feat: seeded RNG with per-label deterministic rolls

src/sim/rng.js

@@ -0,0 +1,28 @@
+// FNV-1a 32-bit string hash.
+export function hash32(str) {
+  let h = 0x811c9dc5;
+  for (let i = 0; i < str.length; i++) {
+    h ^= str.charCodeAt(i);
+    h = Math.imul(h, 0x01000193);
+  }
+  return h >>> 0;
+}
+
+// mulberry32 PRNG: returns function yielding uniforms in [0,1).
+export function makeRng(seed) {
+  let a = seed >>> 0;
+  return function () {
+    a |= 0;
+    a = (a + 0x6d2b79f5) | 0;
+    let t = Math.imul(a ^ (a >>> 15), 1 | a);
+    t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
+    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
+  };
+}
+
+// One deterministic uniform in [0,1) for a (seed, label) pair.
+// This is the whole hidden-variable mechanism: outcomes are fixed
+// the moment their label exists, revealed whenever first asked for.
+export function rollFor(seed, label) {
+  return makeRng(hash32(seed + ':' + label))();
+}

tests/rng.test.js

@@ -0,0 +1,32 @@
+import { describe, it, expect } from 'vitest';
+import { hash32, makeRng, rollFor } from '../src/sim/rng.js';
+
+describe('rng', () => {
+  it('hash32 is deterministic and spreads', () => {
+    expect(hash32('abc')).toBe(hash32('abc'));
+    expect(hash32('abc')).not.toBe(hash32('abd'));
+  });
+
+  it('makeRng produces a deterministic sequence in [0,1)', () => {
+    const a = makeRng(42), b = makeRng(42);
+    for (let i = 0; i < 100; i++) {
+      const v = a();
+      expect(v).toBe(b());
+      expect(v).toBeGreaterThanOrEqual(0);
+      expect(v).toBeLessThan(1);
+    }
+  });
+
+  it('rollFor depends on seed and label, independently', () => {
+    expect(rollFor('s1', 'x')).toBe(rollFor('s1', 'x'));
+    expect(rollFor('s1', 'x')).not.toBe(rollFor('s1', 'y'));
+    expect(rollFor('s1', 'x')).not.toBe(rollFor('s2', 'x'));
+  });
+
+  it('rollFor is roughly uniform', () => {
+    let sum = 0;
+    for (let i = 0; i < 2000; i++) sum += rollFor('u', `l${i}`);
+    expect(sum / 2000).toBeGreaterThan(0.45);
+    expect(sum / 2000).toBeLessThan(0.55);
+  });
+});