判断一个点是否在一个不规则多边形内算法

/**
 * @params:{
 *  checkPoint:已知点
 *  polygonPoints:不规则多边形所有定点坐标
 * }
 */
  
  function isInPolygon(checkPoint, polygonPoints) {
    var counter = 0;
    var i;
    var xinters;
    var p1, p2;
    var pointCount = polygonPoints.length;
    p1 = polygonPoints[0];

    for (i = 1; i <= pointCount; i++) {
      p2 = polygonPoints[i % pointCount];
      if (
        checkPoint[0] > Math.min(p1[0], p2[0]) &&
        checkPoint[0] <= Math.max(p1[0], p2[0])
      ) {
        if (checkPoint[1] <= Math.max(p1[1], p2[1])) {
          if (p1[0] != p2[0]) {
            xinters =
              (checkPoint[0] - p1[0]) *
              (p2[1] - p1[1]) /
              (p2[0] - p1[0]) +
              p1[1];
            if (p1[1] == p2[1] || checkPoint[1] <= xinters) {
              counter++;
            }
          }
        }
      }
      p1 = p2;
    }
    if (counter % 2 == 0) {
      return false;
    } else {
      return true;
    }
  }