sub _pointIsInPolygon {
  my ($a_point, $n, $a_x, $a_y) = @_;

  # point coords
  my ($x, $y) = ($a_point->[0], $a_point->[1]);
  
  # poly coords
  # $n is the number of points in polygon.      
  my @x = @$a_x; # Even indices: x-coordinates.
  my @y = @$a_y; # Odd indices: y-coordinates.
  
  my ($i, $j);                          # Indices.
  my $side = 0;                         # 0 = outside, 1 = inside.
  for ($i = 0, $j = $n - 1 ; $i < $n; $j = $i++) {
    if (
        (
          # If the y is between the (y-) borders ...
          (($y[$i] <= $y) && ($y < $y[$j])) ||
          (($y[$j] <= $y) && ($y < $y[$i]))
        )
        and
          # ...the (x,y) to infinity line crosses the edge
          # from the ith point to the jth point...
          ($x
            <
            ($x[$j] - $x[$i] ) *
            ($y - $y[$i]) / ($y[$j] - $y[$i]) + $x[$i] )) {
         $side = not $side; # Jump the fence.
    }
  }
  return $side ? 1 : 0;
}