gcd求出角度后四舍五入一下(2pi / a),再代回去求一下a

double g = Gcd(Math.Abs(th[1] - th[0]), Math.PI * 2);
g = Gcd(g, Math.Abs(th[2] - th[0]));
g = Gcd(g, Math.Abs(th[2] - th[1]));
double n = Math.Round((2 * Math.PI) / g);
g = 2 * Math.PI / n;