TemperTemper

Seeded Random

Generate reproducible random numbers using a seed value. Same seed always produces the same sequence, essential for testing and procedural generation.

Code

General
// Simple mulberry32 PRNG
let s = seed;
const random = () => {
  s |= 0; s = s + 0x6D2B79F5 | 0;
  let t = Math.imul(s ^ s >>> 15, 1 | s);
  t = t + Math.imul(t ^ t >>> 7, 61 | t) ^ t;
  return ((t ^ t >>> 14) >>> 0) / 4294967296;
};
return Array.from({ length: count }, () => +random().toFixed(4));

Parameters

Seed value for reproducibility

How many random numbers to generate

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Why Use Seeded Random?

Math.random() is non-deterministic — you get different results every time:

Math.random()  // 0.7234... (unpredictable)
Math.random()  // 0.1892... (different every run)

A seeded PRNG is deterministic — same seed = same sequence:

seededRandom(42)  // Always: 0.3745, 0.9507, 0.7320...
seededRandom(42)  // Always: 0.3745, 0.9507, 0.7320...

Use Cases

Use CaseWhy Seeded Random?
Unit testsReproducible test data that doesn't cause flaky tests
Procedural generationSame seed = same game world/level
SimulationsReplay and debug exact scenarios
A/B testingConsistent user bucketing based on user ID
ShufflingReproducible card shuffles, playlist orders

How It Works (Mulberry32)

This snippet uses Mulberry32, a fast 32-bit PRNG:

  1. Takes a 32-bit integer seed
  2. Applies bitwise operations to scramble the state
  3. Returns a float between 0 and 1
  4. Each call advances the internal state

The magic constants (0x6D2B79F5, etc.) are carefully chosen to maximize randomness quality.

Choosing Seeds

// Reproducible: Use fixed seeds
const testSeed = 12345;

// User-specific: Hash user ID
const userSeed = hashCode(userId);

// Time-based but reproducible: Use date
const dailySeed = new Date().toDateString().hashCode();

Limitations

  • Not cryptographically secure — don't use for encryption keys
  • 32-bit state — period of ~2³² before sequence repeats
  • Sequential correlation — for statistical work, consider Mersenne Twister

For cryptographic randomness, use crypto.getRandomValues() instead.

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