import { browser } from '$app/environment';
import log from 'loglevel';

const SCRIPT_SRC = 'https://connect.facebook.net/en_US/fbevents.js';

type QueuedFBQ = ((...args: unknown[]) => void) & {
	queue?: unknown[][];
	callMethod?: (...args: unknown[]) => void;
	loaded?: boolean;
	version?: string;
	push?: unknown;
	disablePushState?: boolean;
	allowDuplicatePageViews?: boolean;
};

/**
 * Loads the Meta Pixel script and configures the `fbq` function to queue
 * commands until the script is fully loaded. You may optionally await the
 * returned Promise to ensure the script has loaded before proceeding.
 *
 * Options:
 * - `disablePushState` (default: true) — when true, sets
 *   `window.fbq.disablePushState = true` before the pixel script loads so the
 *   pixel does not auto-listen to `history.pushState`/`popstate` (recommended
 *   for SPA frameworks like Svelte).
 * - `allowDuplicatePageViews` (default: false) — when true, sets
 *   `window.fbq.allowDuplicatePageViews = true` on the stub.
 */
export const loadMetaPixel = (opts?: {
	disablePushState?: boolean;
	allowDuplicatePageViews?: boolean;
}): Promise<void> => {
	// Make sure we're using the browser
	if (!browser || !window) {
		return Promise.reject(new Error(`Not in browser, can't access window`));
	}

	// Default behavior: disable pushState handling since Svelte apps manage
	// navigation themselves and Meta's auto-patching of history APIs can
	// cause duplicate/incorrect pageview events. Consumers can pass
	// `opts.disablePushState = false` to opt out.
	const disablePushState = opts?.disablePushState ?? true;
	const allowDuplicatePageViews = opts?.allowDuplicatePageViews ?? false;

	// If fbq is already defined, resolve immediately
	const existing = window.fbq as QueuedFBQ | undefined;
	if (existing) {
		// If the existing stub is present but hasn't set these flags yet, set
		// them now so the loaded library (if it inspects them) sees intended
		// behavior. Setting these is a no-op if initialization already
		// completed.
		if (disablePushState) existing.disablePushState = true;
		if (allowDuplicatePageViews) existing.allowDuplicatePageViews = true;
		const existingScript = getExistingScript();
		if (existingScript) {
			return new Promise((resolve, reject) => {
				attachToScript(existingScript, resolve, reject);
			});
		}

		log.debug(
			'Meta Pixel fbq already present, skipping injection',
			existing.version,
			existing.queue
		);
		return Promise.resolve();
	}

	// Configure fbq to queue commands until Meta takes over
	const q = function (...args: unknown[]) {
		if (q.callMethod) {
			q.callMethod(...args);
		} else {
			if (!q.queue) q.queue = [];
			q.queue.push(args);
		}
	} as QueuedFBQ;
	q.queue = [];
	q.push = q;
	q.loaded = true;
	q.version = '2.0';
	// set control flags on the stub before the meta script runs
	if (disablePushState) q.disablePushState = true;
	if (allowDuplicatePageViews) q.allowDuplicatePageViews = true;
	window.fbq = q;
	window._fbq = q;

	return new Promise((resolve, reject) => {
		// Avoid adding the same script twice
		const existingScript = getExistingScript();
		if (existingScript) {
			attachToScript(existingScript, resolve, reject);
			log.debug('Meta Pixel script already present, waiting for load');
			return;
		}

		// Otherwise, create the script element
		const script = document.createElement('script');
		script.src = SCRIPT_SRC;
		script.async = true;
		attachToScript(script, resolve, reject);
		document.head.appendChild(script);
		log.debug('Meta Pixel script added to document');
	});
};

const getExistingScript = (): HTMLScriptElement | null => {
	return document.querySelector(
		`script[src*="connect.facebook.net"][src*="fbevents.js"]`
	) as HTMLScriptElement | null;
};

const attachToScript = (
	el: HTMLScriptElement,
	resolve: () => void,
	reject: (err: Error) => void
) => {
	el.addEventListener('load', () => resolve());
	el.addEventListener('error', () => reject(new Error('Failed to load Meta Pixel script')));
};