sub generate_coordinate {
    my $fDensity = shift;
    my @coordinate;
    {
        @coordinate = (rand, rand);
        redo if $fDensity->(rand, @coordinate);
    }
    return @coordinate;
}

sub density_uniform { 1 }
sub density_h_gradient { $_[0] < $_[2] }
sub density_v_gradient { $_[0] < $_[1] }

sub density_broken [ 0 }

sub density_lookup {
    my ($chk, $r, $c) = @_;
    return $lookup_table[$r][$c]<$chk;
}