Implement the 003-remove-combatant feature that adds the possibility to remove a combatant from an encounter

packages/domain/src/__tests__/remove-combatant.test.ts

@@ -0,0 +1,142 @@
+import { describe, expect, it } from "vitest";
+import { removeCombatant } from "../remove-combatant.js";
+import type { Combatant, Encounter } from "../types.js";
+import { combatantId, isDomainError } from "../types.js";
+
+// --- Helpers ---
+
+function makeCombatant(name: string): Combatant {
+	return { id: combatantId(name), name };
+}
+
+const A = makeCombatant("A");
+const B = makeCombatant("B");
+const C = makeCombatant("C");
+const D = makeCombatant("D");
+
+function enc(
+	combatants: Combatant[],
+	activeIndex = 0,
+	roundNumber = 1,
+): Encounter {
+	return { combatants, activeIndex, roundNumber };
+}
+
+function successResult(encounter: Encounter, id: string) {
+	const result = removeCombatant(encounter, combatantId(id));
+	if (isDomainError(result)) {
+		throw new Error(`Expected success, got error: ${result.message}`);
+	}
+	return result;
+}
+
+// --- Acceptance Scenarios ---
+
+describe("removeCombatant", () => {
+	describe("acceptance scenarios", () => {
+		it("AS-1: remove combatant after active — activeIndex unchanged", () => {
+			// [A*, B, C] remove C → [A*, B], activeIndex stays 0
+			const e = enc([A, B, C], 0, 2);
+			const { encounter, events } = successResult(e, "C");
+
+			expect(encounter.combatants).toEqual([A, B]);
+			expect(encounter.activeIndex).toBe(0);
+			expect(encounter.roundNumber).toBe(2);
+			expect(events).toEqual([
+				{
+					type: "CombatantRemoved",
+					combatantId: combatantId("C"),
+					name: "C",
+				},
+			]);
+		});
+
+		it("AS-2: remove combatant before active — activeIndex decrements", () => {
+			// [A, B, C*] remove A → [B, C*], activeIndex 2→1
+			const e = enc([A, B, C], 2, 3);
+			const { encounter } = successResult(e, "A");
+
+			expect(encounter.combatants).toEqual([B, C]);
+			expect(encounter.activeIndex).toBe(1);
+			expect(encounter.roundNumber).toBe(3);
+		});
+
+		it("AS-3: remove active combatant mid-list — next slides in", () => {
+			// [A, B*, C, D] remove B → [A, C*, D], activeIndex stays 1
+			const e = enc([A, B, C, D], 1, 1);
+			const { encounter } = successResult(e, "B");
+
+			expect(encounter.combatants).toEqual([A, C, D]);
+			expect(encounter.activeIndex).toBe(1);
+		});
+
+		it("AS-4: remove active combatant at end — wraps to 0", () => {
+			// [A, B, C*] remove C → [A, B], activeIndex wraps to 0
+			const e = enc([A, B, C], 2, 1);
+			const { encounter } = successResult(e, "C");
+
+			expect(encounter.combatants).toEqual([A, B]);
+			expect(encounter.activeIndex).toBe(0);
+		});
+
+		it("AS-5: remove only combatant — empty list, activeIndex 0", () => {
+			const e = enc([A], 0, 5);
+			const { encounter } = successResult(e, "A");
+
+			expect(encounter.combatants).toEqual([]);
+			expect(encounter.activeIndex).toBe(0);
+			expect(encounter.roundNumber).toBe(5);
+		});
+
+		it("AS-6: ID not found — returns DomainError", () => {
+			const e = enc([A, B], 0, 1);
+			const result = removeCombatant(e, combatantId("nonexistent"));
+
+			expect(isDomainError(result)).toBe(true);
+			if (isDomainError(result)) {
+				expect(result.code).toBe("combatant-not-found");
+			}
+		});
+	});
+
+	describe("invariants", () => {
+		it("event shape includes combatantId and name", () => {
+			const e = enc([A, B], 0, 1);
+			const { events } = successResult(e, "B");
+
+			expect(events).toHaveLength(1);
+			expect(events[0]).toEqual({
+				type: "CombatantRemoved",
+				combatantId: combatantId("B"),
+				name: "B",
+			});
+		});
+
+		it("roundNumber never changes on removal", () => {
+			const e = enc([A, B, C], 1, 7);
+			const { encounter } = successResult(e, "A");
+			expect(encounter.roundNumber).toBe(7);
+		});
+
+		it("determinism — same input produces same output", () => {
+			const e = enc([A, B, C], 1, 3);
+			const result1 = removeCombatant(e, combatantId("B"));
+			const result2 = removeCombatant(e, combatantId("B"));
+			expect(result1).toEqual(result2);
+		});
+
+		it("every success emits exactly one CombatantRemoved event", () => {
+			const scenarios: [Encounter, string][] = [
+				[enc([A]), "A"],
+				[enc([A, B], 1), "A"],
+				[enc([A, B, C], 2, 5), "C"],
+			];
+
+			for (const [e, id] of scenarios) {
+				const { events } = successResult(e, id);
+				expect(events).toHaveLength(1);
+				expect(events[0].type).toBe("CombatantRemoved");
+			}
+		});
+	});
+});

packages/domain/src/events.ts

@@ -19,4 +19,14 @@ export interface CombatantAdded {
 	readonly position: number;
 }
 
-export type DomainEvent = TurnAdvanced | RoundAdvanced | CombatantAdded;
+export interface CombatantRemoved {
+	readonly type: "CombatantRemoved";
+	readonly combatantId: CombatantId;
+	readonly name: string;
+}
+
+export type DomainEvent =
+	| TurnAdvanced
+	| RoundAdvanced
+	| CombatantAdded
+	| CombatantRemoved;

packages/domain/src/index.ts

@@ -1,12 +1,16 @@
 export { type AddCombatantSuccess, addCombatant } from "./add-combatant.js";
 export { advanceTurn } from "./advance-turn.js";
-
 export type {
 	CombatantAdded,
+	CombatantRemoved,
 	DomainEvent,
 	RoundAdvanced,
 	TurnAdvanced,
 } from "./events.js";
+export {
+	type RemoveCombatantSuccess,
+	removeCombatant,
+} from "./remove-combatant.js";
 export {
 	type Combatant,
 	type CombatantId,

packages/domain/src/remove-combatant.ts

@@ -0,0 +1,65 @@
+import type { DomainEvent } from "./events.js";
+import type { CombatantId, DomainError, Encounter } from "./types.js";
+
+export interface RemoveCombatantSuccess {
+	readonly encounter: Encounter;
+	readonly events: DomainEvent[];
+}
+
+/**
+ * Pure function that removes a combatant from an encounter by ID.
+ *
+ * Adjusts activeIndex to preserve turn integrity:
+ * - Removed after active → unchanged
+ * - Removed before active → decrement
+ * - Removed is active, mid-list → same index (next slides in)
+ * - Removed is active, at end → wrap to 0
+ * - Only combatant removed → 0
+ *
+ * roundNumber is never changed.
+ */
+export function removeCombatant(
+	encounter: Encounter,
+	id: CombatantId,
+): RemoveCombatantSuccess | DomainError {
+	const removedIdx = encounter.combatants.findIndex((c) => c.id === id);
+
+	if (removedIdx === -1) {
+		return {
+			kind: "domain-error",
+			code: "combatant-not-found",
+			message: `No combatant found with ID "${id}"`,
+		};
+	}
+
+	const removed = encounter.combatants[removedIdx];
+	const newCombatants = encounter.combatants.filter((_, i) => i !== removedIdx);
+
+	let newActiveIndex: number;
+	if (newCombatants.length === 0) {
+		newActiveIndex = 0;
+	} else if (removedIdx < encounter.activeIndex) {
+		newActiveIndex = encounter.activeIndex - 1;
+	} else if (removedIdx > encounter.activeIndex) {
+		newActiveIndex = encounter.activeIndex;
+	} else {
+		// removedIdx === activeIndex
+		newActiveIndex =
+			removedIdx >= newCombatants.length ? 0 : encounter.activeIndex;
+	}
+
+	return {
+		encounter: {
+			combatants: newCombatants,
+			activeIndex: newActiveIndex,
+			roundNumber: encounter.roundNumber,
+		},
+		events: [
+			{
+				type: "CombatantRemoved",
+				combatantId: removed.id,
+				name: removed.name,
+			},
+		],
+	};
+}