"use client";

import { useEffect, useState } from "react";

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

const KEY = "vantage-hq-theme";

/*
 * globals.css has always defined the dark tokens under BOTH
 * :root[data-theme="dark"] and :root[data-theme="light"], specifically so an
 * in-page control can win over the OS preference in either direction. Nothing
 * ever set the attribute. This is that missing half.
 *
 * "system" removes the attribute rather than writing a value, which hands the
 * decision back to the prefers-color-scheme block.
 */
export function applyTheme(pref: ThemePref) {
    const root = document.documentElement;
    if (pref === "system") root.removeAttribute("data-theme");
    else root.setAttribute("data-theme", pref);
}

export function readTheme(): ThemePref {
    if (typeof localStorage === "undefined") return "system";
    const v = localStorage.getItem(KEY);
    return v === "light" || v === "dark" ? v : "system";
}

export function useTheme(): [ThemePref, (p: ThemePref) => void] {
    // Starts at "system" on both server and first client render so hydration
    // matches; the real value lands in the effect below. The inline script in
    // app/layout.tsx has already painted the correct colours by then, so there
    // is no flash only this control's own highlight settles a tick late.
    const [pref, setPref] = useState<ThemePref>("system");

    useEffect(() => setPref(readTheme()), []);

    return [
        pref,
        (next: ThemePref) => {
            setPref(next);
            if (next === "system") localStorage.removeItem(KEY);
            else localStorage.setItem(KEY, next);
            applyTheme(next);
        },
    ];
}

/*
 * Runs before first paint, so a dark-preferring user never sees a white flash.
 * Inlined as a string because it has to execute ahead of React.
 */
export const THEME_BOOT_SCRIPT = `try{var t=localStorage.getItem(${JSON.stringify(KEY)});if(t==="light"||t==="dark")document.documentElement.setAttribute("data-theme",t)}catch(e){}`;
