#!/usr/bin/perl -w
use strict;
use Benchmark;
use Math::BigInt;

sub fact1 {
	my $n = Math::BigInt->new(shift);
	return 1 unless $n->bcmp(0);
	return $n->bmul(fact1($n->bsub(1)));
}

sub fact4 {
	my $n = Math::BigInt->new(1);
	foreach my $i (map Math::BigInt->new($_), 1..shift) {
		$n = Math::BigInt->new($i->bmul($n));
	}
	return $n;
}

sub fact7 {
	my @factor = map Math::BigInt->new($_), (1 .. shift);
	while(@factor > 1) {
		my @pair = splice @factor, 1 + $#factor / 2;
		$_ = $_ * pop @pair for @factor[0..$#pair];
	}
	return shift @factor;
}

my $fact = shift || 100;

my ($f1, $f4, $f7) = (fact1($fact), fact4($fact), fact7($fact));

die "something's broke" if grep $_ != $f1, ($f4, $f7);

timethese(shift || 10, {
	f1 => sub { fact1($fact) },
	f4 => sub { fact4($fact) },
	f7 => sub { fact7($fact) },
});
__END__

$ fact 600 5
Deep recursion on subroutine "main::fact1" at fact line 9.
Benchmark: timing 5 iterations of f1, f4, f7...
Deep recursion on subroutine "main::fact1" at fact line 9.
Deep recursion on subroutine "main::fact1" at fact line 9.
Deep recursion on subroutine "main::fact1" at fact line 9.
Deep recursion on subroutine "main::fact1" at fact line 9.
Deep recursion on subroutine "main::fact1" at fact line 9.
        f1: 17 wallclock secs (17.09 usr +  0.10 sys = 17.19 CPU) @  0.29/s (n=5)
        f4: 15 wallclock secs (15.09 usr +  0.06 sys = 15.15 CPU) @  0.33/s (n=5)
        f7:  4 wallclock secs ( 3.99 usr +  0.04 sys =  4.03 CPU) @  1.24/s (n=5)