"Ray Casting Method" to find if a point in a polygon in lua

inside.poly.lua

-- Port of http://www.ariel.com.au/a/python-point-int-poly.html in lua
-- # Determine if a point is inside a given polygon or not
-- # Polygon is a list of (x,y) pairs. This function
-- # returns True or False.  The algorithm is called
-- # the "Ray Casting Method".

function point_inside_poly(x,y,poly)
	-- poly is like { {x1,y1},{x2,y2} .. {xn,yn}}
	-- x,y is the point
	local inside = false
	local p1x = poly[1][1]
	local p1y = poly[1][2]

	for i=0,#poly do
		
		local p2x = poly[((i)%#poly)+1][1]
		local p2y = poly[((i)%#poly)+1][2]
		
		if y > math.min(p1y,p2y) then
			if y <= math.max(p1y,p2y) then
				if x <= math.max(p1x,p2x) then
					if p1y ~= p2y then
						xinters = (y-p1y)*(p2x-p1x)/(p2y-p1y)+p1x
					end
					if p1x == p2x or x <= xinters then
						inside = not inside
					end
				end
			end
		end
		p1x,p1y = p2x,p2y	
	end
	return inside
end


polgon = {{0,10},{10,10},{10,0},{0,0}}
print(point_inside_poly(10,11,polgon))

Returns false for print(point_inside_poly(10,0,polgon))