import {Inject, Injectable} from '@angular/core';
import {DOCUMENT} from '@angular/common';

@Injectable()
export class LazyLoadService {
  private _queue: {
    promise: Promise<void>,
    id: string
  }[];

  constructor(
    @Inject(DOCUMENT) private _document: any
  ) {
    this._queue = [];
  }

  public load(url: string): Promise<void> {
    let request = this.validateRequestExists(url);
    if (!request) {
      request = new Promise<void>((resolve, reject) => {
        if (this.scriptExist(url)) {
          resolve();
          return;
        }
        const onLoad = () => {
          this.removeRequest(url);
          resolve();
        };
        const onError = () => {
          this.removeRequest(url);
          reject();
        };
        const script = this._document.createElement('script') as HTMLScriptElement;
        script.type = 'text/javascript';
        script.src = url;
        script.id = url;
        script.crossOrigin = 'anonymous';
        script.onload = onLoad;
        script.onerror = onError;
        this._document.body.appendChild(script);
      });
      this._queue.push({
        promise: request,
        id: url
      });
    }
    return request;
  }

  private scriptExist(url: string): boolean {
    return this._document.getElementById(url) !== null;
  }

  private validateRequestExists(url: string): Promise<void> | null {
    let result = null;
    this._queue.forEach(request => {
      if (request.id === url) {
        result = request.promise;
      }
    });
    return result;
  }

  private removeRequest(url: string): void {
    let i = -1;
    this._queue.forEach((request, index) => {
      if (request.id === url) {
        i = index;
      }
    });
    if (i > -1) {
      this._queue.splice(i, 1);
    }
  }
}