Javascript random ordering with seed

Question

I want to randomly shuffle a list of 4 items but with a seed so that so long as you have the same seed the you will get the same order of items.

["a", "b", "c", "d"]

I figure I can get the seed with Math.random, I don't need something very exact. How do I sort according to the seed?

Answer 1

You can achieve this with a slight modification to Mike Bostock's implementation of the Fisher–Yates algorithm*:

function shuffle(array, seed) {                // <-- ADDED ARGUMENT
  var m = array.length, t, i;

  // While there remain elements to shuffle…
  while (m) {

    // Pick a remaining element…
    i = Math.floor(random(seed) * m--);        // <-- MODIFIED LINE

    // And swap it with the current element.
    t = array[m];
    array[m] = array[i];
    array[i] = t;
    ++seed                                     // <-- ADDED LINE
  }

  return array;
}

function random(seed) {
  var x = Math.sin(seed++) * 10000; 
  return x - Math.floor(x);
}

*The random function is taken from this SO answer. It is a hack and not entirely random and most importantly not cryptographically secure! Here's a histogram of samples (also in the comments of that response, takes a while to run). Conclusively, you should only use this when these things don't really matter. Alternatively, substitute the random function with a better seedable random number generator.