#!/usr/bin/perl
use warnings;
use strict;

use Math::BigInt;

sub factorial {
    my ($n) = @_;
    my $p = 'Math::BigInt'->new(1);
    $p *= $_ for 1 .. $n;
    $p
}

sub trailing0s {
    my ($n) = @_;
    length((factorial($n) =~ /(0*)$/)[0])
}

sub trailing0s_fast {
    my ($n) = @_;
    my $z = 0;
    my $i = 1;
    $z += int($n / ($i *= 5)) while $i < $n;
    return $z
}

use Test::More;

subtest factorial => sub {
    is factorial(0), 1;
    is factorial(1), 1;
    is factorial(5), 120;
};

subtest trailing0s => sub {
    for my $n (0 .. 1000) {
        is trailing0s($n), trailing0s_fast($n), "fast $n";
    }
};

done_testing();

use Benchmark qw{ cmpthese };
cmpthese(-3, {
         slow => 'trailing0s(2000)',
         fast => 'trailing0s_fast(2000)',
});