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

type Theme = "light" | "dark" | "system";
type Ctx = { theme: Theme; setTheme: (t: Theme) => void; resolved: "light" | "dark" };

const ThemeCtx = createContext<Ctx | null>(null);
const STORAGE_KEY = "votersalert-theme";

function applyTheme(t: Theme) {
  const root = document.documentElement;
  const isDark =
    t === "dark" || (t === "system" && window.matchMedia("(prefers-color-scheme: dark)").matches);
  root.classList.toggle("dark", isDark);
  return isDark ? "dark" : "light";
}

export function ThemeProvider({ children }: { children: ReactNode }) {
  const [theme, setThemeState] = useState<Theme>("system");
  const [resolved, setResolved] = useState<"light" | "dark">("light");

  useEffect(() => {
    const stored = (localStorage.getItem(STORAGE_KEY) as Theme | null) ?? "system";
    setThemeState(stored);
    setResolved(applyTheme(stored) as "light" | "dark");
    const mq = window.matchMedia("(prefers-color-scheme: dark)");
    const handler = () => {
      if ((localStorage.getItem(STORAGE_KEY) as Theme | null) === "system") {
        setResolved(applyTheme("system") as "light" | "dark");
      }
    };
    mq.addEventListener("change", handler);
    return () => mq.removeEventListener("change", handler);
  }, []);

  const setTheme = (t: Theme) => {
    localStorage.setItem(STORAGE_KEY, t);
    setThemeState(t);
    setResolved(applyTheme(t) as "light" | "dark");
  };

  return <ThemeCtx.Provider value={{ theme, setTheme, resolved }}>{children}</ThemeCtx.Provider>;
}

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