sub weights
{
    my ( $self ) = @_;

    #   Step through each stemmed term indexed

    foreach my $stem ( keys %{$self->{'_index_count'}} )
    {
        #   Step through each document in which the stemmed term $stem appears,
        #   calculate its weight and store this ranking in the %weights hash.

        my %weights;
        foreach my $url ( keys %{${$self->{'_index_frequency'}}{$stem}} )
        {
            $weights{$url} = sprintf "%.2f", ${${$self->{'_index_frequency'}}{$stem}}{$url} * log( ( scalar keys %{$self->{'_crawl_visited'}} ) / $self->{'_index_count'}->{$stem} );
        }

        #   Store ranking score in tied hash - Note the fashion by which the hash
        #   reference is built first and then assigned to the MLDBM-tied hash.  This is
        #   required due to the limitations of the Perl TIEHASH interface which has no
        #   support for multi-dimensional ties.

        ${$self->{'_tied_weight'}}{$stem} = \%weights;
    }
}