class Extractor {
    constructor() {
        this._tasks = [];
        this._results = {};
    }
    /**
     * Add a task to Extractor. \n
     * One Extractor could has multiple tasks, which orgnized in a task chian.
     * Later task will use previous task result as input (target url list).
     * So only the first task can have target url arguments, while later tasks can't.
     * @param {...any} args itemsSelector, fieldSelectors, and more args to specify target urls.
     */
    task(...args) {
        if (!testArgs(...args))
            throw new Error(`Invalid call arguments.\n\n${signitures}\n\n`);
        // given >2 arguments means the task specifies target page, 
        // so it won't accept last task result as url list.
        // in this case, former tasks are useless, can be cleared.
        if (args.length > 2) this.clear();
        this._tasks.push(args);
        return this;
    }
    /**
     * Clear tasks and caches.
     */
    clear() {
        this._tasks = [];
        this._results = [];
    }
    /**
     * Start the task chain.
     */
    async start() {
        if (!this._tasks.length) {
            console.log('No task to run.');
            return;
        }
        return this._tasks.reduce((pms, args, i, tasks) => {
            return pms.then(
                result => {
                    if (result === undefined) return getData(...args);
                    this._results[tasks[i - 1]] = result;
                    return getData(...args, result);
                });
        }, Promise.resolve(undefined)).then(
            result => {
                this._results[this._tasks[this._tasks.length - 1]] = result;
                this.save();
            }
        ).catch(err => console.log(err));
    }
    /**
     * restart from specified task, but don't restart the previous tasks.
     * @param {number} taskid from which restart the tasks
     */
    async restart(taskid) {
        taskid = this._checkTaskId(taskid, 1);
        if (!taskid) return;
        if (taskid == 1) {
            this.start();
            return;
        }
        let cache = this._results[this._tasks[taskid - 2]];
        if (!cache) {
            console.log(`No result cache for task (id ${taskid}). \nMake sure call ".start()" before ".restart()"?`);
            return;
        }
        return this._tasks.slice(taskid - 1).reduce((pms, args, i, tasks) => {
            return pms.then(
                result => {
                    this._results[tasks[i - 1]] = result;
                    return getData(...args, result);
                });
        }, Promise.resolve(cache)).then(
            result => {
                this._results[this._tasks[this._tasks.length - 1]] = result;
                this.save();
            }
        ).catch(err => console.log(err));
    }
    /**
     * Save result of a task
     * @param {number} taskid which task id to save.
     */
    save(taskid) {
        taskid = this._checkTaskId(taskid, this._tasks.length);
        if (!taskid) return;
        const result = this._results[this._tasks[taskid - 1]];
        if (!result) {
            console.log(`No task result for id (${taskid}). Forget to call ".start()"?`);
            return;
        }
        if (confirm(
                `Click confirm to download if the sample data looks good (${result.data.length} items)：\n\n${result.toString(50) || "- Empty -"}`
            )) {
            saveFile(result, "text/csv");
        }
    }
    _checkTaskId(id, defaultId) {
        if (!this._tasks.length) {
            console.log("No task found.");
            return 0;
        }
        if (defaultId && id === undefined || this.task === null) id = defaultId;
        if (isNaN(id) || id < 1 || id > this._tasks.length) {
            console.log(`Invalid task id. Rang(1-${this._tasks.length})`);
            return 0;
        }
        return id
    }
}