5. Caching

Faster version:

        {
          my %cmyk;

          sub cmyk {
            my $key = join ',' , @_;
            return $cmyk{$key} if exists $cmyk{$key};
            $cmyk{$key} = real_cmyk(@_);
          }

          sub real_cmyk {
            # as before...
          }
        }

Code in C, C++, Java, etc. will be similar