Implement the 002-add-combatant feature that adds the possibility to add new combatants to an encounter

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

@@ -0,0 +1,200 @@
+import { describe, expect, it } from "vitest";
+import { addCombatant } from "../add-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");
+
+function enc(
+	combatants: Combatant[],
+	activeIndex = 0,
+	roundNumber = 1,
+): Encounter {
+	return { combatants, activeIndex, roundNumber };
+}
+
+function successResult(encounter: Encounter, id: string, name: string) {
+	const result = addCombatant(encounter, combatantId(id), name);
+	if (isDomainError(result)) {
+		throw new Error(`Expected success, got error: ${result.message}`);
+	}
+	return result;
+}
+
+// --- Acceptance Scenarios ---
+
+describe("addCombatant", () => {
+	describe("acceptance scenarios", () => {
+		it("scenario 1: add to empty encounter", () => {
+			const e = enc([], 0, 1);
+			const { encounter, events } = successResult(e, "gandalf", "Gandalf");
+
+			expect(encounter.combatants).toEqual([
+				{ id: combatantId("gandalf"), name: "Gandalf" },
+			]);
+			expect(encounter.activeIndex).toBe(0);
+			expect(encounter.roundNumber).toBe(1);
+			expect(events).toEqual([
+				{
+					type: "CombatantAdded",
+					combatantId: combatantId("gandalf"),
+					name: "Gandalf",
+					position: 0,
+				},
+			]);
+		});
+
+		it("scenario 2: add to encounter with [A, B]", () => {
+			const e = enc([A, B], 0, 1);
+			const { encounter, events } = successResult(e, "C", "C");
+
+			expect(encounter.combatants).toEqual([
+				A,
+				B,
+				{ id: combatantId("C"), name: "C" },
+			]);
+			expect(encounter.activeIndex).toBe(0);
+			expect(encounter.roundNumber).toBe(1);
+			expect(events).toEqual([
+				{
+					type: "CombatantAdded",
+					combatantId: combatantId("C"),
+					name: "C",
+					position: 2,
+				},
+			]);
+		});
+
+		it("scenario 3: add during mid-round does not change active combatant", () => {
+			const e = enc([A, B, C], 2, 3);
+			const { encounter, events } = successResult(e, "D", "D");
+
+			expect(encounter.combatants).toHaveLength(4);
+			expect(encounter.combatants[3]).toEqual({
+				id: combatantId("D"),
+				name: "D",
+			});
+			expect(encounter.activeIndex).toBe(2);
+			expect(encounter.roundNumber).toBe(3);
+			expect(events).toEqual([
+				{
+					type: "CombatantAdded",
+					combatantId: combatantId("D"),
+					name: "D",
+					position: 3,
+				},
+			]);
+		});
+
+		it("scenario 4: two sequential adds preserve order", () => {
+			const e = enc([A]);
+			const first = successResult(e, "B", "B");
+			const second = successResult(first.encounter, "C", "C");
+
+			expect(second.encounter.combatants).toEqual([
+				A,
+				{ id: combatantId("B"), name: "B" },
+				{ id: combatantId("C"), name: "C" },
+			]);
+			expect(first.events).toHaveLength(1);
+			expect(second.events).toHaveLength(1);
+		});
+
+		it("scenario 5: empty name returns error", () => {
+			const e = enc([A, B]);
+			const result = addCombatant(e, combatantId("x"), "");
+
+			expect(isDomainError(result)).toBe(true);
+			if (isDomainError(result)) {
+				expect(result.code).toBe("invalid-name");
+			}
+		});
+
+		it("scenario 6: whitespace-only name returns error", () => {
+			const e = enc([A, B]);
+			const result = addCombatant(e, combatantId("x"), "   ");
+
+			expect(isDomainError(result)).toBe(true);
+			if (isDomainError(result)) {
+				expect(result.code).toBe("invalid-name");
+			}
+		});
+	});
+
+	describe("invariants", () => {
+		it("INV-1: encounter may have zero combatants (adding to empty is valid)", () => {
+			const e = enc([]);
+			const result = addCombatant(e, combatantId("a"), "A");
+			expect(isDomainError(result)).toBe(false);
+		});
+
+		it("INV-2: activeIndex remains valid after adding", () => {
+			const scenarios: Encounter[] = [
+				enc([], 0, 1),
+				enc([A], 0, 1),
+				enc([A, B, C], 2, 3),
+			];
+
+			for (const e of scenarios) {
+				const result = successResult(e, "new", "New");
+				const { combatants, activeIndex } = result.encounter;
+				if (combatants.length > 0) {
+					expect(activeIndex).toBeGreaterThanOrEqual(0);
+					expect(activeIndex).toBeLessThan(combatants.length);
+				} else {
+					expect(activeIndex).toBe(0);
+				}
+			}
+		});
+
+		it("INV-3: roundNumber is preserved (never decreases)", () => {
+			const e = enc([A, B], 1, 5);
+			const { encounter } = successResult(e, "C", "C");
+			expect(encounter.roundNumber).toBe(5);
+		});
+
+		it("INV-4: determinism — same input produces same output", () => {
+			const e = enc([A, B], 1, 3);
+			const result1 = addCombatant(e, combatantId("x"), "X");
+			const result2 = addCombatant(e, combatantId("x"), "X");
+			expect(result1).toEqual(result2);
+		});
+
+		it("INV-5: every success emits exactly one CombatantAdded event", () => {
+			const scenarios: Encounter[] = [enc([]), enc([A]), enc([A, B, C], 2, 5)];
+
+			for (const e of scenarios) {
+				const result = successResult(e, "z", "Z");
+				expect(result.events).toHaveLength(1);
+				expect(result.events[0].type).toBe("CombatantAdded");
+			}
+		});
+
+		it("INV-6: addCombatant does not change activeIndex or roundNumber", () => {
+			const e = enc([A, B, C], 2, 7);
+			const { encounter } = successResult(e, "D", "D");
+			expect(encounter.activeIndex).toBe(2);
+			expect(encounter.roundNumber).toBe(7);
+		});
+
+		it("INV-7: new combatant is always appended at the end", () => {
+			const e = enc([A, B]);
+			const { encounter } = successResult(e, "C", "C");
+			expect(encounter.combatants[encounter.combatants.length - 1]).toEqual({
+				id: combatantId("C"),
+				name: "C",
+			});
+			// Existing combatants preserve order
+			expect(encounter.combatants[0]).toEqual(A);
+			expect(encounter.combatants[1]).toEqual(B);
+		});
+	});
+});

packages/domain/src/add-combatant.ts

@@ -0,0 +1,50 @@
+import type { DomainEvent } from "./events.js";
+import type { CombatantId, DomainError, Encounter } from "./types.js";
+
+export interface AddCombatantSuccess {
+	readonly encounter: Encounter;
+	readonly events: DomainEvent[];
+}
+
+/**
+ * Pure function that adds a combatant to the end of an encounter's list.
+ *
+ * FR-001: Accepts an Encounter, CombatantId, and name; returns next state + events.
+ * FR-002: Appends new combatant to end of combatants list.
+ * FR-004: Rejects empty/whitespace-only names with DomainError.
+ * FR-005: Does not alter activeIndex or roundNumber.
+ * FR-006: Events returned as values, not dispatched via side effects.
+ */
+export function addCombatant(
+	encounter: Encounter,
+	id: CombatantId,
+	name: string,
+): AddCombatantSuccess | DomainError {
+	const trimmed = name.trim();
+
+	if (trimmed === "") {
+		return {
+			kind: "domain-error",
+			code: "invalid-name",
+			message: "Combatant name must not be empty",
+		};
+	}
+
+	const position = encounter.combatants.length;
+
+	return {
+		encounter: {
+			combatants: [...encounter.combatants, { id, name: trimmed }],
+			activeIndex: encounter.activeIndex,
+			roundNumber: encounter.roundNumber,
+		},
+		events: [
+			{
+				type: "CombatantAdded",
+				combatantId: id,
+				name: trimmed,
+				position,
+			},
+		],
+	};
+}

packages/domain/src/events.ts

@@ -12,4 +12,11 @@ export interface RoundAdvanced {
 	readonly newRoundNumber: number;
 }
 
-export type DomainEvent = TurnAdvanced | RoundAdvanced;
+export interface CombatantAdded {
+	readonly type: "CombatantAdded";
+	readonly combatantId: CombatantId;
+	readonly name: string;
+	readonly position: number;
+}
+
+export type DomainEvent = TurnAdvanced | RoundAdvanced | CombatantAdded;

packages/domain/src/index.ts

@@ -1,6 +1,8 @@
+export { type AddCombatantSuccess, addCombatant } from "./add-combatant.js";
 export { advanceTurn } from "./advance-turn.js";
 
 export type {
+	CombatantAdded,
 	DomainEvent,
 	RoundAdvanced,
 	TurnAdvanced,