import { id, registerFire, registerSubscriptions, call } from './util.js';

export function stream(fn, dependencies = [], hash = id) {
	let subscribers = [];
	let isDirty = true;
	let val;
	let oldId;
	let currentProcess;

	// Compute new value, call subscribers if changed.
	const accessor = function _stream() {
		if (!isDirty) {
			return Promise.resolve(val);
		}
		if (!currentProcess) {
			currentProcess = Promise.all(dependencies.map(call))
				.then(params => fn.apply(null, params))
				.then(res => {
					const newId = hash(res);
					isDirty = false;
					if (oldId !== newId) {
						oldId = newId;
						val = res;
						accessor.fire(val);
					}
					return val;
				})
				.finally(_ => {
					isDirty = false;
					currentProcess = null;
				});
		}
		return currentProcess;
	};

	// Add child nodes to the logic graph (value-based)
	accessor.subscribe = registerSubscriptions(subscribers);
	accessor.fire = registerFire(subscribers);

	// Receive dirty flag from parent logic node (dependency).  Pass it down.
	accessor.setDirty = function setDirty() {
		if (!isDirty) {
			isDirty = true;
			subscribers.forEach(s => s.setDirty && s.setDirty());
		}
		return subscribers.length && accessor;
	};

	// Remove this node from the logic graph completely
	accessor.detach = () => {
		subscribers = [];
		dependentSubscriptions.forEach(call);
	};

	// Remove child nodes from the logic graph
	accessor.unsubscribeAll = () => {
		subscribers = [];
	};

	const dependentSubscriptions = dependencies.map(d => d.subscribe(accessor.setDirty));

	return accessor;
}