Question

I am just trying to implement a simple RNG in JS.

What’s happening is javascript evaluates 119106029 * 1103515245 to be 131435318772912110 rather than 131435318772912105. We know it’s wrong since two odd numbers multiplied does not give an even number.

Anyone know what’s up? I just want a reliable repeatable RNG, and because of these incorrect values I can’t get results to match up with my C implementation of the same thing.

Answer 1

Per the ECMAScript standard, all numbers in JavaScript are (64-bit IEEE 754) floating-point numbers by default.

However all 32-bit integers can be exactly represented as floating-point numbers. You can force a result to 32 bits by using the appropriate bitwise operator, like this:

x = (a * b) >>> 0;  // force to unsigned int32
x = (a * b) | 0;    // force to signed int32

Weird, but that’s the standard.

(Incidentally this rounding behavior is one of the most frequently reported "bugs" against Firefox’s JavaScript engine. Looks like it’s been reported 3 times so far this year…)

If you want real integer math, you can use BigInt values, a different type of number, written with an n at the end:

> 119106029n * 1103515245n
131435318772912105n

This is a relatively recent JS feature, and may not be implemented in old browsers.

---

As for reproducible random numbers in JavaScript, the V8 benchmark uses this:

// To make the benchmark results predictable, we replace Math.random
// with a 100% deterministic alternative.
Math.random = (function() {
  var seed = 49734321;
  return function() {
    // Robert Jenkins' 32 bit integer hash function.
    seed = ((seed + 0x7ed55d16) + (seed << 12))  & 0xffffffff;
    seed = ((seed ^ 0xc761c23c) ^ (seed >>> 19)) & 0xffffffff;
    seed = ((seed + 0x165667b1) + (seed << 5))   & 0xffffffff;
    seed = ((seed + 0xd3a2646c) ^ (seed << 9))   & 0xffffffff;
    seed = ((seed + 0xfd7046c5) + (seed << 3))   & 0xffffffff;
    seed = ((seed ^ 0xb55a4f09) ^ (seed >>> 16)) & 0xffffffff;
    return (seed & 0xfffffff) / 0x10000000;
  };
})();