src/plugins/leaflet-draw/ext/LineUtil.Intersect.js

/**
 * @class L.LineUtil
 * @aka Util
 * @aka L.Utils
 */
L.Util.extend(L.LineUtil, {

	// @method segmentsIntersect(): boolean
	// Checks to see if two line segments intersect. Does not handle degenerate cases.
	// http://compgeom.cs.uiuc.edu/~jeffe/teaching/373/notes/x06-sweepline.pdf
	segmentsIntersect: function (/*Point*/ p, /*Point*/ p1, /*Point*/ p2, /*Point*/ p3) {
		return this._checkCounterclockwise(p, p2, p3) !==
			   this._checkCounterclockwise(p1, p2, p3) &&
			   this._checkCounterclockwise(p, p1, p2) !==
			   this._checkCounterclockwise(p, p1, p3);
	},

	// check to see if points are in counterclockwise order
	_checkCounterclockwise: function (/*Point*/ p, /*Point*/ p1, /*Point*/ p2) {
		return (p2.y - p.y) * (p1.x - p.x) > (p1.y - p.y) * (p2.x - p.x);
	}
});