From 605b107c7707836c0f36200f0e1f982ecbea77e9 Mon Sep 17 00:00:00 2001
From: Tim Evans <timevans666@gmail.com>
Date: Mon, 20 Jul 2026 16:18:27 +0100
Subject: [PATCH] feat: CoC 7e d100 rule-pack

---
 src/dice/rulepacks/coc-d100.js | 22 ++++++++++++++++++++++
 src/dice/rulepacks/index.js    |  3 ++-
 tests/dice/coc-d100.test.js    | 32 ++++++++++++++++++++++++++++++++
 3 files changed, 56 insertions(+), 1 deletion(-)
 create mode 100644 src/dice/rulepacks/coc-d100.js
 create mode 100644 tests/dice/coc-d100.test.js

diff --git a/src/dice/rulepacks/coc-d100.js b/src/dice/rulepacks/coc-d100.js
new file mode 100644
index 0000000..173802b
--- /dev/null
+++ b/src/dice/rulepacks/coc-d100.js
@@ -0,0 +1,22 @@
+// Call of Cthulhu 7e success ladder vs a skill %.
+export const cocD100 = {
+  id: 'coc-d100',
+  label: 'd100 (Call of Cthulhu)',
+  params: [{ key: 'target', label: 'Skill %', default: 50 }],
+  interpret(results, params = {}) {
+    const value = results[0]?.value ?? 0;
+    const target = params.target ?? 50;
+    let level;
+    if (value === 1) level = 'critical';
+    else if (value <= Math.floor(target / 5)) level = 'extreme';
+    else if (value <= Math.floor(target / 2)) level = 'hard';
+    else if (value <= target) level = 'success';
+    else if ((target < 50 && value >= 96) || value === 100) level = 'fumble';
+    else level = 'failure';
+    const success = ['critical', 'extreme', 'hard', 'success'].includes(level);
+    return { value, target, level, success };
+  },
+  summary(v) {
+    return `${v.value} vs ${v.target}% → ${v.level.toUpperCase()}`;
+  },
+};
diff --git a/src/dice/rulepacks/index.js b/src/dice/rulepacks/index.js
index 8432605..aefe47f 100644
--- a/src/dice/rulepacks/index.js
+++ b/src/dice/rulepacks/index.js
@@ -1,6 +1,7 @@
 import { yearZero } from './year-zero.js';
+import { cocD100 } from './coc-d100.js';
 
-const packs = new Map([[yearZero.id, yearZero]]);
+const packs = new Map([[yearZero.id, yearZero], [cocD100.id, cocD100]]);
 
 export function getRulePack(id) { return packs.get(id) || null; }
 export function registerRulePack(pack) { packs.set(pack.id, pack); }
diff --git a/tests/dice/coc-d100.test.js b/tests/dice/coc-d100.test.js
new file mode 100644
index 0000000..a4f5e07
--- /dev/null
+++ b/tests/dice/coc-d100.test.js
@@ -0,0 +1,32 @@
+import { describe, it, expect } from 'vitest';
+import { cocD100 } from '../../src/dice/rulepacks/coc-d100.js';
+import { getRulePack } from '../../src/dice/rulepacks/index.js';
+
+const roll = (v) => [{ sides: 100, value: v }];
+
+describe('coc-d100 rule-pack', () => {
+  it('grades a roll against a 50% skill', () => {
+    expect(cocD100.interpret(roll(1), { target: 50 }).level).toBe('critical');
+    expect(cocD100.interpret(roll(10), { target: 50 }).level).toBe('extreme');  // <= 10
+    expect(cocD100.interpret(roll(25), { target: 50 }).level).toBe('hard');     // <= 25
+    expect(cocD100.interpret(roll(50), { target: 50 }).level).toBe('success');
+    expect(cocD100.interpret(roll(51), { target: 50 }).level).toBe('failure');
+    expect(cocD100.interpret(roll(100), { target: 50 }).level).toBe('fumble');
+  });
+  it('fumbles on 96+ when the skill is under 50', () => {
+    expect(cocD100.interpret(roll(96), { target: 40 }).level).toBe('fumble');
+    expect(cocD100.interpret(roll(41), { target: 40 }).level).toBe('failure');
+  });
+  it('marks the four passing bands as success', () => {
+    expect(cocD100.interpret(roll(10), { target: 50 }).success).toBe(true);
+    expect(cocD100.interpret(roll(51), { target: 50 }).success).toBe(false);
+  });
+  it('defaults target to 50 and summarises', () => {
+    const v = cocD100.interpret(roll(5));
+    expect(v.target).toBe(50);
+    expect(cocD100.summary(v)).toBe('5 vs 50% → EXTREME');
+  });
+  it('is registered', () => {
+    expect(getRulePack('coc-d100')).toBe(cocD100);
+  });
+});