// Pin-to-bottom controller for the document scroll.
//
// Tracks whether the user is "near" the bottom of the page. While pinned, new
// content (e.g. streaming tokens) scrolls the viewport to keep up. Once the
// user scrolls up, pinning releases and updates stop forcing scroll until the
// user returns to the bottom.

const PIN_THRESHOLD_PX = 80;

export interface AutoScroll {
  isPinned(): boolean;
  scrollToBottom(behavior?: ScrollBehavior): void;
  maybeScrollToBottom(): void;
}

export function createAutoScroll(): AutoScroll {
  let pinned = true;

  const scrollEl = () => document.scrollingElement || document.documentElement;
  const distanceFromBottom = () => {
    const el = scrollEl();
    return el.scrollHeight - el.scrollTop - el.clientHeight;
  };

  const onScroll = () => {
    pinned = distanceFromBottom() < PIN_THRESHOLD_PX;
  };
  window.addEventListener('scroll', onScroll, { passive: true });

  return {
    isPinned: () => pinned,
    scrollToBottom(behavior: ScrollBehavior = 'auto') {
      window.scrollTo({ top: scrollEl().scrollHeight, behavior });
      pinned = true;
    },
    maybeScrollToBottom() {
      if (!pinned) return;
      requestAnimationFrame(() => {
        window.scrollTo({ top: scrollEl().scrollHeight });
      });
    },
  };
}