line segment intersection

// translation from C code from this answer http://stackoverflow.com/a/1968345/1418097

    type Point2D =
        {
            x: float
            y: float
        }

    let crossing l1start l1finish l2start l2finish =
        let toTuple point = (point.x, point.y)
        let [(p0x, p0y); (p1x, p1y); (p2x, p2y); (p3x, p3y)] = List.map toTuple [l1start; l1finish; l2start; l2finish]

        let s1x = p1x - p0x
        let s1y = p1y - p0y
        let s2x = p3x - p2x
        let s2y = p3y - p2y

        let sdiv = -s2x * s1y + s1x * s2y
        let tdiv = -s2x * s1y + s1x * s2y

        if 0.0 = sdiv || 0.0 = tdiv
            then None
            else
                let s = (-s1y * (p0x - p2x) + s1x * (p0y - p2y)) / sdiv
                let t = ( s2x * (p0y - p2y) - s2y * (p0x - p2x)) / sdiv

                if (s >= 0.0 && s <= 1.0 && t >= 0.0 && t <= 1.0)
                    then Some {x = p0x + (t * s1x); y = p0y + (t * s1y)}
                    else None