点在多边形内的判断

在看zrender源码时，看到点在多边形内的判断代码，特意粘出来，参考下：

        /**
         * 多边形包含判断
         * 采用 non-zero winding rule
         */
        function isInsidePolygon(points, x, y) {
            var N = points.length;
            var w = 0;

            for (var i = 0, j = N - 1; i < N; i++) {
                var x0 = points[j][0];
                var y0 = points[j][1];
                var x1 = points[i][0];
                var y1 = points[i][1];
                w += windingLine(x0, y0, x1, y1, x, y);
                j = i;
            }
            return w !== 0;
        }

        function windingLine(x0, y0, x1, y1, x, y) {
            if ((y > y0 && y > y1) || (y < y0 && y < y1)) {
                return 0;
            }
            if (y1 == y0) {
                return 0;
            }
            var dir = y1 < y0 ? 1 : -1;
            var t = (y - y0) / (y1 - y0);
            var x_ = t * (x1 - x0) + x0;

            return x_ > x ? dir : 0;
        }