src/hooks.server.ts

import type { Handle } from '@sveltejs/kit';
import { getTextDirection, isLocale } from '$lib/paraglide/runtime';
import { paraglideMiddleware } from '$lib/paraglide/server';

const DATA_CACHE_CONTROL = 'private, max-age=300';
const MIN_GZIP_SIZE_BYTES = 1024;

function isBucketDataRequest(request: Request) {
	const segments = new URL(request.url).pathname.split('/').filter(Boolean);
	if (segments.at(-1) !== '__data.json') return false;

	segments.pop();
	if (segments[0] && isLocale(segments[0])) segments.shift();

	const route = segments.join('/');
	return route === '' || route === 'details' || route === 'tools';
}

function acceptsGzip(request: Request) {
	return (request.headers.get('accept-encoding') ?? '').split(',').some((entry) => {
		const [encoding, ...parameters] = entry.trim().toLowerCase().split(';');
		if (encoding !== 'gzip' && encoding !== '*') return false;

		const quality = parameters
			.map((parameter) => parameter.trim())
			.find((parameter) => parameter.startsWith('q='));
		return quality ? Number(quality.slice(2)) > 0 : true;
	});
}

function appendVary(headers: Headers, value: string) {
	const values = new Set(
		(headers.get('vary') ?? '')
			.split(',')
			.map((item) => item.trim())
			.filter(Boolean)
	);
	values.add(value);
	headers.set('vary', [...values].join(', '));
}

function optimizeBucketDataResponse(request: Request, response: Response) {
	if (!response.ok || !response.body || !isBucketDataRequest(request)) return response;

	const headers = new Headers(response.headers);
	headers.set('cache-control', DATA_CACHE_CONTROL);
	appendVary(headers, 'Accept-Encoding');

	const contentLength = Number(headers.get('content-length') ?? 0);
	if (request.method === 'HEAD' || contentLength < MIN_GZIP_SIZE_BYTES || !acceptsGzip(request)) {
		return new Response(response.body, {
			status: response.status,
			statusText: response.statusText,
			headers
		});
	}

	headers.delete('content-length');
	headers.set('content-encoding', 'gzip');

	return new Response(response.body.pipeThrough(new CompressionStream('gzip')), {
		status: response.status,
		statusText: response.statusText,
		headers
	});
}

const handleParaglide: Handle = ({ event, resolve }) =>
	paraglideMiddleware(event.request, async ({ request, locale }) => {
		event.request = request;

		const response = await resolve(event, {
			transformPageChunk: ({ html }) =>
				html
					.replace('%paraglide.lang%', locale)
					.replace('%paraglide.dir%', getTextDirection(locale))
		});

		return optimizeBucketDataResponse(request, response);
	});

export const handle: Handle = handleParaglide;