import { _await_value } from "./_await_value.js";

export function _async_generator(gen) {
    var front, back;

    function send(key, arg) {
        return new Promise(function(resolve, reject) {
            var request = { key: key, arg: arg, resolve: resolve, reject: reject, next: null };
            if (back) back = back.next = request;
            else {
                front = back = request;
                resume(key, arg);
            }
        });
    }

    function resume(key, arg) {
        try {
            var result = gen[key](arg);
            var value = result.value;
            var wrappedAwait = value instanceof _await_value;
            Promise.resolve(wrappedAwait ? value.wrapped : value).then(function(arg) {
                if (wrappedAwait) {
                    resume("next", arg);

                    return;
                }
                settle(result.done ? "return" : "normal", arg);
            }, function(err) {
                resume("throw", err);
            });
        } catch (err) {
            settle("throw", err);
        }
    }

    function settle(type, value) {
        switch (type) {
            case "return":
                front.resolve({ value: value, done: true });
                break;
            case "throw":
                front.reject(value);
                break;
            default:
                front.resolve({ value: value, done: false });
                break;
        }
        front = front.next;
        if (front) resume(front.key, front.arg);
        else back = null;
    }

    this._invoke = send;

    if (typeof gen.return !== "function") this.return = undefined;
}

if (typeof Symbol === "function" && Symbol.asyncIterator) {
    _async_generator.prototype[Symbol.asyncIterator] = function() {
        return this;
    };
}

_async_generator.prototype.next = function(arg) {
    return this._invoke("next", arg);
};

_async_generator.prototype.throw = function(arg) {
    return this._invoke("throw", arg);
};

_async_generator.prototype.return = function(arg) {
    return this._invoke("return", arg);
};
export { _async_generator as _ };