Point in polygon by winding number

point_in_polygon_wn.js

function isLeft (x0, y0, x1, y1, x2, y2) {
  return ((x1 - x0) * (y2 - y0) - (x2 - x0) * (y1 - y0));
}

export default function pointInPoygon (x, y, V) {
  let wn = 0;  // winding number
  for (let i = 0; i < V.length - 1; i++) {   // edge from V[i] to  V[i+1]
    const vi = V[i], vj = V[i + 1];
    if (vi.y <= y) { // start y <= P.y
      if (vj.y > y) {  // an upward crossing
        //if ((vj.x - vi.x) * (y - vi.y) - (x - vi.x) * (vj.y - vi.y) > 0) {
        if (isLeft(vi.x, vi.y, vj.x, vj.y, x, y) > 0) { // P left of  edge
          ++wn;            // have  a valid up intersect
        }
      }
    } else {           // start vy > y (no test needed)
      if (vj.y <= y) {  // a downward crossing
        // if ((vj.x - vi.x) * (y - vi.y) - (x - vi.x) * (vj.y - vi.y) < 0) {
        if (isLeft(vi.x, vi.y, vj.x, vj.y, x, y) < 0) {  // P right of  edge
          --wn;            // have  a valid down intersect
        }
      }
    }
  }
  return wn !== 0;
}