import React, { createContext, useContext, useEffect, useState, useCallback } from "react";

export type Theme = "light" | "dark" | "system";

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

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

const STORAGE_KEY = "theme";

function getSystemTheme(): "light" | "dark" {
  if (typeof window === "undefined") return "light";
  return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
}

function getStoredTheme(): Theme {
  if (typeof window === "undefined") return "system";
  try {
    const stored = localStorage.getItem(STORAGE_KEY);
    if (stored === "light" || stored === "dark" || stored === "system") {
      return stored;
    }
  } catch {
    // localStorage not available
  }
  return "system";
}

function applyTheme(theme: "light" | "dark") {
  const root = document.documentElement;
  root.classList.remove("light", "dark");
  root.classList.add(theme);
}

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

interface ThemeProviderProps {
  children: React.ReactNode;
  defaultTheme?: Theme;
}

export function ThemeProvider({ children, defaultTheme = "system" }: ThemeProviderProps) {
  const [theme, setThemeState] = useState<Theme>(() => getStoredTheme() || defaultTheme);
  const [resolvedTheme, setResolvedTheme] = useState<"light" | "dark">(() => {
    const stored = getStoredTheme() || defaultTheme;
    return stored === "system" ? getSystemTheme() : stored;
  });

  const setTheme = useCallback((newTheme: Theme) => {
    setThemeState(newTheme);
    try {
      localStorage.setItem(STORAGE_KEY, newTheme);
    } catch {
      // localStorage not available
    }
  }, []);

  // Apply theme on mount and when theme changes
  useEffect(() => {
    const resolved = theme === "system" ? getSystemTheme() : theme;
    setResolvedTheme(resolved);
    applyTheme(resolved);
  }, [theme]);

  // Listen for system theme changes
  useEffect(() => {
    if (theme !== "system") return;

    const mediaQuery = window.matchMedia("(prefers-color-scheme: dark)");
    const handleChange = (e: MediaQueryListEvent) => {
      const newTheme = e.matches ? "dark" : "light";
      setResolvedTheme(newTheme);
      applyTheme(newTheme);
    };

    mediaQuery.addEventListener("change", handleChange);
    return () => mediaQuery.removeEventListener("change", handleChange);
  }, [theme]);

  const value: ThemeContextValue = {
    theme,
    resolvedTheme,
    setTheme,
  };

  return React.createElement(ThemeContext.Provider, { value }, children);
}