#
# This software is Copyright 2005 by Elsevier Inc.  You may use it
# under the terms of the license at http://perl.plover.com/hop/LICENSE.txt .
#



###
### powerset_recurse2
###

## Chapter 5 section 4.1.1

sub powerset_recurse ($) {
    my ( $set ) = @_;
    my $null = { };
    my $powerset  = { $null, $null };
    my $keys      = [ keys   %{ $set } ];
    my $values    = [ values %{ $set } ];
    my $nmembers  = keys %{ $set };    # This many rounds.

    for my $i (0 .. $nmembers-1) {

      # Remap.

      my @powerkeys   = keys   %{ $powerset };
      my @powervalues = values %{ $powerset };
      my $powern      = @powerkeys;
      my $j;

      for ( $j = 0; $j < $powern; $j++ ) {
          my %subset = ( );

          # Copy the old set to the subset.
          @subset{keys   %{ $powerset->{ $powerkeys  [ $j ] } }} =
                  values %{ $powerset->{ $powervalues[ $j ] } };

          # Add the new member to the subset.
          $subset{$keys->[ $i ]} = $values->[ $i ];

          # Add the new subset to the powerset.
          $powerset->{ \%subset } = \%subset;
      }
    }

    return $powerset;
}