Commit | Line | Data |
c0b91998 |
1 | |
2 | package Algorithm::C3; |
3 | |
4 | use strict; |
5 | use warnings; |
6 | |
7 | use Carp 'confess'; |
8 | |
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9 | our $VERSION = '0.03'; |
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10 | |
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11 | sub merge { |
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12 | my ($root, $parent_fetcher) = @_; |
13 | |
14 | my @STACK; # stack for simulating recursion |
15 | my %fcache; # cache of _fetcher results |
16 | my %mcache; # cache of merge do-block results |
17 | |
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18 | my $pfetcher_is_coderef = ref($parent_fetcher) eq 'CODE'; |
19 | |
20 | unless ($pfetcher_is_coderef or $root->can($parent_fetcher)) { |
21 | confess "Could not find method $parent_fetcher in $root"; |
22 | } |
23 | |
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24 | my $current_root = $root; |
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25 | my $current_parents = [ $root->$parent_fetcher ]; |
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26 | my $recurse_mergeout = []; |
27 | my $i = 0; |
28 | |
29 | while(1) { |
30 | if($i < @$current_parents) { |
31 | my $new_root = $current_parents->[$i++]; |
32 | |
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33 | unless ($pfetcher_is_coderef or $new_root->can($parent_fetcher)) { |
34 | confess "Could not find method $parent_fetcher in $new_root"; |
35 | } |
36 | |
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37 | push(@STACK, [ |
38 | $current_root, |
39 | $current_parents, |
40 | $recurse_mergeout, |
41 | $i, |
42 | ]); |
43 | |
44 | $current_root = $new_root; |
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45 | $current_parents = $fcache{$current_root} ||= [ $current_root->$parent_fetcher ]; |
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46 | $recurse_mergeout = []; |
47 | $i = 0; |
48 | next; |
49 | } |
50 | |
51 | my $mergeout = $mcache{$current_root} ||= do { |
52 | |
53 | # This do-block is the code formerly known as the function |
54 | # that was a perl-port of the python code at |
55 | # http://www.python.org/2.3/mro.html :) |
56 | |
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57 | # Initial set (make sure everything is copied - it will be modded) |
58 | my @seqs = map { [@$_] } (@$recurse_mergeout, $current_parents); |
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59 | |
60 | # Construct the tail-checking hash |
61 | my %tails; |
62 | foreach my $seq (@seqs) { |
63 | $tails{$_}++ for (@$seq[1..$#$seq]); |
64 | } |
65 | |
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66 | my @res = ( $current_root ); |
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67 | while (1) { |
68 | my $cand; |
69 | my $winner; |
70 | foreach (@seqs) { |
71 | next if !@$_; |
72 | if(!$winner) { # looking for a winner |
73 | $cand = $_->[0]; # seq head is candidate |
74 | next if $tails{$cand}; # he loses if in %tails |
75 | push @res => $winner = $cand; |
76 | } |
77 | if($_->[0] eq $winner) { |
78 | shift @$_; # strip off our winner |
79 | $tails{$_->[0]}-- if @$_; # keep %tails sane |
80 | } |
81 | } |
82 | last if !$cand; |
83 | die q{Inconsistent hierarchy found while merging '} |
84 | . $current_root . qq{':\n\t} |
85 | . qq{current merge results [\n\t\t} |
86 | . (join ",\n\t\t" => @res) |
87 | . qq{\n\t]\n\t} . qq{merging failed on '$cand'\n} |
88 | if !$winner; |
89 | } |
90 | \@res; |
91 | }; |
92 | |
93 | return @$mergeout if !@STACK; |
94 | |
95 | ($current_root, $current_parents, $recurse_mergeout, $i) |
96 | = @{pop @STACK}; |
97 | |
98 | push(@$recurse_mergeout, $mergeout); |
99 | } |
c0b91998 |
100 | } |
101 | |
102 | 1; |
103 | |
104 | __END__ |
105 | |
106 | =pod |
107 | |
108 | =head1 NAME |
109 | |
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110 | Algorithm::C3 - A module for merging hierarchies using the C3 algorithm |
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111 | |
112 | =head1 SYNOPSIS |
113 | |
114 | use Algorithm::C3; |
115 | |
116 | # merging a classic diamond |
117 | # inheritence graph like this: |
118 | # |
119 | # <A> |
120 | # / \ |
121 | # <B> <C> |
122 | # \ / |
123 | # <D> |
124 | |
125 | my @merged = Algorithm::C3::merge( |
126 | 'D', |
127 | sub { |
128 | # extract the ISA array |
129 | # from the package |
130 | no strict 'refs'; |
131 | @{$_[0] . '::ISA'}; |
132 | } |
133 | ); |
134 | |
135 | print join ", " => @merged; # prints D, B, C, A |
136 | |
137 | =head1 DESCRIPTION |
138 | |
139 | This module implements the C3 algorithm. I have broken this out |
140 | into it's own module because I found myself copying and pasting |
141 | it way too often for various needs. Most of the uses I have for |
142 | C3 revolve around class building and metamodels, but it could |
143 | also be used for things like dependency resolution as well since |
144 | it tends to do such a nice job of preserving local precendence |
145 | orderings. |
146 | |
147 | Below is a brief explanation of C3 taken from the L<Class::C3> |
148 | module. For more detailed information, see the L<SEE ALSO> section |
149 | and the links there. |
150 | |
151 | =head2 What is C3? |
152 | |
153 | C3 is the name of an algorithm which aims to provide a sane method |
154 | resolution order under multiple inheritence. It was first introduced |
155 | in the langauge Dylan (see links in the L<SEE ALSO> section), and |
156 | then later adopted as the prefered MRO (Method Resolution Order) |
157 | for the new-style classes in Python 2.3. Most recently it has been |
158 | adopted as the 'canonical' MRO for Perl 6 classes, and the default |
159 | MRO for Parrot objects as well. |
160 | |
161 | =head2 How does C3 work. |
162 | |
163 | C3 works by always preserving local precendence ordering. This |
164 | essentially means that no class will appear before any of it's |
165 | subclasses. Take the classic diamond inheritence pattern for |
166 | instance: |
167 | |
168 | <A> |
169 | / \ |
170 | <B> <C> |
171 | \ / |
172 | <D> |
173 | |
174 | The standard Perl 5 MRO would be (D, B, A, C). The result being that |
175 | B<A> appears before B<C>, even though B<C> is the subclass of B<A>. |
176 | The C3 MRO algorithm however, produces the following MRO (D, B, C, A), |
177 | which does not have this same issue. |
178 | |
179 | This example is fairly trival, for more complex examples and a deeper |
180 | explaination, see the links in the L<SEE ALSO> section. |
181 | |
182 | =head1 FUNCTION |
183 | |
184 | =over 4 |
185 | |
186 | =item B<merge ($root, $func_to_fetch_parent)> |
187 | |
188 | This takes a C<$root> node, which can be anything really it |
189 | is up to you. Then it takes a C<$func_to_fetch_parent> which |
190 | can be either a CODE reference (see L<SYNOPSIS> above for an |
191 | example), or a string containing a method name to be called |
192 | on all the items being linearized. An example of how this |
193 | might look is below: |
194 | |
195 | { |
196 | package A; |
197 | |
198 | sub supers { |
199 | no strict 'refs'; |
200 | @{$_[0] . '::ISA'}; |
201 | } |
202 | |
203 | package C; |
204 | our @ISA = ('A'); |
205 | package B; |
206 | our @ISA = ('A'); |
207 | package D; |
208 | our @ISA = ('B', 'C'); |
209 | } |
210 | |
211 | print join ", " => Algorithm::C3::merge('D', 'supers'); |
212 | |
213 | The purpose of C<$func_to_fetch_parent> is to provide a way |
214 | for C<merge> to extract the parents of C<$root>. This is |
215 | needed for C3 to be able to do it's work. |
216 | |
217 | =back |
218 | |
219 | =head1 CODE COVERAGE |
220 | |
221 | I use B<Devel::Cover> to test the code coverage of my tests, below |
222 | is the B<Devel::Cover> report on this module's test suite. |
223 | |
224 | ------------------------ ------ ------ ------ ------ ------ ------ ------ |
225 | File stmt bran cond sub pod time total |
226 | ------------------------ ------ ------ ------ ------ ------ ------ ------ |
6d8a26f9 |
227 | Algorithm/C3.pm 100.0 100.0 100.0 100.0 100.0 100.0 100.0 |
c0b91998 |
228 | ------------------------ ------ ------ ------ ------ ------ ------ ------ |
6d8a26f9 |
229 | Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 |
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230 | ------------------------ ------ ------ ------ ------ ------ ------ ------ |
231 | |
232 | =head1 SEE ALSO |
233 | |
234 | =head2 The original Dylan paper |
235 | |
236 | =over 4 |
237 | |
238 | =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html> |
239 | |
240 | =back |
241 | |
242 | =head2 The prototype Perl 6 Object Model uses C3 |
243 | |
244 | =over 4 |
245 | |
246 | =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/> |
247 | |
248 | =back |
249 | |
250 | =head2 Parrot now uses C3 |
251 | |
252 | =over 4 |
253 | |
254 | =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631> |
255 | |
256 | =item L<http://use.perl.org/~autrijus/journal/25768> |
257 | |
258 | =back |
259 | |
260 | =head2 Python 2.3 MRO related links |
261 | |
262 | =over 4 |
263 | |
264 | =item L<http://www.python.org/2.3/mro.html> |
265 | |
266 | =item L<http://www.python.org/2.2.2/descrintro.html#mro> |
267 | |
268 | =back |
269 | |
270 | =head2 C3 for TinyCLOS |
271 | |
272 | =over 4 |
273 | |
274 | =item L<http://www.call-with-current-continuation.org/eggs/c3.html> |
275 | |
276 | =back |
277 | |
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278 | =head1 AUTHORS |
c0b91998 |
279 | |
280 | Stevan Little, E<lt>stevan@iinteractive.comE<gt> |
281 | |
f4e5601f |
282 | Brandon L. Black, E<lt>blblack@gmail.comE<gt> |
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283 | |
c0b91998 |
284 | =head1 COPYRIGHT AND LICENSE |
285 | |
286 | Copyright 2006 by Infinity Interactive, Inc. |
287 | |
288 | L<http://www.iinteractive.com> |
289 | |
290 | This library is free software; you can redistribute it and/or modify |
291 | it under the same terms as Perl itself. |
292 | |
293 | =cut |