#
# 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_recurse0
###

## Chapter 5 section 4.1.1

sub powerset_recurse ($;@) {
    my ( $set, $powerset, $keys, $values, $n, $i ) = @_;

    if ( @_ == 1 ) { # Initialize.
        my $null   = { };
        $powerset  = { $null, $null };
        $keys      = [ keys   %{ $set } ];
        $values    = [ values %{ $set } ];
        $nmembers  = keys %{ $set };    # This many rounds.
        $i         = 0;                 # The current round.
    }

    # Ready?
    return $powerset if $i == $nmembers;

    # 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;
    }

    # Recurse.
    powerset_recurse( $set, $powerset, $keys, $values, $nmembers, $i+1 );
}