import type { DomainEvent } from "./events.js";
import type { DomainError, Encounter } from "./types.js";

interface RetreatTurnSuccess {
	readonly encounter: Encounter;
	readonly events: DomainEvent[];
}

export function retreatTurn(
	encounter: Encounter,
): RetreatTurnSuccess | DomainError {
	if (encounter.combatants.length === 0) {
		return {
			kind: "domain-error",
			code: "invalid-encounter",
			message: "Cannot retreat turn on an encounter with no combatants",
		};
	}

	if (encounter.roundNumber === 1 && encounter.activeIndex === 0) {
		return {
			kind: "domain-error",
			code: "no-previous-turn",
			message: "Cannot retreat before the start of the encounter",
		};
	}

	const previousIndex = encounter.activeIndex;
	const wraps = previousIndex === 0;
	const newIndex = wraps ? encounter.combatants.length - 1 : previousIndex - 1;
	const newRoundNumber = wraps
		? encounter.roundNumber - 1
		: encounter.roundNumber;

	const events: DomainEvent[] = [
		{
			type: "TurnRetreated",
			previousCombatantId: encounter.combatants[previousIndex].id,
			newCombatantId: encounter.combatants[newIndex].id,
			roundNumber: newRoundNumber,
		},
	];

	if (wraps) {
		events.push({
			type: "RoundRetreated",
			newRoundNumber,
		});
	}

	return {
		encounter: {
			combatants: encounter.combatants,
			activeIndex: newIndex,
			roundNumber: newRoundNumber,
		},
		events,
	};
}