I will agree that there's a distinct difference between "trivial" and "not too difficult" that applies to converting between iterators and recursive solutions. However, jd and I found it easier to write recursive solutions, so there's a trade-off in difficulty: It might be easier en toto to come up with a recursive one and then convert it to an iterator.

Not even understanding jdporter's algorithm, I converted it to be iterative (or perhaps more precisely, lazily evaluated). I made the derange function print only the first 15 results, for convenient testing of large inputs. 

sub _derange_iter {
  my ($cb, $todo, @v) = @_;
  @$todo or return do { $cb->( @v ); sub {} }; # this line was wrong b
+efore
  my %seen;
  @seen{@v} = ();
  my ( $range, @todo ) = @$todo;
  my @sub_iter = map {
    my $my_ = $_;
    sub { _derange_iter ( $cb, \@todo, @v, $my_ ) }
  } grep { ! exists $seen{$_} } @$range;
  return sub {} unless (@sub_iter);
  # Grab and unwrap an iterator from the list
  my $iter = (shift @sub_iter)->();
  return sub {
      my $rval;
      $iter = (shift @sub_iter)->()
        until ($rval = $iter->() or @sub_iter == 0);
      return $rval;
  }
}

sub derange(&@)
{
    my $cb = shift;
    my $iter = _derange_iter( $cb,
        [ map { my $x = $_; [ grep { $_ ne $x } @_ ] } @_ ] );
    for (1..15) {
      $iter->();
    }
}

derange { print "@_\n" } @ARGV;

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Caution: Contents may have been coded under pressure.

In reply to Re^5: Derangements iterator (callbacks) by Roy Johnson
in thread Derangements iterator by tye

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