import { id } from './util.js';

export function prop(store, hash = id) {
	const subscribers = new Set();
	let id = hash(store);

	const accessor = function _prop(newVal) {
		const newId = hash(newVal);
		if (newVal !== undefined && id !== newId) {
			id = newId;
			store = newVal;
			subscribers.forEach(s => s(store));
		}
		return store;
	};

	accessor.subscribe = accessor._d = fn => {
		subscribers.add(fn);
		return () => {
			subscribers.delete(fn);
			return subscribers.size;
		};
	};

	accessor.unsubscribeAll = () => subscribers.clear();

	return accessor;
}