###
### permute-flop
###

## Chapter 4 section 3.1

sub permute {
  my @items = @_;
  my $n = 0;
  return Iterator {
    $n++, return @items if $n==0;
    my $i;
    my $p = $n;
    for ($i=1; $i<=@items && $p%$i==0; $i++) {
      $p /= $i;
    }
    my $d = $p % $i;
    my $j = @items - $i;
    return if $j < 0;

    @items[$j+1..$#items] = reverse @items[$j+1..$#items];
    @items[$j,$j+$d] = @items[$j+$d,$j];

    $n++;
    return @items;
  };
}