import { browser } from '$app/environment';
import log from 'loglevel';
import { onDestroy, onMount, createContext } from 'svelte';

/**
 * Options for initializing the TrackingManager.
 */
export type TrackingManagerOpts = {
	consent?: boolean | null;
};

/** Definition of a service callback function. */
export type ServiceCallback<T> = (context?: T) => boolean;

/**
 * Definition of a tracking service with consent and revoke callbacks.
 */
export type Service<T> = {
	context?: T;
	/**
	 * Called when the user consents to tracking or immediately if tracking is
	 * already allowed. Return true to mark the service as mounted.
	 */
	onConsent: ServiceCallback<T>;
	/**
	 * Called when the user revokes consent to tracking. Return true to mark
	 * the service as unmounted.
	 */
	onRevoke: ServiceCallback<T>;
};

type InternalService<T> = Service<T> & {
	mounted: boolean;
	sid: string;
};

/**
 * Manages user tracking preferences and services that require consent.
 */
export class TrackingManager {
	/** tracking consent, persisted to localStorage by saveOpts */
	private _consent: boolean | null = $state(null);
	private _services: Record<string, InternalService<unknown>> = {};
	private _changeCallbacks: Array<(consent: boolean | null) => void> = [];
	private _consentQueue: Array<() => void> = [];

	/**
	 * Saves consent state to localStorage if browser storage is available.
	 * Automatically called after updating consent.
	 * @throws Error if storage is not available.
	 */
	saveOpts(): TrackingManager {
		if (!browser || !window?.localStorage) {
			throw new Error('Cannot access localStorage to save tracking state');
		}

		window.localStorage.setItem(
			'trackingOpts',
			JSON.stringify({ consent: this._consent } as TrackingManagerOpts)
		);
		return this;
	}

	/**
	 * Loads tracking options from localStorage if available. Suitable to call
	 * after initialization, e.g. in onMount after a TrackingManager is created.
	 * @throws Error if storage is not available.
	 */
	loadOpts(): TrackingManager {
		if (!browser || !window?.localStorage) {
			throw new Error('Cannot access localStorage to load tracking state');
		}

		const raw = window.localStorage.getItem('trackingOpts');
		if (raw) {
			const opts = JSON.parse(raw) as TrackingManagerOpts;
			if (opts.consent !== undefined && opts.consent !== null) {
				this.setConsent(opts.consent);
			}
			log.debug('[TrackingManager] Loaded tracking options from storage:', opts);
		}

		return this;
	}

	/**
	 * Creates a TrackingManager instance.
	 * @param opts Optional initial options.
	 */
	constructor(opts?: TrackingManagerOpts) {
		if (opts) {
			if (opts.consent !== undefined) this._consent = opts.consent;
		}
	}

	/**
	 * Creates a TrackingManager instance from localStorage data.
	 * @throws Error if storage is not available.
	 */
	static fromLocalStorage(): TrackingManager {
		return new TrackingManager().loadOpts();
	}

	/** Indicates whether tracking is currently allowed. */
	get consent() {
		return this._consent;
	}
	/** Indicates whether tracking permission has been explicitly set. */
	haveUserDecision() {
		return this._consent !== null;
	}
	/**
	 * Checks whether tracking consent has been explicitly granted.
	 * @returns True if consent is granted, false if revoked or not set.
	 */
	haveUserConsent() {
		return this._consent === true;
	}

	/**
	 * Checks whether tracking is currently allowed.
	 * @throws Error if the user has not made a decision yet or has revoked consent.
	 */
	requireConsent() {
		if (this._consent === null) {
			throw new Error('Tracking permission has not been set yet.');
		}
		if (this._consent === false) {
			throw new Error('Tracking permission has been revoked by the user.');
		}
	}

	/**
	 * Sets whether tracking is consented. If set to true, all queued callbacks
	 * will be executed. Automatically persists to localStorage if available.
	 */
	setConsent(value: boolean) {
		if (this._consent === value) return;
		this._consent = value;

		if (this._consent) {
			this.mountServices();
			this._consentQueue.forEach((cb) => cb());
			this._consentQueue = [];
		} else {
			this.destroyServices();
		}

		this._changeCallbacks.forEach((cb) => {
			cb(this._consent);
		});
		this.saveOpts();
	}

	/**
	 * Registers a callback to be notified when the tracking permission changes.
	 */
	notifyChange(callback: (consent: boolean | null) => void) {
		this._changeCallbacks.push(callback);
	}

	/**
	 * Unregisters a previously registered change callback.
	 */
	unnotifyChange(callback: () => void) {
		this._changeCallbacks = this._changeCallbacks.filter((cb) => cb !== callback);
	}

	/**
	 * Runs callback immediately if we have consent already or queues it for later.
	 * @param callback The function to run when consent is granted.
	 */
	runWithConsent(callback: () => void) {
		if (this._consent) callback();
		else this._consentQueue.push(callback);
	}

	/**
	 * Runs callback onMount if we have consent already or queues it for later.
	 * Removes the callback from the queue onDestroy.
	 * @param callback The function to run when consent is granted.
	 */
	lifecycleWithConsent(callback: () => void) {
		onMount(() => {
			if (this._consent) callback();
			else this._consentQueue.push(callback);
		});
		onDestroy(() => {
			this._consentQueue = this._consentQueue.filter((cb) => cb !== callback);
		});
	}

	/**
	 * Records a service to be executed once tracking is allowed. Use this to
	 * defer tracking-related actions until the user has granted permission.
	 * @param service The service definition.
	 * @return The unique identifier for the registered service.
	 */
	createService<T>(service: Service<T>): string {
		const s: InternalService<T> = {
			...service,
			mounted: false,
			sid: crypto.randomUUID()
		};

		if (this._consent) {
			s.mounted = s.onConsent();
		}

		this._services[s.sid] = s as InternalService<unknown>;
		return s.sid;
	}

	/**
	 * Gets the typed context for a registered service.
	 * @param sid The unique identifier of the service.
	 * @returns The context of the service, or undefined if not found.
	 */
	getServiceContext<T>(sid: string): T | undefined {
		const service = this._services[sid];
		if (service) {
			return service.context as T;
		}
		return undefined;
	}

	/**
	 * Removes a registered service by its unique identifier. Calls the
	 * destroy function if the service was mounted.
	 */
	removeService(sid: string) {
		const service = this._services[sid];
		if (service) {
			if (service.mounted) {
				service.onRevoke(service.context);
			}
			delete this._services[sid];
		}
	}

	private mountServices() {
		Object.values(this._services).forEach((service) => {
			if (!service.mounted) {
				service.mounted = service.onConsent(service.context);
			}
		});
	}
	private destroyServices() {
		Object.values(this._services).forEach((service) => {
			if (service.mounted) {
				service.mounted = !service.onRevoke(service.context);
			}
		});
	}
}

const [getTrackingContext, setTrackingContext] = createContext<TrackingManager>();

/**
 * Gets the TrackingManager from context, or creates one if it doesn't exist.
 * If called from the browser, attempts to load saved state from localStorage.
 * @returns The TrackingManager instance.
 */
export const getTrackingManager = (): TrackingManager => {
	try {
		const saved = getTrackingContext();
		if (saved) {
			log.debug('[TrackingManager] Using existing instance from context');
			return saved;
		}
	} catch {
		// ignore missing context, we'll create a new one
	}

	log.debug('[TrackingManager] Creating new instance');
	const manager = $state(new TrackingManager());
	setTrackingContext(manager);

	if (browser) {
		manager.loadOpts();
	}

	return manager;
};