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

type ThemePreference = 'light' | 'dark' | 'system';
type ResolvedTheme = 'light' | 'dark';

interface ThemeContextType {
    theme: ThemePreference;
    resolvedTheme: ResolvedTheme;
    setTheme: (theme: ThemePreference) => void;
}

const getSystemTheme = (): ResolvedTheme =>
    window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';

const ThemeContext = createContext<ThemeContextType | undefined>(undefined);

export function ThemeProvider({ children }: { children: React.ReactNode }) {
    const [theme, setTheme] = useState<ThemePreference>(() => {
        const stored = localStorage.getItem('theme');
        if (stored === 'dark' || stored === 'light' || stored === 'system') {
            return stored;
        }
        return 'system';
    });

    const [systemTheme, setSystemTheme] = useState<ResolvedTheme>(getSystemTheme);

    useEffect(() => {
        const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
        const onChange = (event: MediaQueryListEvent) => {
            setSystemTheme(event.matches ? 'dark' : 'light');
        };
        mediaQuery.addEventListener('change', onChange);
        return () => mediaQuery.removeEventListener('change', onChange);
    }, []);

    const resolvedTheme = useMemo<ResolvedTheme>(
        () => (theme === 'system' ? systemTheme : theme),
        [systemTheme, theme],
    );

    useEffect(() => {
        const root = window.document.documentElement;
        root.classList.remove('light', 'dark');
        root.classList.add(resolvedTheme);
        localStorage.setItem('theme', theme);
    }, [resolvedTheme, theme]);

    return (
        <ThemeContext.Provider value={{ theme, resolvedTheme, setTheme }}>
            {children}
        </ThemeContext.Provider>
    );
}

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