#!/usr/bin/perl -w
# mjd_permute: permute each word of input
use strict;
# Utility function: factorial with memorizing
BEGIN {
my @fact = (1);
sub factorial($) {
my $n = shift;
return $fact[$n] if defined $fact[$n];
$fact[$n] = $n * factorial($n - 1);
}
}
# n2pat($N, $len) : produce the $N-th pattern of length $len
sub n2pat {
my $i = 1;
my $N = shift;
my $len = shift;
my @pat;
while ($i <= $len + 1) { # Should really be just while ($N) { ...
push @pat, $N % $i;
$N = int($N/$i);
$i++;
}
return @pat;
}
# pat2perm(@pat) : turn pattern returned by n2pat() into
# permutation of integers. XXX: splice is already O(N)
sub pat2perm {
my @pat = @_;
my @source = (0 .. $#pat);
my @perm;
push @perm, splice(@source, (pop @pat), 1) while @pat;
return @perm;
}
# n2perm($N, $len) : generate the Nth permutation of S objects
sub n2perm {
pat2perm(n2pat(@_));
}
1;