export class AsyncTaskBatch {
    private added = 0;
    private scheduled = [] as Array<{ index: number; task: () => Promise<any> }>;
    private running = 0;
    private results = [] as any[];
    private doneCallback: (results: any[]) => any;
    private errorCallback: (err: Error) => any;
    private done = false;

    /**
     * Creates a new batch: runs a maximum amount of async tasks simultaniously and waits until they are all resolved.
     * If all tasks succeed, returns the results in the same order tasks were added (like `Promise.all` would do), but
     * cancels any waiting tasks upon failure of one task. Note that the execution order of tasks added after the set
     * limit is unknown.
     * @param limit Max amount of async functions to execute simultaniously. Default is `1000`
     * @param options Additional options
     */
    constructor(public limit = 1000, public options?: {
        /**
         * Whether to wait with processing until `start` is called. Set this to true if you are adding tasks
         * in an async loop and batch processing might be finished already before new tasks are added.
         * @default false
         */
        wait?: boolean,
    }) { }

    private async execute(task: () => Promise<any>, index: number) {
        try {
            this.running++;
            const result = await task();
            this.results[index] = result;
            this.running--;

            if (this.running === 0 && this.scheduled.length === 0) {
                // Finished
                this.done = true;
                this.doneCallback?.(this.results);
            }
            else if (this.scheduled.length > 0) {
                // Run next scheduled task
                const next = this.scheduled.shift();
                this.execute(next.task, next.index);
            }
        }
        catch (err) {
            this.done = true;
            this.errorCallback?.(err);
        }
    }

    add(task: () => Promise<any>) {
        if (this.done) {
            throw new Error(`Cannot add to a batch that has already finished. Use wait option and start batch processing manually if you are adding tasks in an async loop`);
        }
        const index = this.added++;
        if (this.options?.wait !== true && this.running < this.limit) {
            this.execute(task, index);
        }
        else {
            this.scheduled.push({ task, index });
        }
    }

    /**
     * Manually starts batch processing, mus be done if the `wait` option was used
     */
    start() {
        while (this.running < this.limit) {
            const next = this.scheduled.shift();
            this.execute(next.task, next.index);
        }
    }

    async finish() {
        if (this.running === 0 && this.scheduled.length === 0) {
            return this.results;
        }
        await new Promise<any[]>((resolve, reject) => {
            this.doneCallback = resolve;
            this.errorCallback = reject;
        });
        return this.results;
    }
}