/**
 * Inspired by https://codepen.io/oxleberry/pen/BOEBaB
 *
 * With added support for horizontal scrolling, scrolling the window, and linear easing
 *
 * native scrollTo or scrollIntoView conflict with each other if there are different horizontal and vertical scrolls happening
 */

// Easing equations, http://www.gizma.com/easing/
function easeOutCubic(t: number, b: number, c: number, d: number) {
  t /= d;
  t--;
  return c * (t * t * t + 1) + b;
}
// function linear(t: number, b: number, c: number, d: number) {
//   return (c * t) / d + b;
// }

type SmoothScrollToOptions = {
  targetPos: number;
  horizontal?: boolean;
  duration?: number;
  getOffsetParent: () => Element | null;
};

export function smoothScrollToPosition({
  targetPos,
  horizontal = false,
  duration = 250,
  getOffsetParent,
}: SmoothScrollToOptions) {
  let offsetParent = getOffsetParent();

  if (!offsetParent) return;
  if (offsetParent.tagName === 'BODY') {
    offsetParent = document.getElementsByTagName('html')[0];
    if (!offsetParent) return;
  }

  // tracks the current X position in pixels
  const currentPos = horizontal
    ? offsetParent.scrollLeft
    : offsetParent.scrollTop;

  // tracks the remaining distance from target in pixels
  const distance = targetPos - currentPos;

  // track the time, for use with request animation
  let start: null | number = null;

  let animationId: number | undefined = undefined;

  const animation = (timestamp: number) => {
    // timestamp part of reqAF to keep track of animation time
    if (!start) start = timestamp;

    // tracks the time elapsed
    const timeElapsed = timestamp - start;

    // run, calculates how to reach targetPos in an ease trajectory
    // 1) value of current time in the animation
    // 2) currentPos position in pixel
    // 3) how far we need to go till we reach targetPos in pixels
    // 4) target end time of animation
    const run = easeOutCubic(timeElapsed, currentPos, distance, duration);

    // scrollTo, first argument scrolls on the x axis
    // scrollTo, second argument scrolls on the y axis
    // animates till we reach the duration time
    offsetParent?.scrollTo(horizontal ? { left: run } : { top: run });

    if (timeElapsed < duration) {
      requestAnimationFrame(animation);
    } else if (animationId) {
      cancelAnimationFrame(animationId);
    }
  };

  // recursively renders the animation function
  animationId = requestAnimationFrame(animation);
}

export type SmoothScrollOptions = {
  horizontal?: boolean;
  duration?: number;
  getTargetPos?: (target: HTMLElement, offsetParent: Element) => number;
  getOffsetParent?: (target: HTMLElement) => Element | null;
};

export function smoothScrollToTarget(
  target: HTMLElement,
  {
    horizontal = false,
    duration = 250,
    getTargetPos,
    getOffsetParent,
  }: SmoothScrollOptions = {},
) {
  let offsetParent = getOffsetParent
    ? getOffsetParent(target)
    : target.offsetParent;

  if (!offsetParent) return;
  if (offsetParent.tagName === 'BODY') {
    offsetParent = document.getElementsByTagName('html')[0];
    if (!offsetParent) return;
  }

  // tracks the target X positiom in pixels
  const targetPos = getTargetPos
    ? getTargetPos(target, offsetParent)
    : horizontal
    ? target.offsetLeft
    : target.offsetTop;

  return smoothScrollToPosition({
    targetPos,
    horizontal,
    duration,
    getOffsetParent: () => offsetParent,
  });
}