export default {
  ease (t: number, b: number, c: number, d: number): number { // simple linear tween from http://www.gizma.com/easing/;
    return c * t / d + b;
  },
  getStep (destinationPosition: number, startPosition: number, duration: number, timeElapsed: number) {
    const distance = destinationPosition - startPosition;
    let step = this.ease(timeElapsed, startPosition, distance, duration);
    if ((destinationPosition > startPosition) && (step > destinationPosition)) {
      step = destinationPosition;
    }
    if ((destinationPosition < startPosition) && (step < destinationPosition)) {
      step = destinationPosition;
    }
    return step;
  },
  scrollToPositionX (element: HTMLElement, destinationPositionX: number, duration: number): void {
    const startPositionX = element.scrollLeft;
    let startTime: number;
    const animate = (currentTime: number) => {
      startTime = startTime || currentTime;
      const timeElapsed = currentTime - startTime;
      const step = this.getStep(destinationPositionX, startPositionX, duration, timeElapsed);
      element.scrollLeft = step;
      if (step !== destinationPositionX) {
        requestAnimationFrame(animate);
      }
    };
    requestAnimationFrame(animate);
  },
  scrollToPositionY (element: HTMLElement, destinationPositionY: number, duration: number): void {
    const startPositionY = element.scrollTop;
    let startTime: number;
    const animate = (currentTime: number) => {
      startTime = startTime || currentTime;
      const timeElapsed = currentTime - startTime;
      const step = this.getStep(destinationPositionY, startPositionY, duration, timeElapsed);
      element.scrollTop = step;
      if (step !== destinationPositionY) {
        requestAnimationFrame(animate);
      }
    };
    requestAnimationFrame(animate);
  },
};