import { CookieNames } from "$lib/configuration";
import { detectLocale, locales } from '$lib/i18n/i18n-util'
import type { Handle, RequestEvent } from '@sveltejs/kit'
import { sequence } from "@sveltejs/kit/hooks";
import { initAcceptLanguageHeaderDetector } from 'typesafe-i18n/detectors'
import { parse, serialize } from "cookie";
import { logDebug } from "$lib/logger";

const handleLocale: Handle = async ({ event, resolve }) => {
    const cookies = parse(event.request.headers.get("Cookie") ?? '');
    const localeCookie = cookies[CookieNames.locale];
    const preferredLocale = getPreferredLocale(event);
    let finalLocale = localeCookie ?? preferredLocale;

    logDebug("Handling locale", {
        locales,
        localeCookie,
        preferredLocale,
        finalLocale
    });

    if (locales.findIndex((locale) => locale === finalLocale) === -1) finalLocale = "en";
    if (!localeCookie) {
        // Set a locale cookie
        event.setHeaders({
            "Set-Cookie": serialize(CookieNames.locale, finalLocale, {
                path: "/",
                expires: new Date(2099, 1, 1, 0, 0, 0, 0),
                sameSite: "strict"
            })
        });
    }
    // replace html lang attribute with correct language
    return resolve(event, { transformPageChunk: ({ html }) => html.replace('%lang%', finalLocale) });
}

function getPreferredLocale(event: RequestEvent) {
    // detect the preferred language the user has configured in it's browser
    // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Language
    const headers = transformHeaders(event)
    const acceptLanguageDetector = initAcceptLanguageHeaderDetector({ headers })

    return detectLocale(acceptLanguageDetector)
}

function transformHeaders({ request }: RequestEvent) {
    const headers: Record<string, string> = {}
    request.headers.forEach((value, key) => (headers[key] = value))
    return headers
}

export const handle = sequence(handleLocale);