initiative/packages/domain/src/__tests__/retreat-turn.test.ts

import { describe, expect, it } from "vitest";
import type { DomainEvent } from "../events.js";
import { retreatTurn } from "../retreat-turn.js";
import {
	type Combatant,
	combatantId,
	createEncounter,
	type Encounter,
	isDomainError,
} from "../types.js";
import { expectDomainError } from "./test-helpers.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 encounter(
	combatants: Combatant[],
	activeIndex: number,
	roundNumber: number,
): Encounter {
	const result = createEncounter(combatants, activeIndex, roundNumber);
	if (isDomainError(result)) {
		throw new Error(`Test setup failed: ${result.message}`);
	}
	return result;
}

function successResult(enc: Encounter) {
	const result = retreatTurn(enc);
	if (isDomainError(result)) {
		throw new Error(`Expected success, got error: ${result.message}`);
	}
	return result;
}

// --- Acceptance Scenarios ---

describe("retreatTurn", () => {
	describe("acceptance scenarios", () => {
		it("scenario 1: mid-round retreat — retreats from second to first combatant", () => {
			const enc = encounter([A, B, C], 1, 1);
			const { encounter: next, events } = successResult(enc);

			expect(next.activeIndex).toBe(0);
			expect(next.roundNumber).toBe(1);
			expect(events).toEqual([
				{
					type: "TurnRetreated",
					previousCombatantId: combatantId("B"),
					newCombatantId: combatantId("A"),
					roundNumber: 1,
				},
			]);
		});

		it("scenario 2: round-boundary retreat — wraps from first combatant to last, decrements round", () => {
			const enc = encounter([A, B, C], 0, 2);
			const { encounter: next, events } = successResult(enc);

			expect(next.activeIndex).toBe(2);
			expect(next.roundNumber).toBe(1);
			expect(events).toEqual([
				{
					type: "TurnRetreated",
					previousCombatantId: combatantId("A"),
					newCombatantId: combatantId("C"),
					roundNumber: 1,
				},
				{
					type: "RoundRetreated",
					newRoundNumber: 1,
				},
			]);
		});

		it("scenario 3: start-of-encounter error — cannot retreat at round 1 index 0", () => {
			const enc = encounter([A, B, C], 0, 1);
			const result = retreatTurn(enc);

			expectDomainError(result, "no-previous-turn");
		});

		it("scenario 4: single-combatant retreat — wraps to same combatant, decrements round", () => {
			const enc = encounter([A], 0, 2);
			const { encounter: next, events } = successResult(enc);

			expect(next.activeIndex).toBe(0);
			expect(next.roundNumber).toBe(1);
			expect(events).toEqual([
				{
					type: "TurnRetreated",
					previousCombatantId: combatantId("A"),
					newCombatantId: combatantId("A"),
					roundNumber: 1,
				},
				{
					type: "RoundRetreated",
					newRoundNumber: 1,
				},
			]);
		});

		it("scenario 5: empty-encounter error", () => {
			const enc: Encounter = {
				combatants: [],
				activeIndex: 0,
				roundNumber: 1,
			};
			const result = retreatTurn(enc);

			expectDomainError(result, "invalid-encounter");
		});
	});

	describe("invariants", () => {
		it("determinism — same input produces same output", () => {
			const enc = encounter([A, B, C], 1, 3);
			const result1 = retreatTurn(enc);
			const result2 = retreatTurn(enc);
			expect(result1).toEqual(result2);
		});

		it("activeIndex always in bounds after retreat", () => {
			const combatants = [A, B, C];
			// Start at round 4 so we can retreat many times
			let enc = encounter(combatants, 2, 4);

			for (let i = 0; i < 9; i++) {
				const result = successResult(enc);
				expect(result.encounter.activeIndex).toBeGreaterThanOrEqual(0);
				expect(result.encounter.activeIndex).toBeLessThan(combatants.length);
				enc = result.encounter;
			}
		});

		it("roundNumber never goes below 1", () => {
			let enc = encounter([A, B, C], 2, 2);

			// Retreat through rounds — should stop at round 1 index 0
			while (!(enc.roundNumber === 1 && enc.activeIndex === 0)) {
				const result = successResult(enc);
				expect(result.encounter.roundNumber).toBeGreaterThanOrEqual(1);
				enc = result.encounter;
			}
		});

		it("every success emits at least TurnRetreated", () => {
			const scenarios: Encounter[] = [
				encounter([A, B, C], 1, 1),
				encounter([A, B, C], 0, 2),
				encounter([A], 0, 2),
			];

			for (const enc of scenarios) {
				const result = successResult(enc);
				const hasTurnRetreated = result.events.some(
					(e: DomainEvent) => e.type === "TurnRetreated",
				);
				expect(hasTurnRetreated).toBe(true);
			}
		});

		it("event ordering: on wrap, events are [TurnRetreated, RoundRetreated]", () => {
			const enc = encounter([A, B, C], 0, 2);
			const { events } = successResult(enc);

			expect(events).toHaveLength(2);
			expect(events[0].type).toBe("TurnRetreated");
			expect(events[1].type).toBe("RoundRetreated");
		});
	});
});