export function computed(fn, dependencies = []) {
	const subscribers = new Set();
	const dependents = new Set();
	let isDirty = true;
	let val;

	function _computedDirtyReporter() {
		if (!isDirty) {
			isDirty = true;
		}
		dependents.forEach(runParam);

		if (subscribers.size) {
			accessor();
		}
	}

	const dependentSubscriptions = Array.from(dependencies).map(d => d._d(_computedDirtyReporter));

	const accessor = function _computed() {
		if (isDirty) {
			const newVal = fn.apply(null, dependencies.map(runParam));
			isDirty = false;
			if (newVal !== val) {
				val = newVal;
				subscribers.forEach(s => s(val));
			}
		}
		return val;
	};

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

	accessor._d = fn => {
		dependents.add(fn);
		return () => dependents.delete(fn);
	};

	accessor.detach = () => {
		subscribers.clear();
		dependents.clear();
		dependentSubscriptions.forEach(runParam);
	};

	return accessor;
}

const runParam = a => a();