isIntersection (v2 teory)

isIntersection.js

function isIntersection (x1, y1, x2, y2, x3, y3, x4, y4) {
	var angles = [
		getAngleOf(x2, y2, x1, y1, x3, y3) + getAngleOf(x2, y2, x1, y1, x4, y4),
		getAngleOf(x4, y4, x3, y3, x1, y1) + getAngleOf(x4, y4, x3, y3, x2, y2),
		getAngleOf(x1, y1, x2, y2, x3, y3) + getAngleOf(x1, y1, x2, y2, x4, y4),
		getAngleOf(x3, y3, x4, y4, x2, y2) + getAngleOf(x3, y3, x4, y4, x1, y1)
	];

	var sum = angles[0] + angles[1] + angles[2] + angles[3];

	return (sum !== 2 * Math.PI || angles.some(function (a) { return a > Math.PI; })) ? false : (angles.every(function (a) { return a < Math.PI }) || angles.some(function (a) { return a === Math.PI; }));

	function dist (x1, y1, x2, y2) {
		return Math.sqrt(Math.pow(x1 - x2, 2) + Math.pow(y1 - y2, 2));
	}

	function getAngleOf(x1, y1, x2, y2, x3, y3) {
		var a = dist(x1, y1, x2, y2),
			b = dist(x2, y2, x3, y3),
			c = dist(x1, y1, x3, y3);

		return Math.acos((a*a + b*b - c*c) / (2*a*b)) || 0;
	}
}