#! perl -slw
use strict;
use constant X 		=> 0;
use constant Y 		=> 1;
use constant PI		=> atan2(0,-1);
use constant TWOPI	=> 2*PI;

sub mapAdjPairs (&@) {
	my $code = shift;
	map { local ($a, $b) = (shift, $_[0]); $code->() } 0 .. @_-2;
}

sub Angle{
	my ($x1, $y1, $x2, $y2) = @_;
	my $dtheta = atan2($y1, $x1) - atan2($y2, $x2);
	$dtheta -= TWOPI while $dtheta >   PI;
	$dtheta += TWOPI while $dtheta < - PI;
	return $dtheta;
}

sub PtInPoly{
	my ($poly, $pt) = @_;
	my $angle=0;

	mapAdjPairs{
		$angle += Angle(
			$a->[X] - $pt->[X],
			$a->[Y] - $pt->[Y],
			$b->[X] - $pt->[X],
			$b->[Y] - $pt->[Y]
		)
	} @$poly, $poly->[0];

	return !(abs($angle) < PI);
}

my @pts = ( [-1,1], [1,1], [1,-1], [-1,-1] );
print '[0,0] should be in the polygon:', PtInPoly( \@pts, [0,0]) ? 'It is' : 'It is not';
print '[2,0] should be out of polygon:', PtInPoly( \@pts, [2,0]) ? 'It is not' : 'It is';

my @diamond = ( [0,1], [1,2], [2,1], [1,0] );

print "[$_->[0],$_->[1]] is "
	, PtInPoly(\@diamond, $_) ? 'inside' : 'outside'
	for [0.49999999999,0.49999999999], [0.5,0.5], [0.50000000001,0.50000000001];

my @concave = ( [0,3],[3,0],[5,2],[4,3],[3,2],[2,3],[3,4],[2,5] );
print '[2,2] is ', PtInPoly(\@concave, [2,2]) ? 'inside' : 'outside';
print '[3,3] is ', PtInPoly(\@concave, [3,3]) ? 'inside' : 'outside';
print '[2,4] is ', PtInPoly(\@concave, [2,4]) ? 'inside' : 'outside';
print '[4,2] is ', PtInPoly(\@concave, [4,2]) ? 'inside' : 'outside';

__END__
C:\test>temp
[0,0] should be in the polygon:It is
[2,0] should be out of polygon:It is

[0.49999999999,0.49999999999] is outside
[0.5,0.5] is outside
[0.50000000001,0.50000000001] is inside

[2,2] is inside
[3,3] is outside
[2,4] is inside
[4,2] is inside