/**
 * EventBus - Hệ Thống Sự Kiện Trung Tâm
 * Cung cấp giao tiếp dựa trên sự kiện giữa các thành phần
 * Requirements: 2.2, 11.4
 */

const EventBus = (function () {
    // ─── Private State ────────────────────────────────────────────────────────

    /** @type {Map<string, Set<Function>>} */
    const _listeners = new Map();

    // ─── Private Helpers ──────────────────────────────────────────────────────

    /**
     * Validate event name.
     * @param {string} eventName
     */
    function _validateEvent(eventName) {
        if (!eventName || typeof eventName !== 'string') {
            throw new TypeError('[EventBus] Event name must be a non-empty string');
        }
    }

    // ─── Public API ───────────────────────────────────────────────────────────

    return {
        /**
         * Subscribe to an event.
         * @param {string} eventName - Event name to listen for
         * @param {Function} callback - Handler called with event data
         * @returns {Function} Unsubscribe function
         */
        on(eventName, callback) {
            _validateEvent(eventName);
            if (typeof callback !== 'function') {
                throw new TypeError('[EventBus] Callback must be a function');
            }

            if (!_listeners.has(eventName)) {
                _listeners.set(eventName, new Set());
            }
            _listeners.get(eventName).add(callback);

            // Return unsubscribe function
            return function off() {
                const set = _listeners.get(eventName);
                if (set) {
                    set.delete(callback);
                    if (set.size === 0) {
                        _listeners.delete(eventName);
                    }
                }
            };
        },

        /**
         * Unsubscribe a specific callback from an event.
         * @param {string} eventName
         * @param {Function} callback
         */
        off(eventName, callback) {
            _validateEvent(eventName);
            const set = _listeners.get(eventName);
            if (set) {
                set.delete(callback);
                if (set.size === 0) {
                    _listeners.delete(eventName);
                }
            }
        },

        /**
         * Emit an event with optional data.
         * @param {string} eventName
         * @param {*} [data]
         */
        emit(eventName, data) {
            _validateEvent(eventName);

            const set = _listeners.get(eventName);
            if (!set || set.size === 0) return;

            // Iterate over a copy to allow handlers to call off() safely
            const handlers = Array.from(set);
            for (const handler of handlers) {
                try {
                    handler(data);
                } catch (err) {
                    console.error(`[EventBus] Error in handler for "${eventName}":`, err);
                }
            }
        },

        /**
         * Subscribe to an event for a single invocation, then auto-unsubscribe.
         * @param {string} eventName
         * @param {Function} callback
         * @returns {Function} Unsubscribe function
         */
        once(eventName, callback) {
            const unsubscribe = this.on(eventName, (data) => {
                unsubscribe();
                callback(data);
            });
            return unsubscribe;
        },

        /**
         * Remove all listeners for a specific event, or all events if no name given.
         * @param {string} [eventName]
         */
        clear(eventName) {
            if (eventName) {
                _listeners.delete(eventName);
            } else {
                _listeners.clear();
            }
        },

        /**
         * Get the number of listeners for an event (useful for debugging/testing).
         * @param {string} eventName
         * @returns {number}
         */
        listenerCount(eventName) {
            const set = _listeners.get(eventName);
            return set ? set.size : 0;
        }
    };
})();

// Export as global for browser usage
window.EventBus = EventBus;