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

type Theme = 'light' | 'dark';

type ThemeContextValue = {
  theme: Theme;
  setTheme: (theme: Theme) => void;
  toggleTheme: () => void;
};

const STORAGE_KEY = 'trading-dashboard-theme';

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

function applyTheme(theme: Theme) {
  const root = document.documentElement;
  root.classList.toggle('dark', theme === 'dark');
  root.dataset.theme = theme;
}

function readStoredTheme(): Theme | null {
  if (typeof window === 'undefined') return null;
  try {
    const storage = window.localStorage;
    if (typeof storage?.getItem !== 'function') return null;
    const stored = storage.getItem(STORAGE_KEY);
    return stored === 'light' || stored === 'dark' ? stored : null;
  } catch {
    return null;
  }
}

function writeStoredTheme(theme: Theme) {
  try {
    const storage = window.localStorage;
    if (typeof storage?.setItem === 'function') {
      storage.setItem(STORAGE_KEY, theme);
    }
  } catch {
    // Theme persistence is best-effort; restricted storage should not break UI.
  }
}

function resolveInitialTheme(): Theme {
  if (typeof window === 'undefined') return 'light';
  const stored = readStoredTheme();
  if (stored) return stored;
  return window.matchMedia?.('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
}

export function ThemeProvider({ children }: { children: ReactNode }) {
  const [theme, setThemeState] = useState<Theme>(() => resolveInitialTheme());

  useEffect(() => {
    applyTheme(theme);
    writeStoredTheme(theme);
  }, [theme]);

  useEffect(() => {
    const media = window.matchMedia?.('(prefers-color-scheme: dark)');
    if (!media) return;
    const handleChange = () => {
      if (readStoredTheme()) return;
      setThemeState(media.matches ? 'dark' : 'light');
    };
    media.addEventListener?.('change', handleChange);
    return () => media.removeEventListener?.('change', handleChange);
  }, []);

  const value = useMemo<ThemeContextValue>(() => ({
    theme,
    setTheme: setThemeState,
    toggleTheme: () => setThemeState(prev => (prev === 'dark' ? 'light' : 'dark')),
  }), [theme]);

  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;
}
