for ( $d = 0; $d <= $limit; ++$d ) {
$start_diag = max( $value_to_add_forward + $start_forward, -$d );
$end_diag = min( $end_forward, $d );
for ( $d = 0; $d <= $limit; ++$d ) {
$start_diag = max( $value_to_add_forward + $start_forward, -$d );
$end_diag = min( $end_forward, $d );