pub fn cross_product(x1: f64, y1: f64, x2: f64, y2: f64, x: f64, y: f64) -> f64
Cross product of vectors (x2-x1, y2-y1) and (x-x2, y-y2). The sign indicates which side of the line (x1,y1)→(x2,y2) the point (x,y) is on.