How do I create a seedable random number generator in JavaScript?

Implementing a Seedable JavaScript Random Number Generator

It is a common requirement to generate a sequence of random numbers in JavaScript, but the built-in Math.random() function lacks the ability to set a specific seed. This can be problematic when we want to reproduce a particular sequence of numbers or create deterministic randomness.

Using a Custom Random Number Generator

To overcome this limitation, we can implement a custom random number generator that allows us to specify a seed value. Here's an example using a Linear Congruential Generator (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;
}</code>

This LCG produces a sequence of integers between 0 and (2**31 - 1). Other types of random number generators, such as Mersenne Twister, provide stronger randomness guarantees but are more complex to implement.

Setting a Seed Value

To set a seed value, we can pass it as an argument to the RNG constructor:

<code class="js">var rng = new RNG(20);</code>

This will initialize the internal state of the generator with the specified seed, ensuring that it produces a repeatable sequence of numbers.

Using the Random Number Generator

The custom RNG provides the following methods:

• nextInt(): Generates a random integer between 0 and (2**31 - 1).
• nextFloat(): Generates a random floating-point number between 0 and 1.
• nextRange(start, end): Generates a random number within a specified range (inclusive start, exclusive end).
• choice(array): Chooses a random element from an array.

For example, to generate a sequence of random numbers between 10 and 50:

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

Additional Considerations

• Consider the characteristics and limitations of the chosen RNG before using it.
• Use helper functions around the RNG to simplify its usage, such as randRange(start, end) for generating numbers within a specific range.
• Refer to the documentation of the RNG for further details and usage instructions.

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