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  return map $pop->[$_], keys %s;
}

# sub topN():
# fittest N individuals.
# arguments are anon list of pop, and N (def = 1).
# return anon list of top N individuals.

sub topN {
  my ($pop, $N) = @_;

  $N ||= 1;

  # hmm .. are inputs already sorted?
  return [(sort {$b->score <=> $a->score}
	   map {$_->score; $_}  # This avoids a bug in Perl. See Genetic.pm.
	   @$pop)[0 .. $N-1]];
}

1;

__END__

=head1 NAME

AI::Genetic::OpSelection - A class that implements various selection operators.

=head1 SYNOPSIS

See L<AI::Genetic>.

=head1 DESCRIPTION

This package implements a few selection mechanisms that can be used in user-defined
strategies. The methods in this class are to be called as static class methods,
rather than instance methods, which means you must call them as such:

  AI::Genetic::OpSelection::MethodName(arguments)

=head1 SELECTION OPERATORS AND THEIR METHODS

The following selection operators are defined:

=over

=item Roulette Wheel

Here, the probability of an individual being selected is proportional to its fitness
score. The following methods are defined:

=over

=item B<initWheel>(I<population>)

This method initializes the roulette wheel. It expects an anonymous list of individuals,
as returned by the I<people()> method as described in L<AI::Genetic/CLASS METHODS>.
This B<must> be called only once.

=item B<roulette>(?I<N>?)

This method selects I<N> individuals. I<N> defaults to 2. Note that the same individual
can be selected multiple times.

=item B<rouletteUnique>(?I<N>?)

This method selects I<N> unique individuals. I<N> defaults to 2. Any individual
can be selected only once per call to this method.

=back

=item Tournament

Here, I<N> individuals are randomly selected, and the fittest one of
them is returned. The following method is defined:

=over

=item B<tournament>(?I<N>?)

I<N> defaults to 2. Note that only one individual is returned per call to this
method.

=back

=item Random

Here, I<N> individuals are randomly selected and returned.
The following method is defined:

=over

=item B<random>(?I<N>?)

I<N> defaults to 1.

=back

=item Fittest

Here, the fittest I<N> individuals of the whole population are returned.
The following method is defined:

=over

=item B<topN>(?I<N>?)

I<N> defaults to 1.

=back

=back

=head1 AUTHOR

Written by Ala Qumsieh I<aqumsieh@cpan.org>.

=head1 COPYRIGHTS

(c) 2003,2004 Ala Qumsieh. All rights reserved.
This module is distributed under the same terms as Perl itself.

=cut