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=head1 NAME

Algorithm::Merge - Three-way merge and diff

=head1 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);


=head1 USAGE

This module complements L<Algorithm::Diff|Algorithm::Diff> by 
providing three-way merge and diff functions.

In this documentation, the first list to C<diff3>, C<merge>, and 
C<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 
L<Algorithm::Diff|Algorithm::Diff>.  See L<Algorithm::Diff> for more 
information.

=head2 diff3

Given references to three lists of items, C<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 C<@ancestor>, C<@a>, and C<@b>, respectively).

=head2 merge

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

The only callback currently used is C<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 C<CONFLICT> callback returns the following:

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

=head2 traverse_sequences3

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

The following callbacks are supported.

=over 4

=item 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.

=item 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.

=item B_DIFF

This is called if the second sequence is different than the other two