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 = [];
27 my %seen = ( $root => 1 );
30 if($i < @$current_parents) {
31 my $new_root = $current_parents->[$i++];
33 if($seen{$new_root}) {
35 (map { $_->[0] } @STACK),
39 shift @isastack while $isastack[0] ne $new_root;
40 my $isastack = join(q{ -> }, @isastack);
41 die "Infinite loop detected in parents of '$root': $isastack";
45 unless ($pfetcher_is_coderef or $new_root->can($parent_fetcher)) {
46 confess "Could not find method $parent_fetcher in $new_root";
56 $current_root = $new_root;
57 $current_parents = $cache->{pfetch}->{$current_root} ||= [ $current_root->$parent_fetcher ];
58 $recurse_mergeout = [];
63 $seen{$current_root} = 0;
65 my $mergeout = $cache->{merge}->{$current_root} ||= do {
67 # This do-block is the code formerly known as the function
68 # that was a perl-port of the python code at
69 # http://www.python.org/2.3/mro.html :)
71 # Initial set (make sure everything is copied - it will be modded)
72 my @seqs = map { [@$_] } (@$recurse_mergeout, $current_parents);
74 # Construct the tail-checking hash
76 foreach my $seq (@seqs) {
77 $tails{$_}++ for (@$seq[1..$#$seq]);
80 my @res = ( $current_root );
86 if(!$winner) { # looking for a winner
87 $cand = $_->[0]; # seq head is candidate
88 next if $tails{$cand}; # he loses if in %tails
89 push @res => $winner = $cand;
91 if($_->[0] eq $winner) {
92 shift @$_; # strip off our winner
93 $tails{$_->[0]}-- if @$_; # keep %tails sane
97 die q{Inconsistent hierarchy found while merging '}
98 . $current_root . qq{':\n\t}
99 . qq{current merge results [\n\t\t}
100 . (join ",\n\t\t" => @res)
101 . qq{\n\t]\n\t} . qq{merging failed on '$cand'\n}
107 return @$mergeout if !@STACK;
109 ($current_root, $current_parents, $recurse_mergeout, $i)
112 push(@$recurse_mergeout, $mergeout);
124 Algorithm::C3 - A module for merging hierarchies using the C3 algorithm
130 # merging a classic diamond
131 # inheritence graph like this:
139 my @merged = Algorithm::C3::merge(
142 # extract the ISA array
149 print join ", " => @merged; # prints D, B, C, A
153 This module implements the C3 algorithm. I have broken this out
154 into it's own module because I found myself copying and pasting
155 it way too often for various needs. Most of the uses I have for
156 C3 revolve around class building and metamodels, but it could
157 also be used for things like dependency resolution as well since
158 it tends to do such a nice job of preserving local precendence
161 Below is a brief explanation of C3 taken from the L<Class::C3>
162 module. For more detailed information, see the L<SEE ALSO> section
167 C3 is the name of an algorithm which aims to provide a sane method
168 resolution order under multiple inheritence. It was first introduced
169 in the langauge Dylan (see links in the L<SEE ALSO> section), and
170 then later adopted as the prefered MRO (Method Resolution Order)
171 for the new-style classes in Python 2.3. Most recently it has been
172 adopted as the 'canonical' MRO for Perl 6 classes, and the default
173 MRO for Parrot objects as well.
175 =head2 How does C3 work.
177 C3 works by always preserving local precendence ordering. This
178 essentially means that no class will appear before any of it's
179 subclasses. Take the classic diamond inheritence pattern for
188 The standard Perl 5 MRO would be (D, B, A, C). The result being that
189 B<A> appears before B<C>, even though B<C> is the subclass of B<A>.
190 The C3 MRO algorithm however, produces the following MRO (D, B, C, A),
191 which does not have this same issue.
193 This example is fairly trival, for more complex examples and a deeper
194 explaination, see the links in the L<SEE ALSO> section.
200 =item B<merge ($root, $func_to_fetch_parent, $cache)>
202 This takes a C<$root> node, which can be anything really it
203 is up to you. Then it takes a C<$func_to_fetch_parent> which
204 can be either a CODE reference (see L<SYNOPSIS> above for an
205 example), or a string containing a method name to be called
206 on all the items being linearized. An example of how this
222 our @ISA = ('B', 'C');
225 print join ", " => Algorithm::C3::merge('D', 'supers');
227 The purpose of C<$func_to_fetch_parent> is to provide a way
228 for C<merge> to extract the parents of C<$root>. This is
229 needed for C3 to be able to do it's work.
231 The C<$cache> parameter is an entirely optional performance
232 measure, and should not change behavior.
234 If supplied, it should be a hashref that merge can use as a
235 private cache between runs to speed things up. Generally
236 speaking, if you will be calling merge many times on related
237 things, and the parent fetching function will return constant
238 results given the same arguments during all of these calls,
239 you can and should reuse the same shared cache hash for all
240 of the calls. Example:
242 sub do_some_merging {
244 my @foo_mro = Algorithm::C3::Merge('Foo', \&get_supers, \%merge_cache);
245 my @bar_mro = Algorithm::C3::Merge('Bar', \&get_supers, \%merge_cache);
246 my @baz_mro = Algorithm::C3::Merge('Baz', \&get_supers, \%merge_cache);
247 my @quux_mro = Algorithm::C3::Merge('Quux', \&get_supers, \%merge_cache);
255 I use B<Devel::Cover> to test the code coverage of my tests, below
256 is the B<Devel::Cover> report on this module's test suite.
258 ------------------------ ------ ------ ------ ------ ------ ------ ------
259 File stmt bran cond sub pod time total
260 ------------------------ ------ ------ ------ ------ ------ ------ ------
261 Algorithm/C3.pm 100.0 100.0 100.0 100.0 100.0 100.0 100.0
262 ------------------------ ------ ------ ------ ------ ------ ------ ------
263 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0
264 ------------------------ ------ ------ ------ ------ ------ ------ ------
268 =head2 The original Dylan paper
272 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
276 =head2 The prototype Perl 6 Object Model uses C3
280 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
284 =head2 Parrot now uses C3
288 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
290 =item L<http://use.perl.org/~autrijus/journal/25768>
294 =head2 Python 2.3 MRO related links
298 =item L<http://www.python.org/2.3/mro.html>
300 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
304 =head2 C3 for TinyCLOS
308 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
314 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
316 Brandon L. Black, E<lt>blblack@gmail.comE<gt>
318 =head1 COPYRIGHT AND LICENSE
320 Copyright 2006 by Infinity Interactive, Inc.
322 L<http://www.iinteractive.com>
324 This library is free software; you can redistribute it and/or modify
325 it under the same terms as Perl itself.