// Loaded from https://deno.land/x/dnit@dnit-v1.11.0/asyncQueue.ts



export type Action<T> = ( ()=>Promise<T> );

// based on https://medium.com/@karenmarkosyan/how-to-manage-promises-into-dynamic-queue-with-vanilla-javascript-9d0d1f8d4df5
export class AsyncQueue {
  inProgress : number = 0;
  concurrency: number;

  queue: {
    action: Action<any>,
    resolve: (t:any)=>void,
    reject: (err:any)=>void
  }[] = [];

  constructor(concurrency: number) {
    this.concurrency = concurrency;
  }

  /// Schedule an action for start later.  Immediately returns a Promise<T> but actual
  /// work of the original action->promise starts later
  async schedule<T>(t: Action<T>) : Promise<T> {
    return new Promise<T>((resolve,reject)=>{
      this.queue.push({
        action: t,
        resolve,
        reject
      });
      this.startQueuedItem();
    });
  }

  /// Start an action from the front of the queue.
  private startQueuedItem() : void {
    if(this.inProgress >= this.concurrency) {
      return;
    }
    const item = this.queue.shift();
    if(item === undefined) {
      // is empty
      return;
    }

    this.inProgress += 1;
    item.action()
    .then( (val:any)=>{
      item.resolve(val);
    })
    .catch( err => {
      item.reject(err);
    })
    .finally(()=>{
      this.inProgress -= 1;
      this.startQueuedItem();
    });
  }
};