const storageKey = "app_lang";
const defaultLang = "en";
const translationsAssetUrl = new URL(
    "../data/i18n/translations.json",
    import.meta.url,
);
const faqAssetUrl = new URL("../data/i18n/faq.json", import.meta.url);

let currentLang = defaultLang;
const listeners = [];
let assetsLoadingPromise = null;
let assetsLoaded = false;

export let translations = {};
export let faqMarkdown = {};

function isRecord(value) {
    return value !== null && typeof value === "object" && !Array.isArray(value);
}

function validateTranslations(raw) {
    if (!isRecord(raw)) {
        throw new Error("Invalid i18n translations payload.");
    }

    const languages = Object.keys(raw);
    if (languages.length === 0 || !isRecord(raw[defaultLang])) {
        throw new Error(
            `Missing default translation language "${defaultLang}".`,
        );
    }

    const defaultDict = raw[defaultLang];
    const defaultKeys = Object.keys(defaultDict);

    languages.forEach((lang) => {
        const dict = raw[lang];
        if (!isRecord(dict)) {
            throw new Error(
                `Invalid translation dictionary for language "${lang}".`,
            );
        }

        Object.entries(dict).forEach(([key, value]) => {
            if (typeof value !== "string") {
                throw new Error(
                    `Invalid translation value for "${lang}.${key}".`,
                );
            }
        });

        const missing = defaultKeys.filter(
            (key) => typeof dict[key] !== "string",
        );
        if (missing.length > 0) {
            throw new Error(
                `Missing translation keys for "${lang}": ${missing.join(", ")}`,
            );
        }
    });

    return raw;
}

function validateFaq(raw) {
    if (!isRecord(raw) || typeof raw[defaultLang] !== "string") {
        throw new Error(`Missing default FAQ language "${defaultLang}".`);
    }

    Object.entries(raw).forEach(([lang, value]) => {
        if (typeof value !== "string") {
            throw new Error(`Invalid FAQ value for "${lang}".`);
        }
    });

    return raw;
}

async function loadJson(url, label) {
    const response = await fetch(url, { cache: "no-store" });
    if (!response.ok) {
        throw new Error(`Failed to load ${label}: ${response.status}`);
    }

    return response.json();
}

async function ensureAssetsLoaded() {
    if (assetsLoaded) return;
    if (assetsLoadingPromise) return assetsLoadingPromise;

    assetsLoadingPromise = (async () => {
        const [translationsRaw, faqRaw] = await Promise.all([
            loadJson(translationsAssetUrl, "translations"),
            loadJson(faqAssetUrl, "faq"),
        ]);

        translations = validateTranslations(translationsRaw);
        faqMarkdown = validateFaq(faqRaw);
        assetsLoaded = true;
    })().finally(() => {
        assetsLoadingPromise = null;
    });

    return assetsLoadingPromise;
}

function interpolate(template, params = {}) {
    return template.replace(
        /\{(\w+)\}/g,
        (_, key) => params[key] ?? `{${key}}`,
    );
}

function t(key, params) {
    const fallback = translations[defaultLang]?.[key] ?? key;
    const phrase = translations[currentLang]?.[key] ?? fallback;
    return interpolate(phrase, params);
}

function detectLanguage() {
    const stored = localStorage.getItem(storageKey);
    if (stored && translations[stored]) return stored;
    const nav = navigator.language?.slice(0, 2);
    if (nav && translations[nav]) return nav;
    return defaultLang;
}

function applyTranslations(root = document) {
    root.querySelectorAll("[data-i18n]").forEach((el) => {
        const key = el.dataset.i18n;
        const attrs = (el.dataset.i18nAttr || "")
            .split(",")
            .map((a) => a.trim())
            .filter(Boolean);
        const text = t(key);
        if (attrs.length === 0) {
            el.textContent = text;
        } else {
            attrs.forEach((attr) => el.setAttribute(attr, text));
        }
    });
}

function notify() {
    listeners.forEach((fn) => fn(currentLang));
}

function setLanguage(lang) {
    if (!translations[lang]) lang = defaultLang;
    currentLang = lang;
    localStorage.setItem(storageKey, lang);
    document.documentElement.lang = lang;
    applyTranslations();
    notify();
}

function getLanguage() {
    return currentLang;
}

async function initI18n() {
    await ensureAssetsLoaded();
    currentLang = detectLanguage();
    document.documentElement.lang = currentLang;
    applyTranslations();
    notify();
    return currentLang;
}

function onLanguageChange(fn) {
    listeners.push(fn);
}

export {
    t,
    setLanguage,
    getLanguage,
    initI18n,
    applyTranslations,
    onLanguageChange,
};