``````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

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.