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

sub check_monotonicity {
        # Return 0 if series is non-monotonic, +1 if it's
        # an increasing series, -1 if it decreases.

        my $dir = shift;

        $dir = shift            if ! $dir;
        return 0                if ! $dir;

        my $next = shift;
        return 0                if ! ($next - $dir);

        my $sign = ($next - $dir) / abs($next - $dir);
        $dir = $next;

        while ($next = shift) {
                return 0        unless ($next - $dir) * $sign > 0;
                $dir = $next;
        }
        return $sign;
}

my @a = (0, 0, 0);
print check_monotonicity(@a), ": ", join(', ',@a), "\n";

@a = (0, 2, 4, 6, 8);
print check_monotonicity(@a),": ", join(', ',@a),  "\n";

@a = (1, 3, 5, 7, 9, 11);
print check_monotonicity(@a),": ", join(', ',@a),  "\n";

@a = (10, 9, 8, 7, 6, 5);
print check_monotonicity(@a),": ", join(', ',@a),  "\n";

@a = (5, 3, 1, -1, -3, -5);
print check_monotonicity(@a), ": ", join(', ',@a), "\n";

@a = (1, 2, 3, 3, 4);
print check_monotonicity(@a), ": ", join(', ',@a), "\n";