code: function isLeft(a, b, c) {
    return ((b[0] - a[0])*(c[1] - a[1]) - (b[1] - a[1])*(c[0] - a[0])) < 0;
  }
docs: Check if C is on left side of [AB]
  @param {Object} a startpoint of the segment
  @param {Object} b endpoint of the segment
  @param {Object} c the point we want to locate
  @return {Boolean} true if on left side