const delay = (callback) => setTimeout(callback, 0);

const STATEMAP = {
	PENDING: 0,
	FULFILLED: 1,
	REJECTED: 2,
}

class _Promise {
	constructor(callback) {
		this._state = STATEMAP.PENDING;
		this._worker = callback;
		this._handlers = [];
		this._resolve = (value) => {
			this._value = value;
			this._state = STATEMAP.FULFILLED;
			this._handlers.forEach((obj) => {
				this._runHandler(obj.onFulfilled, obj.onRejected);
			});
		};
		this._reject = (error) => {
			this._error = error;
			this._state = STATEMAP.REJECTED;
			this._handlers.forEach((obj) => {
				this._runHandler(obj.onFulfilled, obj.onRejected);
			});
		}
		this._run();
	}

	_run() {
		if (this._worker) {
			try {
				this._worker(this._resolve, this._reject);
			} catch (err) {
				this._reject(err);
			}
		}
	}

	_runHandler(onFulfilled, onRejected) {
		if (this._state === STATEMAP.PENDING) {
			this._handlers.push({onFulfilled, onRejected});
		} else {
			if (this._state === STATEMAP.FULFILLED) {
				onFulfilled(this._value);
			} else if (this._state === STATEMAP.REJECTED) {
				onRejected(this._error);
			}
		}
	}

	then(onSucceeded, onFailed) {
		const self = this;
		const callback = (resolve, reject) => delay(() => {
			self._runHandler((value) => {
				try {
					if (onSucceeded) {
					resolve(onSucceeded(value));
					} else {
						resolve(value);
					}
				} catch (e) {
					reject(e);
				}
			}, (error) => {
				try {
					if (onFailed) {
						reject(onFailed(error));
					} else {
						reject(error);
					}
				} catch (e) {
					reject(e);
				}
			});
		});
		return new _Promise(callback);
	}

	catch(onFailed) {
		this._onRejected = onFailed;
		const self = this;
		const callback = (resolve, reject) => delay(() => {
			self._runHandler(() => {},
			(error) => {
				try {
					if (onFailed) {
						reject(onFailed(error));
					} else {
						reject(error);
					}
				} catch (e) {
					reject(e);
				}
			});
		});
		return new _Promise(callback);
	}
}