js判斷點在多邊形內

/**
 * 判斷點在多邊形內
 * param checkPoint被測點，polygonPoints範圍點
 */

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;
    }
}