Implement the 026-roll-initiative feature that adds d20 roll buttons for bestiary combatants' initiative using a click-to-edit pattern (d20 icon when empty, plain text when set), plus a Roll All button in the top bar that batch-rolls for all unrolled bestiary combatants, with randomness confined to the adapter layer and the domain receiving pre-resolved dice values

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/domain/src/__tests__/roll-initiative.test.ts

@@ -0,0 +1,70 @@
+import { describe, expect, it } from "vitest";
+import { rollInitiative } from "../roll-initiative.js";
+import { isDomainError } from "../types.js";
+
+describe("rollInitiative", () => {
+	describe("valid rolls", () => {
+		it("normal roll: 15 + modifier 7 = 22", () => {
+			expect(rollInitiative(15, 7)).toBe(22);
+		});
+
+		it("boundary: roll 1 + modifier 0 = 1", () => {
+			expect(rollInitiative(1, 0)).toBe(1);
+		});
+
+		it("boundary: roll 20 + modifier 0 = 20", () => {
+			expect(rollInitiative(20, 0)).toBe(20);
+		});
+
+		it("negative modifier: roll 1 + (−3) = −2", () => {
+			expect(rollInitiative(1, -3)).toBe(-2);
+		});
+
+		it("zero modifier: roll 10 + 0 = 10", () => {
+			expect(rollInitiative(10, 0)).toBe(10);
+		});
+
+		it("large positive modifier: roll 20 + 12 = 32", () => {
+			expect(rollInitiative(20, 12)).toBe(32);
+		});
+	});
+
+	describe("invalid dice rolls", () => {
+		it("rejects 0", () => {
+			const result = rollInitiative(0, 5);
+			expect(isDomainError(result)).toBe(true);
+			if (isDomainError(result)) {
+				expect(result.code).toBe("invalid-dice-roll");
+			}
+		});
+
+		it("rejects 21", () => {
+			const result = rollInitiative(21, 5);
+			expect(isDomainError(result)).toBe(true);
+			if (isDomainError(result)) {
+				expect(result.code).toBe("invalid-dice-roll");
+			}
+		});
+
+		it("rejects non-integer (3.5)", () => {
+			const result = rollInitiative(3.5, 0);
+			expect(isDomainError(result)).toBe(true);
+		});
+
+		it("rejects negative dice roll", () => {
+			const result = rollInitiative(-1, 0);
+			expect(isDomainError(result)).toBe(true);
+		});
+
+		it("rejects NaN", () => {
+			const result = rollInitiative(Number.NaN, 0);
+			expect(isDomainError(result)).toBe(true);
+		});
+	});
+
+	describe("determinism", () => {
+		it("same input produces same output", () => {
+			expect(rollInitiative(10, 5)).toBe(rollInitiative(10, 5));
+		});
+	});
+});

packages/domain/src/index.ts

@@ -57,6 +57,7 @@ export {
 	removeCombatant,
 } from "./remove-combatant.js";
 export { retreatTurn } from "./retreat-turn.js";
+export { rollInitiative } from "./roll-initiative.js";
 export { type SetAcSuccess, setAc } from "./set-ac.js";
 export { type SetHpSuccess, setHp } from "./set-hp.js";
 export {

packages/domain/src/roll-initiative.ts

@@ -0,0 +1,21 @@
+import type { DomainError } from "./types.js";
+
+/**
+ * Pure function that computes initiative from a resolved dice roll and modifier.
+ * The dice roll must be an integer in [1, 20].
+ * Returns the sum (diceRoll + modifier) or a DomainError for invalid inputs.
+ */
+export function rollInitiative(
+	diceRoll: number,
+	modifier: number,
+): number | DomainError {
+	if (!Number.isInteger(diceRoll) || diceRoll < 1 || diceRoll > 20) {
+		return {
+			kind: "domain-error",
+			code: "invalid-dice-roll",
+			message: `Dice roll must be an integer between 1 and 20, got ${diceRoll}`,
+		};
+	}
+
+	return diceRoll + modifier;
+}