sub want ($) {
my ($colournum) = @_;
- my $knife = ($colournum-1) % $nknives;
+ my $knife = $nknives-1 - ($colournum-1) % $nknives;
return 0 unless $knife == $wknife;
my $part = $part_order[ floor(($colournum-1) / $nknives) ];
die "huh colour $colournum?" unless defined $part;