const LOOPBACK_HOSTS = new Set(["localhost", "127.0.0.1", "::1"]);

function trimTrailingSlash(value: string) {
  return value.replace(/\/+$/, "");
}

function getDefaultApiBaseUrl() {
  if (typeof window === "undefined") {
    return "http://localhost:5000";
  }

  const protocol = window.location.protocol === "https:" ? "https:" : "http:";
  return `${protocol}//${window.location.hostname}:5000`;
}

function shouldUseBrowserHostForApi(url: URL) {
  if (typeof window === "undefined") {
    return false;
  }

  return LOOPBACK_HOSTS.has(url.hostname) && !LOOPBACK_HOSTS.has(window.location.hostname);
}

function resolveApiBaseUrl() {
  const configuredApiUrl = import.meta.env.VITE_API_URL?.trim();
  if (!configuredApiUrl) {
    return getDefaultApiBaseUrl();
  }

  try {
    const apiUrl = new URL(configuredApiUrl);
    if (shouldUseBrowserHostForApi(apiUrl)) {
      apiUrl.hostname = window.location.hostname;
    }
    return trimTrailingSlash(apiUrl.toString());
  } catch {
    return trimTrailingSlash(configuredApiUrl);
  }
}

export const API_BASE_URL = resolveApiBaseUrl();