'use client';

import { useSyncExternalStore } from 'react';

const MOBILE_QUERY = '(max-width: 1023.98px)';

function subscribe(callback: () => void): () => void {
  const mq = window.matchMedia(MOBILE_QUERY);
  mq.addEventListener('change', callback);
  return () => mq.removeEventListener('change', callback);
}

function getSnapshot(): boolean {
  return window.matchMedia(MOBILE_QUERY).matches;
}

function getServerSnapshot(): boolean {
  // Server has no window — default to desktop. Client hydrates to the
  // true viewport state without a flash because useSyncExternalStore
  // is React 18's "this is server-mismatch safe" primitive.
  return false;
}

/**
 * Returns true when the viewport is below the `lg` Tailwind breakpoint.
 * Backed by useSyncExternalStore so render reads stay pure (no
 * useEffect → setState cascade); React Compiler-safe.
 *
 * Not unit-tested: the repo's vitest is configured for environment='node'
 * (no @testing-library/react / DOM env). Verified through the mobile-shell
 * Playwright visual snapshots in Task 23.
 */
export function useIsMobile(): boolean {
  return useSyncExternalStore(subscribe, getSnapshot, getServerSnapshot);
}