"use client";

import { useCallback, useEffect, useRef, useState } from "react";
import { useRouter } from "next/navigation";

/**
 * Guards a form against accidental loss of in-progress data.
 *
 * While `isDirty` is true it:
 *  - shows the browser's native warning on hard unloads (reload / tab close /
 *    external navigation), and
 *  - intercepts in-app link clicks so the caller can show a custom prompt
 *    offering to save the work as a draft before leaving.
 *
 * Programmatic navigations (e.g. a "Cancel" button calling `router.back()`)
 * should be wrapped with the returned `guard()` so they are intercepted too.
 */
export function useUnsavedChanges(isDirty: boolean) {
  const router = useRouter();
  const dirtyRef = useRef(isDirty);
  dirtyRef.current = isDirty;
  // Set once the user has chosen to leave, so the pending navigation isn't
  // re-intercepted on the way out.
  const bypassRef = useRef(false);
  const [pendingNav, setPendingNav] = useState<null | (() => void)>(null);
  const pendingRef = useRef<null | (() => void)>(null);
  pendingRef.current = pendingNav;

  // Native warning for reload / tab-close / external links.
  useEffect(() => {
    if (!isDirty) return;
    function onBeforeUnload(e: BeforeUnloadEvent) {
      e.preventDefault();
      e.returnValue = "";
    }
    window.addEventListener("beforeunload", onBeforeUnload);
    return () => window.removeEventListener("beforeunload", onBeforeUnload);
  }, [isDirty]);

  // Intercept clicks on in-app links so we can prompt before leaving.
  useEffect(() => {
    function onClick(e: MouseEvent) {
      if (!dirtyRef.current || bypassRef.current) return;
      if (e.defaultPrevented || e.button !== 0) return;
      if (e.metaKey || e.ctrlKey || e.shiftKey || e.altKey) return;

      const anchor = (e.target as HTMLElement | null)?.closest?.("a");
      if (!anchor) return;
      if (anchor.hasAttribute("download")) return;
      const target = anchor.getAttribute("target");
      if (target && target !== "_self") return;
      const href = anchor.getAttribute("href");
      if (!href) return;

      let url: URL;
      try {
        url = new URL(anchor.href, window.location.href);
      } catch {
        return;
      }
      if (url.origin !== window.location.origin) return;

      const dest = url.pathname + url.search + url.hash;
      const here = window.location.pathname + window.location.search;
      // Same page (e.g. an in-page #anchor) — let it through.
      if (url.pathname + url.search === here) return;

      e.preventDefault();
      setPendingNav(() => () => router.push(dest));
    }

    document.addEventListener("click", onClick, true);
    return () => document.removeEventListener("click", onClick, true);
  }, [router]);

  /** Wrap a programmatic navigation so it is intercepted while dirty. */
  const guard = useCallback((run: () => void) => {
    if (!dirtyRef.current || bypassRef.current) {
      run();
      return;
    }
    setPendingNav(() => run);
  }, []);

  /** Proceed with the pending navigation, discarding changes. */
  const leave = useCallback(() => {
    bypassRef.current = true;
    const run = pendingRef.current;
    setPendingNav(null);
    run?.();
  }, []);

  /** Cancel the prompt and stay on the page. */
  const stay = useCallback(() => setPendingNav(null), []);

  return { promptOpen: pendingNav !== null, guard, leave, stay };
}