import { cookies, headers } from "next/headers";
import { getRequestConfig } from "next-intl/server";

// i18n WITHOUT routing: the locale is chosen by a `NEXT_LOCALE` cookie, then
// the Accept-Language header, and finally English as fallback.
export const SUPPORTED_LOCALES = [
	"en",
	"it",
	"nl",
	"de",
	"fr",
	"es",
	"pt",
	"pl",
	"sv",
	"tr",
	"ro",
	"hu",
	"cs",
	"sk",
	"da",
	"no",
	"el",
	"bg",
	"hr",
	"sr",
	"uk",
	"ru",
] as const;
export type AppLocale = (typeof SUPPORTED_LOCALES)[number];
export const DEFAULT_LOCALE: AppLocale = "en";

export function isSupportedLocale(
	value: string | undefined,
): value is AppLocale {
	return !!value && (SUPPORTED_LOCALES as readonly string[]).includes(value);
}

function detectLocaleFromAcceptLanguage(
	acceptLanguage: string | null,
): AppLocale | null {
	if (!acceptLanguage) return null;
	const locales = acceptLanguage
		.split(",")
		.map((entry) => {
			const [tag, qPart] = entry.split(";");
			const quality = qPart ? parseFloat(qPart.replace("q=", "")) : 1;
			const lang = tag?.trim().split("-")[0] ?? "";
			return { lang, quality };
		})
		.filter((l) => l.lang)
		.sort((a, b) => b.quality - a.quality);

	for (const { lang } of locales) {
		if (isSupportedLocale(lang)) return lang;
	}
	return null;
}

export default getRequestConfig(async () => {
	const store = await cookies();
	const cookieLocale = store.get("NEXT_LOCALE")?.value;

	let locale: AppLocale;
	if (isSupportedLocale(cookieLocale)) {
		locale = cookieLocale;
	} else {
		const headersList = await headers();
		const detected = detectLocaleFromAcceptLanguage(
			headersList.get("Accept-Language"),
		);
		locale = detected ?? DEFAULT_LOCALE;
	}

	// Safe: `locale` is validated against SUPPORTED_LOCALES above (only en/it/nl/de/fr/es).
	const messages = (await import(`../messages/${locale}.json`)).default;
	// English is the source of truth; fall back to it for any key missing from a
	// translation so the UI never shows a raw key path.
	const fallback =
		locale === DEFAULT_LOCALE
			? messages
			: (await import(`../messages/${DEFAULT_LOCALE}.json`)).default;

	return {
		locale,
		messages,
		// Never throw on a missing message — use the English value, or a humanised
		// key as a last resort.
		getMessageFallback({ key }: { key: string }) {
			const fromEnglish = key
				.split(".")
				.reduce<unknown>(
					(o, k) =>
						o && typeof o === "object"
							? (o as Record<string, unknown>)[k]
							: undefined,
					fallback,
				);
			if (typeof fromEnglish === "string") return fromEnglish;
			const seg = key.split(".").pop() ?? key;
			return seg
				.replace(/[._-]/g, " ")
				.replace(/\b\w/g, (c) => c.toUpperCase());
		},
		onError() {
			// Swallow MISSING_MESSAGE etc. — getMessageFallback already degrades.
		},
	};
});