Line intersections

window.intersectionCount = 0;
function trimLine(polyline, polygon){
  if (!polyline.length){
    return {true:[],false:[]};
  }
  let zero = segmentIntersectPoly(...polyline[0],polyline[0][0]+Math.E,polyline[0][1]+PI,polygon,true).length % 2 != 0;
  let out = {
    'true' :[[]],
    'false':[[]],
  }
  let io = zero;
  let hasIsect = false;
  for (let i = 0; i < polyline.length; i++){
    let a= polyline[i];
    let b= polyline[i+1];
    out[io][out[io].length-1].push(a);
    if (!b) break;

    let isects = segmentIntersectPoly(...a,...b,polygon,false);
    if(!hasIsect && isects) {
      hasIsect = true;
    }
    for (let j = 0; j < isects.length; j++){
      out[io][out[io].length-1].push(isects[j].xy);
      io = !io;
      out[io].push([isects[j].xy]);
    }
  }
  out.true = out.true.filter(x=>x.length);
  out.false = out.false.filter(x=>x.length);
  if(!hasIsect) {
    return [];
  }
  return out.true;
}
function segmentIntersectPoly(x0,y0,x1,y1,poly,is_ray=false){
  let n = poly.length;
  let isects = [];
  for (let i = 0; i < poly.length; i++){
    let a = poly[i];
    let b = poly[(i+1)%n];
    let xx = segmentIntersects(x0,y0,x1,y1,...a,...b,is_ray);
    if (xx){
      isects.push(xx);
    }
  }
  isects.sort((a,b)=>a.t-b.t);
  return isects;
}

function segmentIntersects(p0x, p0y, p1x, p1y, q0x, q0y, q1x, q1y, is_ray = false) {
  let d0x = p1x - p0x;
  let d0y = p1y - p0y;
  let d1x = q1x - q0x;
  let d1y = q1y - q0y;
  let vc = d0x * d1y - d0y * d1x;
  if (vc == 0) {
    return null;
  }
  let vcn = vc * vc;
  let q0x_p0x = q0x - p0x;
  let q0y_p0y = q0y - p0y;
  let vc_vcn = vc / vcn;
  let t = (q0x_p0x * d1y - q0y_p0y * d1x) * vc_vcn;
  let s = (q0x_p0x * d0y - q0y_p0y * d0x) * vc_vcn;
  if (0 <= t && (is_ray || t < 1) && 0 <= s && s < 1) {
    let ret = {t, s, side: null, other: null, xy: null};
    ret.xy = [p1x * t + p0x * (1 - t), p1y * t + p0y * (1 - t)];
    ret.side = pointInside(p0x, p0y, p1x, p1y, q0x, q0y) < 0 ? 1 : -1;
    return ret;
  }
  window.intersectionCount++;
  return null;
}
function pointInside(x, y, x0, y0, x1, y1) {
  let dx = x1 - x0;
  let dy = y1 - y0;
  let e = (x - x0) * dy - (y - y0) * dx;
  return e;
}