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,
			},
		],
	};
}