// Sample from a normal distribution with mean 0, stddev 1.

function normal() {

  var x = 0, y = 0, rds, c;

  do {

    x = Math.random() * 2 - 1;

    y = Math.random() * 2 - 1;

    rds = x * x + y * y;

  } while (rds == 0 || rds > 1);

  c = Math.sqrt(-2 * Math.log(rds) / rds); // Box-Muller transform

  return x * c; // throw away extra sample y * c

}

// Simple 1D Gaussian (normal) distribution

function normal1(mean, deviation) {

  return function() {

    return mean + deviation * normal();

  };

}

// Gaussian Mixture Model (k=3) fit using E-M algorithm

function normal3(dd) {

  return function() {

    var r = Math.random(),

        i = r < dd[0][2] ? 0 : r < dd[0][2] + dd[1][2] ? 1 : 2,

        d = dd[i];

    return d[0] + Math.sqrt(d[1]) * normal();

  }

}