diff --git a/src/sim/rng.ts b/src/sim/rng.ts
new file mode 100644
index 0000000..603494e
--- /dev/null
+++ b/src/sim/rng.ts
@@ -0,0 +1,35 @@
+export interface RngHolder {
+  rngState: number;
+}
+
+/** mulberry32 — mutates holder.rngState, returns a float in [0, 1). */
+export function random(h: RngHolder): number {
+  let t = (h.rngState = (h.rngState + 0x6d2b79f5) | 0);
+  t = Math.imul(t ^ (t >>> 15), t | 1);
+  t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
+  return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
+}
+
+export function randomInt(h: RngHolder, min: number, max: number): number {
+  return min + Math.floor(random(h) * (max - min + 1));
+}
+
+export function weightedPick<T>(
+  h: RngHolder,
+  items: readonly T[],
+  weight: (item: T) => number,
+): T | undefined {
+  const weights = items.map((it) => Math.max(0, weight(it)));
+  const total = weights.reduce((sum, w) => sum + w, 0);
+  if (total <= 0) return undefined;
+  let r = random(h) * total;
+  let lastPositive: T | undefined;
+  for (let i = 0; i < items.length; i++) {
+    const w = weights[i];
+    if (w === 0) continue;
+    lastPositive = items[i];
+    if (r < w) return items[i];
+    r -= w;
+  }
+  return lastPositive;
+}
diff --git a/tests/sim/rng.test.ts b/tests/sim/rng.test.ts
new file mode 100644
index 0000000..5ae2c96
--- /dev/null
+++ b/tests/sim/rng.test.ts
@@ -0,0 +1,48 @@
+import { describe, it, expect } from 'vitest';
+import { random, randomInt, weightedPick } from '../../src/sim/rng';
+
+describe('rng', () => {
+  it('is deterministic for a given seed', () => {
+    const a = { rngState: 42 };
+    const b = { rngState: 42 };
+    const seqA = [random(a), random(a), random(a)];
+    const seqB = [random(b), random(b), random(b)];
+    expect(seqA).toEqual(seqB);
+  });
+
+  it('returns values in [0, 1)', () => {
+    const h = { rngState: 7 };
+    for (let i = 0; i < 1000; i++) {
+      const v = random(h);
+      expect(v).toBeGreaterThanOrEqual(0);
+      expect(v).toBeLessThan(1);
+    }
+  });
+
+  it('randomInt stays within inclusive bounds', () => {
+    const h = { rngState: 1 };
+    const seen = new Set<number>();
+    for (let i = 0; i < 1000; i++) {
+      const v = randomInt(h, 3, 5);
+      expect(v).toBeGreaterThanOrEqual(3);
+      expect(v).toBeLessThanOrEqual(5);
+      seen.add(v);
+    }
+    expect(seen.size).toBe(3);
+  });
+
+  it('weightedPick never picks zero-weight items', () => {
+    const h = { rngState: 3 };
+    const items = ['a', 'b', 'c'];
+    for (let i = 0; i < 500; i++) {
+      const v = weightedPick(h, items, (x) => (x === 'b' ? 0 : 1));
+      expect(v).not.toBe('b');
+    }
+  });
+
+  it('weightedPick returns undefined when all weights are zero', () => {
+    const h = { rngState: 3 };
+    expect(weightedPick(h, ['a'], () => 0)).toBeUndefined();
+    expect(weightedPick(h, [], () => 1)).toBeUndefined();
+  });
+});