/**
 * Optional utility classes can use when states need to be partially updated
 */

class StateUpdateManager {
	constructor(initialState) {
		this._highestConfirmedId = 0;
		this._currentId = 0;
		this._nextId = 1;

		this._updateHistory = [{ id: 0, state: initialState }];
	}

	get highestConfirmedId() {
		return this._highestConfirmedId;
	}

	/**
	 * Returns the latest (unconfirmed state)
	 */
	get state() {
		return this._updateHistory.find(su => su.id == this._currentId).state;
	}

	/**
	 * Returns the latest confirmed state
	 */
	get confirmedState() {
		return this._updateHistory.find(su => su.id == this._highestConfirmedId).state;
	}

	/**
	 * Registers the potential update of the state
	 * @param {Object} state 
	 * @returns {Number} id which is used to confirm or reject the update later
	 */
	registerUpdate(state) {
		let updateId = this._nextId;
		this._nextId += 1;
		this._currentId = updateId; // until rejected, this is the most recent id

		this._updateHistory.push({
			id: updateId,
			state
		});

		return updateId;
	}

	/**
	 * Add a new state and instantly confirm it
	 * @param {Object} state 
	 */
	insertConfirmedState(state) {
		let suid = this.registerUpdate();
		this.confirmUpdate(suid);
	}

	/**
	 * Confirm the update with the specified id
	 * Also remove all updates with lower ids, as these are now confirmed to be outdated
	 * @param {Number} uid 
	 */
	confirmUpdate(uid) {
		if (uid < this._highestConfirmedId) return; // already outdated

		this._highestConfirmedId = uid;

		// remove history entries with lower ids
		this._updateHistory = this._updateHistory.filter(su => su.id >= this._highestConfirmedId);
	}

	/**
	 * Rejects the update with the specified id
	 * If the id is outdated it is ignored
	 * Otherwise it is thrown out, and the currentId decremented if required
	 * @param {Number} uid 
	 */
	rejectUpdate(uid) {
		if (uid < this._highestConfirmedId) return; // already outdated

		// remove element from history
		this._updateHistory = this._updateHistory.filter(su => su.id != uid);

		if (this._currentId == uid) {
			// set current id to highest known uid
			this._currentId = this._updateHistory.reduce((highest, su) => (su.id > highest) ? su.id : highest, -1);
		}
	}
}

module.exports = StateUpdateManager;