Polygon splitting

This script is used for splitting **convex** polygons.

-- Usage:
--[[
table_of_polygons = polygoncut(cut_vertices,polygon_vertices)

polygons = polygoncut({x1,y1,x2,y2},{px,py,px1,py1,px2,py2,...})

for i,polygon in ipairs(polygons) do
   love.graphics.polygon('fill',unpack(polygon))
end
--]]

local insert= table.insert

local function cross(dx,dy,dx2,dy2)
	return dx*dy2-dy*dx2
end

local polygonCut = function(cvertices,pvertices)
	local plen     = #pvertices
	local x,y,x2,y2= unpack(cvertices)
	local dx,dy    = x2-x,y2-y
	local polygons = {{},{}}
	local set      = polygons[1]
	local intersections = 0
	for i = 1,plen,2 do
		local x3,y3,x4,y4= pvertices[i],pvertices[i+1],pvertices[i+2] or pvertices[1],pvertices[i+3] or pvertices[2]
		insert(set,x3) insert(set,y3)
		local dx2,dy2    = x4-x3,y4-y3
		local denom      = cross(dx,dy,dx2,dy2)
		if denom ~= 0 then
			local tnum = cross((x3-x),(y3-y),dx2,dy2)
			local unum = cross((x3-x),(y3-y),dx,dy)
			local ct,pt= tnum/denom,unum/denom
			if ct >= 0 and ct <= 1 and pt >= 0 and pt <= 1 then
				local ix,iy = x+ct*dx,y+ct*dy
				if pt ~= 1 then
					if pt ~= 0 then
						insert(set,ix) insert(set,iy)
					end
					set = set == polygons[1] and polygons[2] or polygons[1]
					insert(set,ix) insert(set,iy)
					intersections = intersections+1
					
				end
			end
		end
	end
	if intersections == 2 then return polygons end
end

return polygonCut