'use client';

import { createContext, useContext, useEffect, useMemo, useState } from 'react';

type Theme = 'light' | 'dark' | 'system';

interface ThemeContextValue {
  theme: Theme;
  resolvedTheme: 'light' | 'dark';
  setTheme: (theme: Theme) => void;
}

const ThemeContext = createContext<ThemeContextValue | null>(null);

export function ThemeProvider({ children }: { children: React.ReactNode }) {
  const [theme, setThemeState] = useState<Theme>('system');
  const [resolvedTheme, setResolvedTheme] = useState<'light' | 'dark'>('light');

  useEffect(() => {
    const stored = localStorage.getItem('docs-theme') as Theme | null;
    if (stored) setThemeState(stored);
  }, []);

  useEffect(() => {
    const root = document.documentElement;
    const media = window.matchMedia('(prefers-color-scheme: dark)');
    const apply = () => {
      const next = theme === 'system' ? (media.matches ? 'dark' : 'light') : theme;
      setResolvedTheme(next);
      root.classList.toggle('dark', next === 'dark');
    };
    apply();
    media.addEventListener('change', apply);
    return () => media.removeEventListener('change', apply);
  }, [theme]);

  const setTheme = (next: Theme) => {
    setThemeState(next);
    localStorage.setItem('docs-theme', next);
  };

  const value = useMemo(() => ({ theme, resolvedTheme, setTheme }), [theme, resolvedTheme]);

  return <ThemeContext.Provider value={value}>{children}</ThemeContext.Provider>;
}

export function useTheme() {
  const ctx = useContext(ThemeContext);
  if (!ctx) throw new Error('useTheme must be used within ThemeProvider');
  return ctx;
}