sub can_play_card {
  my $cost = shift; # hashref of type => amount required
  my $mana = shift; # arrayref of mana types player holds

  my $pool = join '', map {printf "%2s", $_} sort @$mana;
  my $check = join '(?:..)*?', map {"(?:.$_|$_.)"} map {("$_") x $cost->{$_}} sort keys %$cost;

  return $pool =~ m/$check/;
}