Commit | Line | Data |
c0b91998 |
1 | |
2 | package Algorithm::C3; |
3 | |
4 | use strict; |
5 | use warnings; |
6 | |
7 | use Carp 'confess'; |
8 | |
21f53c4c |
9 | our $VERSION = '0.06'; |
c0b91998 |
10 | |
c0b91998 |
11 | sub merge { |
ca604ce2 |
12 | my ($root, $parent_fetcher, $cache) = @_; |
cf85d7d3 |
13 | |
ca604ce2 |
14 | $cache ||= {}; |
cf85d7d3 |
15 | my @STACK; # stack for simulating recursion |
cf85d7d3 |
16 | |
aeed4a60 |
17 | my $pfetcher_is_coderef = ref($parent_fetcher) eq 'CODE'; |
18 | |
19 | unless ($pfetcher_is_coderef or $root->can($parent_fetcher)) { |
20 | confess "Could not find method $parent_fetcher in $root"; |
21 | } |
22 | |
cf85d7d3 |
23 | my $current_root = $root; |
aeed4a60 |
24 | my $current_parents = [ $root->$parent_fetcher ]; |
cf85d7d3 |
25 | my $recurse_mergeout = []; |
26 | my $i = 0; |
0f7ef7b1 |
27 | my %seen = ( $root => 1 ); |
cf85d7d3 |
28 | |
29 | while(1) { |
30 | if($i < @$current_parents) { |
31 | my $new_root = $current_parents->[$i++]; |
fa27b316 |
32 | |
33 | if($seen{$new_root}) { |
0f7ef7b1 |
34 | my @isastack = ( |
35 | (map { $_->[0] } @STACK), |
36 | $current_root, |
37 | $new_root |
38 | ); |
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"; |
fa27b316 |
42 | } |
bb0280c5 |
43 | $seen{$new_root} = 1; |
cf85d7d3 |
44 | |
aeed4a60 |
45 | unless ($pfetcher_is_coderef or $new_root->can($parent_fetcher)) { |
46 | confess "Could not find method $parent_fetcher in $new_root"; |
47 | } |
48 | |
cf85d7d3 |
49 | push(@STACK, [ |
50 | $current_root, |
51 | $current_parents, |
52 | $recurse_mergeout, |
53 | $i, |
54 | ]); |
55 | |
56 | $current_root = $new_root; |
ca604ce2 |
57 | $current_parents = $cache->{pfetch}->{$current_root} ||= [ $current_root->$parent_fetcher ]; |
cf85d7d3 |
58 | $recurse_mergeout = []; |
59 | $i = 0; |
60 | next; |
61 | } |
62 | |
bb0280c5 |
63 | $seen{$current_root} = 0; |
64 | |
ca604ce2 |
65 | my $mergeout = $cache->{merge}->{$current_root} ||= do { |
cf85d7d3 |
66 | |
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 :) |
70 | |
7946639b |
71 | # Initial set (make sure everything is copied - it will be modded) |
5a07d049 |
72 | my @seqs = map { [@$_] } (@$recurse_mergeout, $current_parents); |
73 | |
74 | # Construct the tail-checking hash |
75 | my %tails; |
76 | foreach my $seq (@seqs) { |
77 | $tails{$_}++ for (@$seq[1..$#$seq]); |
cf85d7d3 |
78 | } |
79 | |
7946639b |
80 | my @res = ( $current_root ); |
cf85d7d3 |
81 | while (1) { |
5a07d049 |
82 | my $cand; |
83 | my $winner; |
cf85d7d3 |
84 | foreach (@seqs) { |
5a07d049 |
85 | next if !@$_; |
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 = $cand; |
cf85d7d3 |
90 | } |
91 | if($_->[0] eq $winner) { |
92 | shift @$_; # strip off our winner |
5a07d049 |
93 | $tails{$_->[0]}-- if @$_; # keep %tails sane |
cf85d7d3 |
94 | } |
95 | } |
96 | last if !$cand; |
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} |
102 | if !$winner; |
103 | } |
104 | \@res; |
105 | }; |
106 | |
107 | return @$mergeout if !@STACK; |
108 | |
109 | ($current_root, $current_parents, $recurse_mergeout, $i) |
110 | = @{pop @STACK}; |
111 | |
112 | push(@$recurse_mergeout, $mergeout); |
113 | } |
c0b91998 |
114 | } |
115 | |
116 | 1; |
117 | |
118 | __END__ |
119 | |
120 | =pod |
121 | |
122 | =head1 NAME |
123 | |
8fe16bec |
124 | Algorithm::C3 - A module for merging hierarchies using the C3 algorithm |
c0b91998 |
125 | |
126 | =head1 SYNOPSIS |
127 | |
128 | use Algorithm::C3; |
129 | |
130 | # merging a classic diamond |
131 | # inheritence graph like this: |
132 | # |
133 | # <A> |
134 | # / \ |
135 | # <B> <C> |
136 | # \ / |
137 | # <D> |
138 | |
139 | my @merged = Algorithm::C3::merge( |
140 | 'D', |
141 | sub { |
142 | # extract the ISA array |
143 | # from the package |
144 | no strict 'refs'; |
145 | @{$_[0] . '::ISA'}; |
146 | } |
147 | ); |
148 | |
149 | print join ", " => @merged; # prints D, B, C, A |
150 | |
151 | =head1 DESCRIPTION |
152 | |
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 |
159 | orderings. |
160 | |
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 |
163 | and the links there. |
164 | |
165 | =head2 What is C3? |
166 | |
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. |
174 | |
175 | =head2 How does C3 work. |
176 | |
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 |
180 | instance: |
181 | |
182 | <A> |
183 | / \ |
184 | <B> <C> |
185 | \ / |
186 | <D> |
187 | |
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. |
192 | |
193 | This example is fairly trival, for more complex examples and a deeper |
194 | explaination, see the links in the L<SEE ALSO> section. |
195 | |
196 | =head1 FUNCTION |
197 | |
198 | =over 4 |
199 | |
ca604ce2 |
200 | =item B<merge ($root, $func_to_fetch_parent, $cache)> |
c0b91998 |
201 | |
202 | This takes a C<$root> node, which can be anything really it |
203 | is up to you. Then it takes a C<$func_to_fetch_parent> which |
204 | can be either a CODE reference (see L<SYNOPSIS> above for an |
205 | example), or a string containing a method name to be called |
206 | on all the items being linearized. An example of how this |
207 | might look is below: |
208 | |
209 | { |
210 | package A; |
211 | |
212 | sub supers { |
213 | no strict 'refs'; |
214 | @{$_[0] . '::ISA'}; |
215 | } |
216 | |
217 | package C; |
218 | our @ISA = ('A'); |
219 | package B; |
220 | our @ISA = ('A'); |
221 | package D; |
222 | our @ISA = ('B', 'C'); |
223 | } |
224 | |
225 | print join ", " => Algorithm::C3::merge('D', 'supers'); |
226 | |
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. |
230 | |
ca604ce2 |
231 | The C<$cache> parameter is an entirely optional performance |
232 | measure, and should not change behavior. |
233 | |
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: |
241 | |
242 | sub do_some_merging { |
243 | my %merge_cache; |
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); |
248 | # ... |
249 | } |
250 | |
c0b91998 |
251 | =back |
252 | |
253 | =head1 CODE COVERAGE |
254 | |
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. |
257 | |
258 | ------------------------ ------ ------ ------ ------ ------ ------ ------ |
259 | File stmt bran cond sub pod time total |
260 | ------------------------ ------ ------ ------ ------ ------ ------ ------ |
6d8a26f9 |
261 | Algorithm/C3.pm 100.0 100.0 100.0 100.0 100.0 100.0 100.0 |
c0b91998 |
262 | ------------------------ ------ ------ ------ ------ ------ ------ ------ |
6d8a26f9 |
263 | Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 |
c0b91998 |
264 | ------------------------ ------ ------ ------ ------ ------ ------ ------ |
265 | |
266 | =head1 SEE ALSO |
267 | |
268 | =head2 The original Dylan paper |
269 | |
270 | =over 4 |
271 | |
272 | =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html> |
273 | |
274 | =back |
275 | |
276 | =head2 The prototype Perl 6 Object Model uses C3 |
277 | |
278 | =over 4 |
279 | |
280 | =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/> |
281 | |
282 | =back |
283 | |
284 | =head2 Parrot now uses C3 |
285 | |
286 | =over 4 |
287 | |
288 | =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631> |
289 | |
290 | =item L<http://use.perl.org/~autrijus/journal/25768> |
291 | |
292 | =back |
293 | |
294 | =head2 Python 2.3 MRO related links |
295 | |
296 | =over 4 |
297 | |
298 | =item L<http://www.python.org/2.3/mro.html> |
299 | |
300 | =item L<http://www.python.org/2.2.2/descrintro.html#mro> |
301 | |
302 | =back |
303 | |
304 | =head2 C3 for TinyCLOS |
305 | |
306 | =over 4 |
307 | |
308 | =item L<http://www.call-with-current-continuation.org/eggs/c3.html> |
309 | |
310 | =back |
311 | |
934d071b |
312 | =head1 AUTHORS |
c0b91998 |
313 | |
314 | Stevan Little, E<lt>stevan@iinteractive.comE<gt> |
315 | |
f4e5601f |
316 | Brandon L. Black, E<lt>blblack@gmail.comE<gt> |
934d071b |
317 | |
c0b91998 |
318 | =head1 COPYRIGHT AND LICENSE |
319 | |
320 | Copyright 2006 by Infinity Interactive, Inc. |
321 | |
322 | L<http://www.iinteractive.com> |
323 | |
324 | This library is free software; you can redistribute it and/or modify |
325 | it under the same terms as Perl itself. |
326 | |
327 | =cut |