From cf028e9a08d46233fefcb5ff3cb75d164c8f66ca Mon Sep 17 00:00:00 2001
From: Tim Evans <timevans666@gmail.com>
Date: Mon, 20 Jul 2026 13:49:54 +0100
Subject: [PATCH] feat: seedable dice core with pools

---
 src/dice/roller.js        | 18 ++++++++++++++++++
 tests/dice/roller.test.js | 26 ++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 src/dice/roller.js
 create mode 100644 tests/dice/roller.test.js

diff --git a/src/dice/roller.js b/src/dice/roller.js
new file mode 100644
index 0000000..9669f0b
--- /dev/null
+++ b/src/dice/roller.js
@@ -0,0 +1,18 @@
+export function expandDice(diceSpecs) {
+  const out = [];
+  for (const d of diceSpecs) {
+    const count = d.count ?? 1;
+    for (let i = 0; i < count; i++) out.push({ sides: d.sides });
+  }
+  return out;
+}
+
+export function rollDie(sides, rng) {
+  return Math.floor(rng() * sides) + 1;
+}
+
+export function rollPool(diceSpecs, rng) {
+  const results = expandDice(diceSpecs).map((d) => ({ sides: d.sides, value: rollDie(d.sides, rng) }));
+  const total = results.reduce((s, r) => s + r.value, 0);
+  return { results, total };
+}
diff --git a/tests/dice/roller.test.js b/tests/dice/roller.test.js
new file mode 100644
index 0000000..b5251b2
--- /dev/null
+++ b/tests/dice/roller.test.js
@@ -0,0 +1,26 @@
+import { describe, it, expect } from 'vitest';
+import { expandDice, rollDie, rollPool } from '../../src/dice/roller.js';
+
+// deterministic rng that replays a fixed sequence
+function seq(values) { let i = 0; return () => values[i++ % values.length]; }
+
+describe('dice core', () => {
+  it('expands count into individual dice', () => {
+    expect(expandDice([{ sides: 8, count: 3 }])).toEqual([{ sides: 8 }, { sides: 8 }, { sides: 8 }]);
+    expect(expandDice([{ sides: 6 }])).toEqual([{ sides: 6 }]);
+  });
+
+  it('maps rng to inclusive 1..sides', () => {
+    expect(rollDie(8, () => 0)).toBe(1);
+    expect(rollDie(8, () => 0.999)).toBe(8);
+    expect(rollDie(6, () => 0.5)).toBe(4);
+  });
+
+  it('rolls a pool and totals it', () => {
+    const rng = seq([0.9, 0.0, 0.7]); // d8 -> 8, 1, 6
+    const { results, total } = rollPool([{ sides: 8, count: 3 }], rng);
+    expect(results.map((r) => r.value)).toEqual([8, 1, 6]);
+    expect(total).toBe(15);
+    expect(results[0]).toEqual({ sides: 8, value: 8 });
+  });
+});