import { goto } from '$app/navigation';
import { session } from '$lib/stores/session.svelte';
import { ApiError } from './client';

/**
 * Turns a failed call into something to show the user.
 *
 * A 401 means the 16-minute JWT lapsed mid-session: there is nothing useful to
 * display, so the session is dropped and the user is sent back to `/login`, and
 * this returns null.
 */
export function toErrorMessage(cause: unknown, fallback: string): string | null {
	if (cause instanceof ApiError && cause.status === 401) {
		session.clear();
		void goto('/login', { replaceState: true });
		return null;
	}

	// ApiError already carries the API's own `message`, or "unreachable" for status 0.
	return cause instanceof ApiError ? cause.message : fallback;
}