code: function randn(mean, variance) {
    var V1, V2, S;
    do {
      var U1 = Math.random();
      var U2 = Math.random();
      V1 = 2 * U1 - 1;
      V2 = 2 * U2 - 1;
      S = V1 * V1 + V2 * V2;
    } while (S > 1);
    X = Math.sqrt(-2 * Math.log(S) / S) * V1;
    X = mean + Math.sqrt(variance) * X;
    return X;
  }
docs: normal distribution random number