12 my ($root, $parent_fetcher, $cache) = @_;
15 my @STACK; # stack for simulating recursion
17 my $pfetcher_is_coderef = ref($parent_fetcher) eq 'CODE';
19 unless ($pfetcher_is_coderef or $root->can($parent_fetcher)) {
20 confess "Could not find method $parent_fetcher in $root";
23 my $current_root = $root;
24 my $current_parents = [ $root->$parent_fetcher ];
25 my $recurse_mergeout = [];
30 if($i < @$current_parents) {
31 my $new_root = $current_parents->[$i++];
32 die "Infinite loop detected" if $seen{$new_root}++;
34 unless ($pfetcher_is_coderef or $new_root->can($parent_fetcher)) {
35 confess "Could not find method $parent_fetcher in $new_root";
45 $current_root = $new_root;
46 $current_parents = $cache->{pfetch}->{$current_root} ||= [ $current_root->$parent_fetcher ];
47 $recurse_mergeout = [];
52 my $mergeout = $cache->{merge}->{$current_root} ||= do {
54 # This do-block is the code formerly known as the function
55 # that was a perl-port of the python code at
56 # http://www.python.org/2.3/mro.html :)
58 # Initial set (make sure everything is copied - it will be modded)
59 my @seqs = map { [@$_] } (@$recurse_mergeout, $current_parents);
61 # Construct the tail-checking hash
63 foreach my $seq (@seqs) {
64 $tails{$_}++ for (@$seq[1..$#$seq]);
67 my @res = ( $current_root );
73 if(!$winner) { # looking for a winner
74 $cand = $_->[0]; # seq head is candidate
75 next if $tails{$cand}; # he loses if in %tails
76 push @res => $winner = $cand;
78 if($_->[0] eq $winner) {
79 shift @$_; # strip off our winner
80 $tails{$_->[0]}-- if @$_; # keep %tails sane
84 die q{Inconsistent hierarchy found while merging '}
85 . $current_root . qq{':\n\t}
86 . qq{current merge results [\n\t\t}
87 . (join ",\n\t\t" => @res)
88 . qq{\n\t]\n\t} . qq{merging failed on '$cand'\n}
94 return @$mergeout if !@STACK;
96 ($current_root, $current_parents, $recurse_mergeout, $i)
99 push(@$recurse_mergeout, $mergeout);
111 Algorithm::C3 - A module for merging hierarchies using the C3 algorithm
117 # merging a classic diamond
118 # inheritence graph like this:
126 my @merged = Algorithm::C3::merge(
129 # extract the ISA array
136 print join ", " => @merged; # prints D, B, C, A
140 This module implements the C3 algorithm. I have broken this out
141 into it's own module because I found myself copying and pasting
142 it way too often for various needs. Most of the uses I have for
143 C3 revolve around class building and metamodels, but it could
144 also be used for things like dependency resolution as well since
145 it tends to do such a nice job of preserving local precendence
148 Below is a brief explanation of C3 taken from the L<Class::C3>
149 module. For more detailed information, see the L<SEE ALSO> section
154 C3 is the name of an algorithm which aims to provide a sane method
155 resolution order under multiple inheritence. It was first introduced
156 in the langauge Dylan (see links in the L<SEE ALSO> section), and
157 then later adopted as the prefered MRO (Method Resolution Order)
158 for the new-style classes in Python 2.3. Most recently it has been
159 adopted as the 'canonical' MRO for Perl 6 classes, and the default
160 MRO for Parrot objects as well.
162 =head2 How does C3 work.
164 C3 works by always preserving local precendence ordering. This
165 essentially means that no class will appear before any of it's
166 subclasses. Take the classic diamond inheritence pattern for
175 The standard Perl 5 MRO would be (D, B, A, C). The result being that
176 B<A> appears before B<C>, even though B<C> is the subclass of B<A>.
177 The C3 MRO algorithm however, produces the following MRO (D, B, C, A),
178 which does not have this same issue.
180 This example is fairly trival, for more complex examples and a deeper
181 explaination, see the links in the L<SEE ALSO> section.
187 =item B<merge ($root, $func_to_fetch_parent, $cache)>
189 This takes a C<$root> node, which can be anything really it
190 is up to you. Then it takes a C<$func_to_fetch_parent> which
191 can be either a CODE reference (see L<SYNOPSIS> above for an
192 example), or a string containing a method name to be called
193 on all the items being linearized. An example of how this
209 our @ISA = ('B', 'C');
212 print join ", " => Algorithm::C3::merge('D', 'supers');
214 The purpose of C<$func_to_fetch_parent> is to provide a way
215 for C<merge> to extract the parents of C<$root>. This is
216 needed for C3 to be able to do it's work.
218 The C<$cache> parameter is an entirely optional performance
219 measure, and should not change behavior.
221 If supplied, it should be a hashref that merge can use as a
222 private cache between runs to speed things up. Generally
223 speaking, if you will be calling merge many times on related
224 things, and the parent fetching function will return constant
225 results given the same arguments during all of these calls,
226 you can and should reuse the same shared cache hash for all
227 of the calls. Example:
229 sub do_some_merging {
231 my @foo_mro = Algorithm::C3::Merge('Foo', \&get_supers, \%merge_cache);
232 my @bar_mro = Algorithm::C3::Merge('Bar', \&get_supers, \%merge_cache);
233 my @baz_mro = Algorithm::C3::Merge('Baz', \&get_supers, \%merge_cache);
234 my @quux_mro = Algorithm::C3::Merge('Quux', \&get_supers, \%merge_cache);
242 I use B<Devel::Cover> to test the code coverage of my tests, below
243 is the B<Devel::Cover> report on this module's test suite.
245 ------------------------ ------ ------ ------ ------ ------ ------ ------
246 File stmt bran cond sub pod time total
247 ------------------------ ------ ------ ------ ------ ------ ------ ------
248 Algorithm/C3.pm 100.0 100.0 100.0 100.0 100.0 100.0 100.0
249 ------------------------ ------ ------ ------ ------ ------ ------ ------
250 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0
251 ------------------------ ------ ------ ------ ------ ------ ------ ------
255 =head2 The original Dylan paper
259 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
263 =head2 The prototype Perl 6 Object Model uses C3
267 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
271 =head2 Parrot now uses C3
275 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
277 =item L<http://use.perl.org/~autrijus/journal/25768>
281 =head2 Python 2.3 MRO related links
285 =item L<http://www.python.org/2.3/mro.html>
287 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
291 =head2 C3 for TinyCLOS
295 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
301 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
303 Brandon L. Black, E<lt>blblack@gmail.comE<gt>
305 =head1 COPYRIGHT AND LICENSE
307 Copyright 2006 by Infinity Interactive, Inc.
309 L<http://www.iinteractive.com>
311 This library is free software; you can redistribute it and/or modify
312 it under the same terms as Perl itself.