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 $recurse_mergeout = [];
29 if($i < @$current_parents) {
30 my $new_root = $current_parents->[$i++];
32 unless ($pfetcher_is_coderef or $new_root->can($parent_fetcher)) {
33 confess "Could not find method $parent_fetcher in $new_root";
43 $current_root = $new_root;
44 $current_parents = $cache->{pfetch}->{$current_root} ||= [ $current_root->$parent_fetcher ];
45 $recurse_mergeout = [];
50 my $mergeout = $cache->{merge}->{$current_root} ||= do {
52 # This do-block is the code formerly known as the function
53 # that was a perl-port of the python code at
54 # http://www.python.org/2.3/mro.html :)
56 # Initial set (make sure everything is copied - it will be modded)
58 for my $d (@$recurse_mergeout, $current_parents){
61 # Construct the tail-checking hash
62 $tails{$_}++ for (@$d[1..$#$d]);
66 my @res = ( $current_root );
78 next if $tails{ $cand = $_->[0] };
79 push(@res, $winner = $cand);
82 if($_->[0] eq $winner) {
83 shift @$_; # strip off our winner
84 $tails{ $_->[0] }-- if @$_; # keep %tails sane
89 die q{Inconsistent hierarchy found while merging '}
90 . $current_root . qq{':\n\t}
91 . qq{current merge results [\n\t\t}
92 . (join ",\n\t\t" => @res)
93 . qq{\n\t]\n\t} . qq{merging failed on '$cand'\n}
99 return @$mergeout if !@STACK;
101 ($current_root, $current_parents, $recurse_mergeout, $i)
104 push(@$recurse_mergeout, $mergeout);
116 Algorithm::C3 - A module for merging hierarchies using the C3 algorithm
122 # merging a classic diamond
123 # inheritence graph like this:
131 my @merged = Algorithm::C3::merge(
134 # extract the ISA array
141 print join ", " => @merged; # prints D, B, C, A
145 This module implements the C3 algorithm. I have broken this out
146 into it's own module because I found myself copying and pasting
147 it way too often for various needs. Most of the uses I have for
148 C3 revolve around class building and metamodels, but it could
149 also be used for things like dependency resolution as well since
150 it tends to do such a nice job of preserving local precendence
153 Below is a brief explanation of C3 taken from the L<Class::C3>
154 module. For more detailed information, see the L<SEE ALSO> section
159 C3 is the name of an algorithm which aims to provide a sane method
160 resolution order under multiple inheritence. It was first introduced
161 in the langauge Dylan (see links in the L<SEE ALSO> section), and
162 then later adopted as the prefered MRO (Method Resolution Order)
163 for the new-style classes in Python 2.3. Most recently it has been
164 adopted as the 'canonical' MRO for Perl 6 classes, and the default
165 MRO for Parrot objects as well.
167 =head2 How does C3 work.
169 C3 works by always preserving local precendence ordering. This
170 essentially means that no class will appear before any of it's
171 subclasses. Take the classic diamond inheritence pattern for
180 The standard Perl 5 MRO would be (D, B, A, C). The result being that
181 B<A> appears before B<C>, even though B<C> is the subclass of B<A>.
182 The C3 MRO algorithm however, produces the following MRO (D, B, C, A),
183 which does not have this same issue.
185 This example is fairly trival, for more complex examples and a deeper
186 explaination, see the links in the L<SEE ALSO> section.
192 =item B<merge ($root, $func_to_fetch_parent, $cache)>
194 This takes a C<$root> node, which can be anything really it
195 is up to you. Then it takes a C<$func_to_fetch_parent> which
196 can be either a CODE reference (see L<SYNOPSIS> above for an
197 example), or a string containing a method name to be called
198 on all the items being linearized. An example of how this
214 our @ISA = ('B', 'C');
217 print join ", " => Algorithm::C3::merge('D', 'supers');
219 The purpose of C<$func_to_fetch_parent> is to provide a way
220 for C<merge> to extract the parents of C<$root>. This is
221 needed for C3 to be able to do it's work.
223 The C<$cache> parameter is an entirely optional performance
224 measure, and should not change behavior.
226 If supplied, it should be a hashref that merge can use as a
227 private cache between runs to speed things up. Generally
228 speaking, if you will be calling merge many times on related
229 things, and the parent fetching function will return constant
230 results given the same arguments during all of these calls,
231 you can and should reuse the same shared cache hash for all
232 of the calls. Example:
234 sub do_some_merging {
236 my @foo_mro = Algorithm::C3::Merge('Foo', \&get_supers, \%merge_cache);
237 my @bar_mro = Algorithm::C3::Merge('Bar', \&get_supers, \%merge_cache);
238 my @baz_mro = Algorithm::C3::Merge('Baz', \&get_supers, \%merge_cache);
239 my @quux_mro = Algorithm::C3::Merge('Quux', \&get_supers, \%merge_cache);
247 I use B<Devel::Cover> to test the code coverage of my tests, below
248 is the B<Devel::Cover> report on this module's test suite.
250 ------------------------ ------ ------ ------ ------ ------ ------ ------
251 File stmt bran cond sub pod time total
252 ------------------------ ------ ------ ------ ------ ------ ------ ------
253 Algorithm/C3.pm 100.0 100.0 100.0 100.0 100.0 100.0 100.0
254 ------------------------ ------ ------ ------ ------ ------ ------ ------
255 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0
256 ------------------------ ------ ------ ------ ------ ------ ------ ------
260 =head2 The original Dylan paper
264 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
268 =head2 The prototype Perl 6 Object Model uses C3
272 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
276 =head2 Parrot now uses C3
280 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
282 =item L<http://use.perl.org/~autrijus/journal/25768>
286 =head2 Python 2.3 MRO related links
290 =item L<http://www.python.org/2.3/mro.html>
292 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
296 =head2 C3 for TinyCLOS
300 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
306 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
308 Brandon L. Black, E<lt>blblack@gmail.comE<gt>
310 =head1 COPYRIGHT AND LICENSE
312 Copyright 2006 by Infinity Interactive, Inc.
314 L<http://www.iinteractive.com>
316 This library is free software; you can redistribute it and/or modify
317 it under the same terms as Perl itself.