sub is_alive {
   my $self = shift;
   return 0 + exists $self->{Cells}->{ join ':', @_ };
}

# Return the list of dead cells surrounding a cell
sub get_dead_cells {
   my ( $self, $x, $y ) = @_;
   ( (join ':', $x - 1, $y - 1) x !$self->is_alive($x - 1, $y - 1),
     (join ':', $x - 1, $y    ) x !$self->is_alive($x - 1, $y    ),
     (join ':', $x - 1, $y + 1) x !$self->is_alive($x - 1, $y + 1),
     (join ':', $x    , $y - 1) x !$self->is_alive($x    , $y - 1),
     (join ':', $x    , $y + 1) x !$self->is_alive($x    , $y + 1),
     (join ':', $x + 1, $y - 1) x !$self->is_alive($x + 1, $y - 1),
     (join ':', $x + 1, $y    ) x !$self->is_alive($x + 1, $y    ),
     (join ':', $x + 1, $y + 1) x !$self->is_alive($x + 1, $y + 1) );
}

sub get_num_live {
   my ( $self, $x, $y ) = @_;
     $self->is_alive( $x - 1, $y - 1 )
   + $self->is_alive( $x - 1, $y     )
   + $self->is_alive( $x - 1, $y + 1 )
   + $self->is_alive( $x    , $y - 1 )
   + $self->is_alive( $x    , $y + 1 )
   + $self->is_alive( $x + 1, $y - 1 )
   + $self->is_alive( $x + 1, $y     )
   + $self->is_alive( $x + 1, $y + 1 );
}


##</code><code>##

# Return the list of dead cells surrounding a cell
sub get_dead_cells {
   my ( $cells, $x0, $y0 ) = ( shift->{Cells}, @_ );
   my ( $x1, $x2, $y1, $y2 ) = ( $x0 - 1, $x0 + 1, $y0 - 1, $y0 + 1 );
   ( ( "$x1:$y1" ) x !( 0 + exists $cells->{ "$x1:$y1" } ),
     ( "$x1:$y0" ) x !( 0 + exists $cells->{ "$x1:$y0" } ),
     ( "$x1:$y2" ) x !( 0 + exists $cells->{ "$x1:$y2" } ),
     ( "$x0:$y1" ) x !( 0 + exists $cells->{ "$x0:$y1" } ),
     ( "$x0:$y2" ) x !( 0 + exists $cells->{ "$x0:$y2" } ),
     ( "$x2:$y1" ) x !( 0 + exists $cells->{ "$x2:$y1" } ),
     ( "$x2:$y0" ) x !( 0 + exists $cells->{ "$x2:$y0" } ),
     ( "$x2:$y2" ) x !( 0 + exists $cells->{ "$x2:$y2" } ) );
}

sub get_num_live {
   my ( $cells, $x0, $y0 ) = ( shift->{Cells}, @_ );
   my ( $x1, $x2, $y1, $y2 ) = ( $x0 - 1, $x0 + 1, $y0 - 1, $y0 + 1 );
     ( 0 + exists $cells->{ "$x1:$y1" } )
   + ( 0 + exists $cells->{ "$x1:$y0" } )
   + ( 0 + exists $cells->{ "$x1:$y2" } )
   + ( 0 + exists $cells->{ "$x0:$y1" } )
   + ( 0 + exists $cells->{ "$x0:$y2" } )
   + ( 0 + exists $cells->{ "$x2:$y1" } )
   + ( 0 + exists $cells->{ "$x2:$y0" } )
   + ( 0 + exists $cells->{ "$x2:$y2" } );
}