import { useState, useEffect, useCallback } from "react";

type Theme = "light" | "dark";

const STORAGE_KEY = "theme";

function getInitialTheme(): Theme {
  const stored = localStorage.getItem(STORAGE_KEY);
  if (stored === "light" || stored === "dark") return stored;
  return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
}

function applyTheme(theme: Theme) {
  document.documentElement.classList.toggle("dark", theme === "dark");
}

export function useTheme() {
  const [theme, setTheme] = useState<Theme>(getInitialTheme);

  useEffect(() => {
    applyTheme(theme);
    localStorage.setItem(STORAGE_KEY, theme);
  }, [theme]);

  // Listen for OS preference changes (only when no explicit user choice)
  useEffect(() => {
    const mq = window.matchMedia("(prefers-color-scheme: dark)");
    const handler = (e: MediaQueryListEvent) => {
      if (!localStorage.getItem(STORAGE_KEY)) {
        setTheme(e.matches ? "dark" : "light");
      }
    };
    mq.addEventListener("change", handler);
    return () => mq.removeEventListener("change", handler);
  }, []);

  const toggleTheme = useCallback(() => {
    setTheme((prev) => (prev === "light" ? "dark" : "light"));
  }, []);

  return { theme, toggleTheme } as const;
}