点是否在多边形区域

/**
 *  判断 点是否在多边形区域
 * @param point  点
 * @param vs    多边形区域的点的集合
 *               三种传参方式
 *                  1. [[1,2],[2,3]]
 *                  2. [{x:1,y:2},{x:2,y:3}]
 *                  3. [1,2,2,3,3,4]
 * @param start
 * @param end
 * @returns {*|boolean}
 */
function pointInPolygon(point, vs, start, end) {
    if (vs.length > 0)
        if (Array.isArray(vs[0])) {
            return pointInPolygonNested(point, vs, start, end, false);
        } else {
            if (typeof vs[0].x !== 'undefined')
                return pointInPolygonNested(point, vs, start, end, true);
            else
                return pointInPolygonFlat(point, vs, start, end);
        }
}

// private
function pointInPolygonFlat(point, vs, start, end) {
    var x = point[0], y = point[1];
    var inside = false;
    if (start === undefined) start = 0;
    if (end === undefined) end = vs.length;
    var len = (end - start) / 2;
    for (var i = 0, j = len - 1; i < len; j = i++) {
        var xi = vs[start + i * 2 + 0], yi = vs[start + i * 2 + 1];
        var xj = vs[start + j * 2 + 0], yj = vs[start + j * 2 + 1];
        var intersect = ((yi > y) !== (yj > y))
            && (x < (xj - xi) * (y - yi) / (yj - yi) + xi);
        if (intersect) inside = !inside;
    }
    return inside;
}

// private
function pointInPolygonNested(point, vs, start, end, isObject) {
    var x = point[0], y = point[1];
    var inside = false;
    if (start === undefined) start = 0;
    if (end === undefined) end = vs.length;
    var len = end - start;
    for (var i = 0, j = len - 1; i < len; j = i++) {
        var xi, yi, xj, yj;
        if (isObject) {
            xi = vs[i + start].x;
            yi = vs[i + start].y;
            xj = vs[j + start].x;
            yj = vs[j + start].y;
        } else {
            xi = vs[i + start][0];
            yi = vs[i + start][1];
            xj = vs[j + start][0];
            yj = vs[j + start][1];
        }
        var intersect = ((yi > y) !== (yj > y))
            && (x < (xj - xi) * (y - yi) / (yj - yi) + xi);
        if (intersect) inside = !inside;
    }
    return inside;
}

// var polygon = [[1, 1], [1, 2], [2, 2], [2, 1]];
//
// console.log(pointInPolygon([1.5, 1.5], polygon)); // true
// console.log(pointInPolygon([4.9, 1.2], polygon)); // false
// console.log(pointInPolygon([1.8, 1.1], polygon)); // true