#!/usr/bin/perl

use strict;
use warnings 'all';
use integer;

=head1 Euclid's Algorithm

Given two positive integers m and n, find their greatest common 
divisor.

=cut

sub p
{
    my ($m, $n) = ($_[0], $_[1]);
    my $r;
  
    # m and n must be positive integers.
    if (($m < 1) || ($n < 1)) {
        die("Values must be positive integers.");
    }

    # E0: If m < n, exchange m and n
    #
    # As Knuth points out, this step can be avoided,
    # but it will force the algorithm to go through
    # another loop as it exchanges m and n when n is
    # greater.
    if ($m < $n) {
        ($m, $n) = ($n, $m);
    }

    do {{
        # E1: Divide m by n and let r be the remainder
        $r = $m % $n;
        # E2: If r = 0, the algorithm terminates; $n is the answer
        if ($r == 0) {
            return $n;
        }
        # E3: Set m = n, n = r, and go back to step E1.
        $m = $n;
        $n = $r;
        redo;
    }} 
}