function resolveElement(value) {
  return typeof value === 'function' ? value() : value;
}

function buildArrowIcon() {
  const icon = document.createElement('span');
  icon.className = 'scroll-to-bottom-btn__icon';
  icon.setAttribute('aria-hidden', 'true');
  icon.textContent = '↓';
  return icon;
}

export function attachScrollToBottomButton({
  scrollContainer,
  mountTarget = document.querySelector('.app-shell'),
  thresholdPx = 160,
  title = 'Вниз',
} = {}) {
  const target = resolveElement(mountTarget);
  if (!(target instanceof Element)) {
    return {
      button: null,
      refresh() {},
      cleanup() {},
    };
  }

  const button = document.createElement('button');
  button.type = 'button';
  button.className = 'scroll-to-bottom-btn';
  button.title = title;
  button.setAttribute('aria-label', 'Прокрутить ленту вниз');
  button.tabIndex = -1;
  button.append(buildArrowIcon());
  target.append(button);

  let disposed = false;
  let boundContainer = null;
  let resizeObserver = null;
  let mutationObserver = null;
  let refreshFrame = 0;

  const hide = () => {
    button.classList.remove('is-visible');
    button.setAttribute('aria-hidden', 'true');
    button.tabIndex = -1;
  };

  const scheduleRefresh = () => {
    if (disposed || refreshFrame) return;
    refreshFrame = window.requestAnimationFrame(() => {
      refreshFrame = 0;
      refresh();
    });
  };

  const unbindContainer = () => {
    boundContainer?.removeEventListener('scroll', scheduleRefresh);
    resizeObserver?.disconnect();
    mutationObserver?.disconnect();
    resizeObserver = null;
    mutationObserver = null;
    boundContainer = null;
  };

  const bindContainer = () => {
    const nextContainer = resolveElement(scrollContainer);
    if (!(nextContainer instanceof Element)) {
      if (boundContainer) unbindContainer();
      return null;
    }
    if (nextContainer === boundContainer) return boundContainer;

    unbindContainer();
    boundContainer = nextContainer;
    boundContainer.addEventListener('scroll', scheduleRefresh, { passive: true });

    if (typeof ResizeObserver === 'function') {
      resizeObserver = new ResizeObserver(scheduleRefresh);
      resizeObserver.observe(boundContainer);
    }

    if (typeof MutationObserver === 'function') {
      mutationObserver = new MutationObserver(scheduleRefresh);
      mutationObserver.observe(boundContainer, { childList: true, subtree: true });
    }

    return boundContainer;
  };

  function refresh() {
    if (disposed) return;
    const container = bindContainer();
    if (!container) {
      hide();
      return;
    }

    const scrollHeight = Number(container.scrollHeight || 0);
    const clientHeight = Number(container.clientHeight || 0);
    const scrollTop = Number(container.scrollTop || 0);
    const maxScrollTop = Math.max(0, scrollHeight - clientHeight);
    const distanceToBottom = Math.max(0, maxScrollTop - scrollTop);
    const shouldShow = maxScrollTop > 8 && distanceToBottom > Math.max(24, Number(thresholdPx || 0));

    button.classList.toggle('is-visible', shouldShow);
    button.setAttribute('aria-hidden', shouldShow ? 'false' : 'true');
    button.tabIndex = shouldShow ? 0 : -1;
  }

  const scrollToBottom = () => {
    const container = bindContainer();
    if (!container) return;
    if (typeof container.scrollTo === 'function') {
      container.scrollTo({ top: container.scrollHeight, behavior: 'smooth' });
    } else {
      container.scrollTop = container.scrollHeight;
    }
    scheduleRefresh();
  };

  button.addEventListener('click', scrollToBottom);
  window.addEventListener('resize', scheduleRefresh);
  window.visualViewport?.addEventListener('resize', scheduleRefresh);
  window.requestAnimationFrame(refresh);

  return {
    button,
    refresh: scheduleRefresh,
    cleanup() {
      if (disposed) return;
      disposed = true;
      if (refreshFrame) {
        window.cancelAnimationFrame(refreshFrame);
        refreshFrame = 0;
      }
      unbindContainer();
      window.removeEventListener('resize', scheduleRefresh);
      window.visualViewport?.removeEventListener('resize', scheduleRefresh);
      button.removeEventListener('click', scrollToBottom);
      button.remove();
    },
  };
}