"use client";

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

function fragmentTarget(hash: string) {
  if (!hash) return null;
  try {
    return document.getElementById(decodeURIComponent(hash.slice(1)));
  } catch {
    return null;
  }
}

function focusFragment(target: HTMLElement) {
  if (!target.hasAttribute("tabindex")) target.tabIndex = -1;
  target.focus({ preventScroll: true });
}

/**
 * A document owns one outstanding homepage arrival. Later mounts are same-document
 * history, where the browser restores its own position.
 */
let arrivalSettled = false;

/** Tests replay several documents inside one jsdom document. */
export function resetHomeArrivalState() {
  arrivalSettled = false;
}

/** Correct only an outstanding arrival; ordinary links and history keep native scrolling. */
export function useHomeFragmentNavigation() {
  const ready = useRef(false);
  const resolveArrival = useRef<(() => void) | null>(null);

  useEffect(() => {
    const navigation = performance.getEntriesByType?.("navigation")[0] as
      | PerformanceNavigationTiming
      | undefined;
    // A history restoration owns its saved position, not the URL's old fragment.
    const restoration = navigation?.type === "back_forward";
    const arrival = !restoration && !arrivalSettled;
    let pending = restoration ? "" : window.location.hash;
    let pendingTop = arrival && !window.location.hash;
    if (arrival) window.history.scrollRestoration = "manual";

    const resolve = () => {
      if (!ready.current) return;
      if (pendingTop) {
        pendingTop = false;
        if (!window.location.hash)
          window.scrollTo({ top: 0, left: 0, behavior: "instant" });
      }
      if (!pending) return;
      const hash = pending;
      pending = "";
      if (hash !== window.location.hash) return;
      const target = fragmentTarget(hash);
      if (!target) return;
      focusFragment(target);
      const padding =
        Number.parseFloat(
          getComputedStyle(document.documentElement).scrollPaddingTop,
        ) || 0;
      const margin =
        Number.parseFloat(getComputedStyle(target).scrollMarginTop) || 0;
      window.scrollTo({
        top: Math.max(
          0,
          window.scrollY +
            target.getBoundingClientRect().top -
            padding -
            margin,
        ),
        left: 0,
        behavior: "instant",
      });
    };

    const cancelArrival = () => {
      pending = "";
      pendingTop = false;
      window.history.scrollRestoration = "auto";
    };
    const onKeyDown = (event: KeyboardEvent) => {
      if (
        [
          "ArrowUp",
          "ArrowDown",
          "PageUp",
          "PageDown",
          "Home",
          "End",
          " ",
          "Tab",
        ].includes(event.key)
      ) {
        cancelArrival();
      }
    };
    const onClick = (event: MouseEvent) => {
      if (
        event.defaultPrevented ||
        event.button !== 0 ||
        event.metaKey ||
        event.ctrlKey ||
        event.shiftKey ||
        event.altKey
      )
        return;
      const link =
        event.target instanceof Element
          ? event.target.closest("a[href]")
          : null;
      if (
        !(link instanceof HTMLAnchorElement) ||
        link.hasAttribute("download") ||
        (link.target && link.target !== "_self")
      )
        return;
      const url = new URL(link.href);
      if (
        url.origin !== window.location.origin ||
        url.pathname !== window.location.pathname ||
        url.search !== window.location.search
      )
        return;
      const target = fragmentTarget(url.hash);
      if (!target) return;
      pendingTop = false;
      pending = ready.current ? "" : url.hash;
      // Capture before a mobile menu removes its activated link. Do not cancel navigation.
      focusFragment(target);
    };
    const onHashChange = () => {
      pendingTop = false;
      if (pending && pending !== window.location.hash) cancelArrival();
      if (!ready.current) return;
      const target = fragmentTarget(window.location.hash);
      if (target) focusFragment(target);
    };

    resolveArrival.current = resolve;
    resolve();
    document.addEventListener("click", onClick, true);
    window.addEventListener("hashchange", onHashChange);
    window.addEventListener("wheel", cancelArrival, { passive: true });
    window.addEventListener("touchmove", cancelArrival, { passive: true });
    window.addEventListener("keydown", onKeyDown);
    window.addEventListener("pointerdown", cancelArrival, { passive: true });
    return () => {
      resolveArrival.current = null;
      window.history.scrollRestoration = "auto";
      document.removeEventListener("click", onClick, true);
      window.removeEventListener("hashchange", onHashChange);
      window.removeEventListener("wheel", cancelArrival);
      window.removeEventListener("touchmove", cancelArrival);
      window.removeEventListener("keydown", onKeyDown);
      window.removeEventListener("pointerdown", cancelArrival);
    };
  }, []);

  return useCallback(() => {
    ready.current = true;
    resolveArrival.current?.();
    arrivalSettled = true;
    window.history.scrollRestoration = "auto";
  }, []);
}