the
        other two sequences, but the other two sequences have corresponding
        elements.

    B_DIFF
        This is called if the second sequence is different than the other
        two sequences at the current position. This callback will be called
        with one, two, or three arguments.

        If one argument, then only the element at the given position from
        the second sequence is not in either of the other two sequences.

        If two arguments, then there is no element in the second sequence
        that corresponds to the elements at the given positions in the first
        and third sequences.

        If three arguments, then the element at the given position in the
        second sequence is different than the corresponding element in the
        other two sequences, but the other two sequences have corresponding
        elements.

    C_DIFF
        This is called if the third sequence is different than the other two
        sequences at the current position. This callback will be called with
        one, two, or three arguments.

        If one argument, then only the element at the given position from
        the third sequence is not in either of the other two sequences.

        If two arguments, then there is no element in the third sequence
        that corresponds to the elements at the given positions in the first
        and second sequences.

        If three arguments, then the element at the given position in the
        third sequence is different than the corresponding element in the
        other two sequences, but the other two sequences have corresponding
        elements.

    CONFLICT
        This is called if all three sequences have different elements at the
        current position. The three arguments are the current positions
        within each sequence.

BUGS
    Most assuredly there are bugs. If a pattern similar to the above example
    does not work, send it to <jsmith@cpan.org> or report it on
    <http://rt.cpan.org/>, the CPAN bug tracker.

    Algorithm::Diff's implementation of "traverse_sequences" may not be
    symmetric with respect to the input sequences if the second and third
    sequence are of different lengths. Because of this,
    "traverse_sequences3" will calculate the diffs of the second and third
    sequences as passed and swapped. If the differences are not the same, it
    will issue an `Algorithm::Diff::diff is not symmetric for second and
    third sequences...' warning. It will try to handle this, but there may
    be some cases where it can't.

SEE ALSO
    the Algorithm::Diff manpage.

AUTHOR
    James G. Smith, <jsmith@cpan.org>

COPYRIGHT
    Copyright (C) 2003 Texas A&M University. All Rights Reserved.

    This module is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.

