import type { ServiceConstructor, ServiceUid } from './types';

const LOG_PREFIX = '[lightweight-sdk:service-registry]';

/**
 * Central singleton registry that holds the single shared instance of every
 * service registered via {@link RegisterService}, keyed by the uid the
 * developer assigned at registration time. Prefer the ready-to-use
 * {@link services} constant over calling {@link ServiceRepository.getInstance}
 * directly.
 */
export class ServiceRepository {
	private static instance: ServiceRepository | undefined;

	private instances = new Map<ServiceUid, unknown>();

	/** Private to enforce the singleton pattern — use {@link ServiceRepository.getInstance} or {@link services}. */
	private constructor() {}

	/**
	 * Returns the single shared instance of the repository, creating it on first access.
	 * @returns the singleton {@link ServiceRepository}
	 */
	public static getInstance(): ServiceRepository {
		if (ServiceRepository.instance === undefined) {
			ServiceRepository.instance = new ServiceRepository();
		}
		return ServiceRepository.instance;
	}

	/**
	 * Clears every registered service. The singleton instance itself is kept
	 * (not discarded), so the {@link services} constant stays valid across
	 * resets. Intended for resetting state between test cases — application
	 * code should not call this.
	 */
	public static resetForTesting(): void {
		ServiceRepository.getInstance().instances.clear();
	}

	/**
	 * Instantiates `target` (its constructor must take no arguments) and
	 * registers the resulting instance under `uid`. Called by
	 * {@link RegisterService} — most callers should use that decorator
	 * instead of calling this method directly.
	 * @param uid the unique identifier the caller assigns to this service
	 * @param target the service class to instantiate and register
	 * @throws {Error} if `uid` is already registered
	 */
	public register<T>(uid: ServiceUid, target: ServiceConstructor<T>): void {
		if (this.instances.has(uid)) {
			throw new Error(`Service uid "${uid}" is already registered.`);
		}

		const instance = new target();
		this.instances.set(uid, instance);

		console.debug(`${LOG_PREFIX} Registered service "${uid}" (${target.name}).`);
	}

	/**
	 * Resolves the instance registered under `uid`.
	 * @param uid the service's unique identifier, as passed to {@link RegisterService}
	 * @returns the instance registered under `uid`
	 * @throws {Error} if no service is registered under `uid`
	 * @example
	 * ```ts
	 * const logger = services.get<LoggerService>(LOGGER_UID);
	 * ```
	 */
	public get<T>(uid: ServiceUid): T {
		if (!this.instances.has(uid)) {
			throw new Error(`No service registered with uid "${uid}". Did you forget @RegisterService?`);
		}
		return this.instances.get(uid) as T;
	}

	/**
	 * Checks whether a service is currently registered under `uid`.
	 * @param uid the unique identifier to check
	 * @returns `true` if a service is registered under `uid`
	 */
	public has(uid: ServiceUid): boolean {
		return this.instances.has(uid);
	}
}

/**
 * Ready-to-use singleton instance of {@link ServiceRepository}. Import this
 * wherever a service needs to be registered or resolved, instead of calling
 * {@link ServiceRepository.getInstance} directly.
 * @example
 * ```ts
 * import { services } from '@lightweight-sdk/service-registry';
 *
 * const logger = services.get<LoggerService>(LOGGER_UID);
 * ```
 */
export const services = ServiceRepository.getInstance();