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	    $closest = [0,  0,  $distance] unless $closest;
	    $closest = [$x, $y, $distance] if $distance < $closest->[2];
	}
    }
    return @$closest;
}

sub neighbors {                                                               # http://www.ai-junkie.com/ann/som/som3.html
    my $self = shift;
    my $sigma = shift;
    my $X     = shift;
    my $Y     = shift;     

    my @neighbors;
    for my $x (0 .. $self->{_D}-1) {
        for my $y (0 .. $self->{_D}-1){
            my $distance = _hexa_distance ($X, $Y, $x, $y);
##warn "$X, $Y, $x, $y: distance: $distance";
	    next if $distance > $sigma;
	    push @neighbors, [ $x, $y, $distance ];                                    # we keep the distances
	}
    }
    return \@neighbors;
}

sub _hexa_distance {
    my ($x1, $y1) = (shift, shift);   # one point
    my ($x2, $y2) = (shift, shift);   # another

    ($x1, $y1, $x2, $y2) = ($x2, $y2, $x1, $y1)  # swapping
	if ( $x1+$y1 > $x2+$y2 );

    my $dx = $x2 - $x1;
    my $dy = $y2 - $y1;

    if ($dx < 0 || $dy < 0) {
	return abs ($dx) + abs ($dy);
    } else {
	return $dx < $dy ? $dy : $dx;
    }
}

=pod

=item I<map>

I<$m> = I<$nn>->map

This method returns the 2-dimensional array of vectors in the grid (as a reference to an array of
references to arrays of vectors).

Example:

   my $m = $nn->map;
   for my $x (0 .. $nn->diameter -1) {
       for my $y (0 .. $nn->diameter -1){
           warn "vector at $x, $y: ". Dumper $m->[$x]->[$y];
       }
   }

This array represents a hexagon like this (ASCII drawing is so cool):

               <0,0>
           <0,1>   <1,0>
       <0,2>   <1,1>   <2,0>
   <0,3>   <1,2>   <2,1>   <3,0>
  ...............................


=item I<as_string>

Not implemented.

=cut

## TODO: pretty printing of this as hexagon ?
sub as_string { die "not implemented"; }

=pod

=item I<as_data>

Not implemented.

=cut

sub as_data { die "not implemented"; }

=pod

=back

=head1 AUTHOR

Robert Barta, E<lt>rho@devc.atE<gt>

=head1 COPYRIGHT AND LICENSE

Copyright (C) 200[78] by Robert Barta

This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself, either Perl version 5.8.8 or,
at your option, any later version of Perl 5 you may have available.

=cut

our $VERSION = '0.02';

1;

__END__



sub _get_coordinates {
    my $self = shift;
    my $D1 = $self->{_D}-1;
    my $t;
    return map { $t = $_ ; map { [ $t, $_ ] } (0 .. $D1) } (0 .. $D1)
}