import type { Object3D } from "three";

import type { IComponent } from "./engine_types.js";

/** Typed event map for {@link Context.events}.
 * Known events get full autocomplete; custom events can be typed at the call site via generic parameter.
 */
export interface ContextEventMap {
	"scene-content-changed": {
		/** The component that triggered the change (e.g. SceneSwitcher, DropListener) */
		readonly source: IComponent;
		/** The root object that was added/loaded */
		readonly object: Object3D;
	};
}

/** Options for {@link EventBus.on}. */
export interface EventBusListenerOptions {
	/** If true the listener is automatically removed after the first invocation. */
	once?: boolean;
}

/** Typed event bus. Known {@link ContextEventMap} events get full autocomplete.
 * Custom events can be typed at the call site via generic parameter.
 * @example Known events
 * ```ts
 * context.events.on("scene-content-changed", e => e.object);
 * ```
 * @example Custom events — type at call site
 * ```ts
 * context.events.emit<{ pts: number }>("scored", { pts: 10 });
 * context.events.on<{ pts: number }>("scored", e => e.pts);
 * ```
 * @example Once
 * ```ts
 * context.events.on("scene-content-changed", e => { ... }, { once: true });
 * ```
 */
export class EventBus {
	private _listeners = new Map<string, Function[]>();

	/** Emit a known {@link ContextEventMap} event */
	emit<K extends keyof ContextEventMap & string>(type: K, detail?: ContextEventMap[K]): void;
	/** Emit a custom event with user-provided type */
	emit<T>(type: string, detail?: T): void;
	emit(type: string, detail?: unknown): void {
		const arr = this._listeners.get(type);
		if (arr) for (const cb of [...arr]) cb(detail);
	}

	/** Subscribe to a known {@link ContextEventMap} event. Returns an unsubscribe function. */
	on<K extends keyof ContextEventMap & string>(type: K, callback: (args: ContextEventMap[K]) => void, options?: EventBusListenerOptions): () => void;
	/** Subscribe to a custom event with user-provided type. Returns an unsubscribe function. */
	on<T>(type: string, callback: (args: T) => void, options?: EventBusListenerOptions): () => void;
	on(type: string, callback: Function, options?: EventBusListenerOptions): () => void {
		let arr = this._listeners.get(type);
		if (!arr) { arr = []; this._listeners.set(type, arr); }
		const unsub = () => {
			const i = arr.indexOf(wrapped);
			if (i >= 0) arr.splice(i, 1);
		};
		const wrapped = options?.once
			? (...args: unknown[]) => { unsub(); callback(...args); }
			: callback;
		arr.push(wrapped);
		return unsub;
	}

	/** Remove all listeners. Called when the context is cleared or destroyed. */
	clear(): void {
		this._listeners.clear();
	}
}