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determine if a point is inside a polygon with a ray intersection counting algorithm

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// ray-casting algorithm based on
// https://wrf.ecse.rpi.edu/Research/Short_Notes/pnpoly.html

module.exports = function pointInPolygonNested (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;
    for (var i = 0, j = len - 1; i < len; j = i++) {
        var xi = vs[i+start][0], yi = vs[i+start][1];
        var 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;
};