# The following algorithm is due to Bob Floyd and is
    # described in Jon Bentley's, _More Programming Pearls_
    # (1988), on p. 143.
    #
    # In this Perl implementation, I use a hash as an efficient,
    # randomly-addressable series that supports insertion in
    # O(1) time.  This implementation selects subsets of size $m
    # from the range [0,$n], without replacement.  (The original
    # from Bentley's book used the range [1,n]).

    sub randomly_permuted_subset_of_range {
        my ($m, $n) = @_;
        my (@result, %series, $head);
        for (my $j = $n-$m+1; $j <= $n; $j++) {
            my $t = int rand($j+1);
            if (exists $series{$t}) {
                # insert $j into series after $t
                @series{$t,$j} = ($j, $series{$t});
            }
            else {
                # prefix $t to series
                ($head, $series{$t}) = ($t, $head);
            }
        }
        for ($n = $head; defined $n; $n = $series{$n}) {
            push @result, $n;
        }
        return \@result;
    }