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Pseudo-random generator

There are many areas where we need random data.

One of them is testing. We may need random data: text, numbers, etc. to test things out well.

In JavaScript, we could use Math.random(). But if something goes wrong, we’d like to be able to repeat the test, using exactly the same data.

For that, so called “seeded pseudo-random generators” are used. They take a “seed”, the first value, and then generate the next ones using a formula so that the same seed yields the same sequence, and hence the whole flow is easily reproducible. We only need to remember the seed to repeat it.

An example of such formula, that generates somewhat uniformly distributed values:

next = previous * 16807 % 2147483647

If we use 1 as the seed, the values will be:

  1. 16807
  2. 282475249
  3. 1622650073
  4. …and so on…

The task is to create a generator function pseudoRandom(seed) that takes seed and creates the generator with this formula.

Usage example:

let generator = pseudoRandom(1);

alert(generator.next().value); // 16807
alert(generator.next().value); // 282475249
alert(generator.next().value); // 1622650073

Open a sandbox with tests.

function* pseudoRandom(seed) {
  let value = seed;

  while(true) {
    value = value * 16807 % 2147483647
    yield value;
  }

};

let generator = pseudoRandom(1);

alert(generator.next().value); // 16807
alert(generator.next().value); // 282475249
alert(generator.next().value); // 1622650073

Please note, the same can be done with a regular function, like this:

function pseudoRandom(seed) {
  let value = seed;

  return function() {
    value = value * 16807 % 2147483647;
    return value;
  }
}

let generator = pseudoRandom(1);

alert(generator()); // 16807
alert(generator()); // 282475249
alert(generator()); // 1622650073

That also works. But then we lose ability to iterate with for..of and to use generator composition, that may be useful elsewhere.

Open the solution with tests in a sandbox.