rogue/src/engine/ecs/EventBus.ts

import { type EntityId } from "../../core/types";
import { type ComponentType } from "./components";

/**
 * Game events for cross-system communication.
 * Systems can emit and subscribe to these events to react to gameplay changes.
 */
export type GameEvent =
  // Combat events
  | { type: "damage"; entityId: EntityId; amount: number; source?: EntityId }
  | { type: "heal"; entityId: EntityId; amount: number; source?: EntityId }
  | { type: "death"; entityId: EntityId; killedBy?: EntityId }

  // Component lifecycle events
  | { type: "component_added"; entityId: EntityId; componentType: ComponentType }
  | { type: "component_removed"; entityId: EntityId; componentType: ComponentType }
  | { type: "entity_created"; entityId: EntityId }
  | { type: "entity_destroyed"; entityId: EntityId }

  // Movement & trigger events
  | { type: "stepped_on"; entityId: EntityId; x: number; y: number }
  | { type: "trigger_activated"; triggerId: EntityId; activatorId: EntityId }

  // Status effect events
  | { type: "status_applied"; entityId: EntityId; status: string; duration: number }
  | { type: "status_expired"; entityId: EntityId; status: string }
  | { type: "status_tick"; entityId: EntityId; status: string; remaining: number }

  // World events
  | { type: "tile_changed"; x: number; y: number };

export type GameEventType = GameEvent["type"];

type EventHandler<T extends GameEvent = GameEvent> = (event: T) => void;

/**
 * Lightweight event bus for cross-system communication.
 * Enables reactive gameplay like status effects, triggers, and combat feedback.
 */
export class EventBus {
  private listeners: Map<string, Set<EventHandler>> = new Map();
  private onceListeners: Map<string, Set<EventHandler>> = new Map();
  private eventQueue: GameEvent[] = [];

  /**
   * Subscribe to events of a specific type.
   * @returns Unsubscribe function
   */
  on<T extends GameEventType>(
    eventType: T,
    handler: EventHandler<Extract<GameEvent, { type: T }>>
  ): () => void {
    if (!this.listeners.has(eventType)) {
      this.listeners.set(eventType, new Set());
    }
    this.listeners.get(eventType)!.add(handler as EventHandler);

    // Return unsubscribe function
    return () => {
      this.listeners.get(eventType)?.delete(handler as EventHandler);
    };
  }

  /**
   * Subscribe to a single occurrence of an event type.
   * The handler is automatically removed after being called once.
   */
  once<T extends GameEventType>(
    eventType: T,
    handler: EventHandler<Extract<GameEvent, { type: T }>>
  ): void {
    if (!this.onceListeners.has(eventType)) {
      this.onceListeners.set(eventType, new Set());
    }
    this.onceListeners.get(eventType)!.add(handler as EventHandler);
  }

  /**
   * Emit an event to all registered listeners.
   */
  emit(event: GameEvent): void {
    const eventType = event.type;

    // Call regular listeners
    const handlers = this.listeners.get(eventType);
    if (handlers) {
      for (const handler of handlers) {
        handler(event);
      }
    }

    // Call once listeners and remove them
    const onceHandlers = this.onceListeners.get(eventType);
    if (onceHandlers) {
      for (const handler of onceHandlers) {
        handler(event);
      }
      this.onceListeners.delete(eventType);
    }

    // Add to queue for drain()
    this.eventQueue.push(event);
  }

  /**
   * Drain all queued events and return them.
   * Clears the internal queue.
   */
  drain(): GameEvent[] {
    const events = [...this.eventQueue];
    this.eventQueue = [];
    return events;
  }

  /**
   * Remove all listeners for a specific event type.
   */
  off(eventType: GameEventType): void {
    this.listeners.delete(eventType);
    this.onceListeners.delete(eventType);
  }

  /**
   * Remove all listeners for all event types.
   */
  clear(): void {
    this.listeners.clear();
    this.onceListeners.clear();
  }

  /**
   * Check if there are any listeners for a specific event type.
   */
  hasListeners(eventType: GameEventType): boolean {
    return (
      (this.listeners.get(eventType)?.size ?? 0) > 0 ||
      (this.onceListeners.get(eventType)?.size ?? 0) > 0
    );
  }
}

// Singleton instance for global event bus (optional - can also create instances per world)
let globalEventBus: EventBus | null = null;

export function getEventBus(): EventBus {
  if (!globalEventBus) {
    globalEventBus = new EventBus();
  }
  return globalEventBus;
}

export function resetEventBus(): void {
  globalEventBus?.clear();
  globalEventBus = null;
}