import { Injectable } from '@angular/core';

@Injectable()
export class WindowScrollService {

  constructor() { }

  public currentYPosition() {
    // Firefox, Chrome, Opera, Safari
    if (self.pageYOffset) {
      return self.pageYOffset;
    }
    // Internet Explorer 6 - standards mode
    if (document.documentElement && document.documentElement.scrollTop) {
      return document.documentElement.scrollTop;
    }
    // Internet Explorer 6, 7 and 8
    if (document.body.scrollTop) {
      return document.body.scrollTop;
    }
    return 0;
  }

  public elmYPosition(eID) {
    const elm: any = document.getElementById(eID);
    if (elm) {
      let y = elm.offsetTop;
      let node = elm;
      while (node.offsetParent && node.offsetParent !== document.body) {
        node = node.offsetParent;
        y += node.offsetTop;
      } return y;
    }
  }

  public smoothScroll(eID, timeOut = 0) {
    setTimeout(() => {
        const startY = this.currentYPosition();
        const stopY = this.elmYPosition(eID);
        const distance = stopY > startY ? stopY - startY : startY - stopY;
        if (distance < 100) {
          scrollTo(0, stopY); return;
        }
        let speed = Math.round(distance / 100);
        if (speed >= 20) { speed = 20; }
        const step = Math.round(distance / 25);
        let leapY = stopY > startY ? startY + step : startY - step;
        let timer = 0;
        if (stopY > startY) {
          for (let i = startY; i < stopY; i += step) {
            setTimeout('window.scrollTo(0, ' + leapY + ')', timer * speed);
            leapY += step; if (leapY > stopY) { leapY = stopY; } timer++;
          } return;
        }
        for (let i = startY; i > stopY; i -= step) {
          setTimeout('window.scrollTo(0, ' + leapY + ')', timer * speed);
          leapY -= step; if (leapY < stopY) { leapY = stopY; } timer++;
        }
        return false;
    }, timeOut);
  }

}