"use client";

import { useEffect, useRef, useState } from "react";

export type TOCSection = { id: string; title: string };

// How long a click "wins" over the scroll-spy observer — long enough to
// cover the native smooth-scroll animation triggered by the anchor href,
// so sections passing through the viewport mid-scroll can't flicker the
// active state away from what was actually clicked.
const CLICK_OVERRIDE_MS = 1000;

// Shared between SectionTOC (desktop sidebar nav) and MobileSectionTOC
// (below lg: collapsible accordion, added 2026-07-24) — both need the same
// scroll-spy "active" state and click-override handling, just render it
// completely differently, so the logic lives here once instead of being
// duplicated per component.
function useActiveSection(sections: TOCSection[]) {
  const [active, setActive] = useState<string>(sections[0]?.id ?? "");
  // Not state — read inside the IntersectionObserver callback without
  // needing to re-subscribe it on every click, and cleared by its own
  // timeout rather than a render.
  const overrideRef = useRef<string | null>(null);
  const overrideTimeoutRef = useRef<ReturnType<typeof setTimeout> | undefined>(undefined);

  useEffect(() => () => clearTimeout(overrideTimeoutRef.current), []);

  useEffect(() => {
    const observer = new IntersectionObserver(
      (entries) => {
        // A click just fired — trust it over whatever the scroll-spy sees
        // mid-animation (sections flying past the intersection band while
        // the browser's native smooth-scroll is still catching up to the
        // clicked target), including the case where the target was
        // already fully visible and no scroll happens at all.
        if (overrideRef.current) return;
        const visible = entries.filter((entry) => entry.isIntersecting);
        if (visible.length > 0) setActive(visible[0].target.id);
      },
      { rootMargin: "-120px 0px -65% 0px", threshold: 0 }
    );
    sections.forEach(({ id }) => {
      const el = document.getElementById(id);
      if (el) observer.observe(el);
    });
    return () => observer.disconnect();
  }, [sections]);

  function handleClick(id: string) {
    setActive(id);
    overrideRef.current = id;
    clearTimeout(overrideTimeoutRef.current);
    overrideTimeoutRef.current = setTimeout(() => {
      overrideRef.current = null;
    }, CLICK_OVERRIDE_MS);
  }

  return { active, handleClick };
}

// lg:-only sidebar — same "wide fixed-width block next to content" shape
// as the cart's order-summary sidebar (see figma-to-nextjs skill Gotcha
// #5): a 360px TOC card plus a readable content column already exceeds
// even the site's 640px structural floor, so sm: wouldn't leave room for
// a real 2-column split at Tablet widths — a deliberate exception to the
// site-wide sm: consolidation, not a leftover of it.
//
// Generic over `sections` — originally written just for /versand
// (VersandTOC), generalized once /datenschutz needed the identical
// scroll-spy sidebar but driven by CMS-authored headings instead of a
// hardcoded array. Any future long legal/content page reuses this too.
//
// Not sticky itself — every caller wraps this in its own `hidden lg:flex
// ... lg:sticky lg:top-32 lg:self-start` div (Impressum/Datenschutz also
// stack a second "Nachhaltigkeit" card below this in that same wrapper, so
// the sticky behavior has to live on the wrapper for the two to travel
// together as one unit — putting it on this <nav> instead would leave
// that card behind as a plain-flow sibling scrolling past a now-fixed nav).
export function SectionTOC({ sections }: { sections: TOCSection[] }) {
  const { active, handleClick } = useActiveSection(sections);
  if (sections.length === 0) return null;

  return (
    <nav className="hidden lg:flex flex-col gap-1 w-[22.5rem] shrink-0 bg-bg-base border border-border rounded-md p-6">
      <p className="text-label font-semibold text-text-muted uppercase tracking-wide mb-2">
        Inhaltsübersicht
      </p>
      {sections.map(({ id, title }) => (
        <a
          key={id}
          href={`#${id}`}
          onClick={() => handleClick(id)}
          className={
            "px-3 py-2 rounded-sm text-body-sm transition-colors border-l-2 " +
            (active === id
              ? "border-toc-active-border bg-bg-muted text-text-primary font-semibold"
              : "border-transparent text-text-muted hover:text-text-primary")
          }
        >
          {title}
        </a>
      ))}
    </nav>
  );
}

// Below lg: only — a collapsible accordion instead of the sidebar nav
// (which is `hidden` entirely below lg:, see SectionTOC's own comment on
// why a real 2-column split doesn't fit there). Added 2026-07-24: these
// legal pages had no on-page navigation aid at all on Mobile/Tablet, which
// is exactly where scanning a long legal document by scrolling is hardest.
// Native <details>/<summary> — no extra open/close state needed, and it
// stays open after a click so jumping between sections doesn't require
// reopening it each time. Render this as its own element in the page
// (typically right after the heading, before the two-column content row),
// not nested inside a parent that's itself `hidden lg:...` — that would
// hide this too regardless of its own lg:hidden class.
export function MobileSectionTOC({ sections }: { sections: TOCSection[] }) {
  const { active, handleClick } = useActiveSection(sections);
  if (sections.length === 0) return null;

  return (
    <details className="lg:hidden w-full bg-bg-base border border-border rounded-md p-4 open:pb-2">
      <summary className="text-label font-semibold text-text-muted uppercase tracking-wide cursor-pointer select-none">
        Inhaltsübersicht
      </summary>
      <div className="flex flex-col gap-1 mt-3">
        {sections.map(({ id, title }) => (
          <a
            key={id}
            href={`#${id}`}
            onClick={() => handleClick(id)}
            className={
              "px-3 py-2 rounded-sm text-body-sm transition-colors border-l-2 " +
              (active === id
                ? "border-toc-active-border bg-bg-muted text-text-primary font-semibold"
                : "border-transparent text-text-muted hover:text-text-primary")
            }
          >
            {title}
          </a>
        ))}
      </div>
    </details>
  );
}