How can I create a repeatable sequence of random numbers in JavaScript?

Providing a Custom Seed for JavaScript Random Number Generator

The default JavaScript Math.random() function generates random values within the range [0, 1], but it does not allow you to set a custom seed. Consequently, the generated sequence of random numbers is non-repeatable.

To create a JavaScript random number generator with a customizable seed, we can explore multiple options:

Math.random() Helper Functions:

If you don't need custom seeding, you can utilize Math.random() with helper functions to create a repeatable range of values (e.g., randRange(start, end)).

Pseudorandom Number Generators (PRNGs):

For more control over the randomness, consider using PRNGs like the Mersenne Twister. However, its implementation is complex. An alternative is a Linear Congruential Generator (LCG), which is easier to implement and provides decent randomness.

LCG Implementation (Short Seedable RNG):

Below is an example implementation of a short seedable RNG using LCG:

<code class="js">function RNG(seed) {
  this.m = 0x80000000; // 2**31;
  this.a = 1103515245;
  this.c = 12345;

  this.state = seed ? seed : Math.floor(Math.random() * (this.m - 1));
}

RNG.prototype.nextInt = function() {
  this.state = (this.a * this.state + this.c) % this.m;
  return this.state;
};

RNG.prototype.nextFloat = function() {
  // returns in range [0,1]
  return this.nextInt() / (this.m - 1);
};

RNG.prototype.nextRange = function(start, end) {
  // returns in range [start, end): including start, excluding end
  // can't modulu nextInt because of weak randomness in lower bits
  var rangeSize = end - start;
  var randomUnder1 = this.nextInt() / this.m;
  return start + Math.floor(randomUnder1 * rangeSize);
};

RNG.prototype.choice = function(array) {
  return array[this.nextRange(0, array.length)];
};</code>

To use the RNG:

<code class="js">var rng = new RNG(20);
for (var i = 0; i < 10; i++)
  console.log(rng.nextRange(10, 50));

var digits = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'];
for (var i = 0; i < 10; i++)
  console.log(rng.choice(digits));</code>

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