// src/lib/api-base.ts
// Server-only helper: resolve the backend API base URL at runtime.
//
// Why:
//   The frontend proxies /api/* to the backend via NEXT_PUBLIC_API_URL /
//   INTERNAL_API_URL. In Docker Swarm deployments these may point to a
//   task-instance hostname like `backone-inspect-backend-bun-0:3001`. When the
//   Swarm service is redeployed the task index can change (`-0` -> `-1`), orphaning
//   the configured URL and breaking every proxied request ("fetch failed").
//
// Solution:
//   Probe candidates in order and cache the first healthy one. Also auto-derive
//   the stable service alias by stripping a trailing `-<digits>` suffix, so a
//   stale `-0` URL transparently recovers to the plain service DNS name.

let cachedBase: string | null = null;

function pushUnique(list: string[], value?: string | null) {
  if (!value) return;
  const clean = value.replace(/\/+$/, '');
  if (clean && !list.includes(clean)) list.push(clean);
}

function buildCandidates(): string[] {
  const list: string[] = [];
  pushUnique(list, process.env.INTERNAL_API_URL);
  pushUnique(list, process.env.NEXT_PUBLIC_API_URL);

  // Derive stable Swarm service alias (strip task index: ...-0 -> ...)
  for (const base of [...list]) {
    try {
      const u = new URL(base);
      const strippedHost = u.host.replace(/-\d+$/, '');
      if (strippedHost && strippedHost !== u.host) {
        pushUnique(list, `${u.protocol}//${strippedHost}`);
      }
    } catch {
      // ignore malformed URLs
    }
  }

  pushUnique(list, 'http://127.0.0.1:3001');
  return list;
}

async function isHealthy(base: string): Promise<boolean> {
  try {
    const ctrl = new AbortController();
    const timer = setTimeout(() => ctrl.abort(), 2500);
    const res = await fetch(`${base}/api/health`, { signal: ctrl.signal });
    clearTimeout(timer);
    return res.ok;
  } catch {
    return false;
  }
}

export async function getApiBase(): Promise<string> {
  if (cachedBase) return cachedBase;

  for (const base of buildCandidates()) {
    if (await isHealthy(base)) {
      cachedBase = base;
      return base;
    }
  }

  // Fallback: use the first configured candidate even if the probe failed.
  cachedBase = buildCandidates()[0] || 'http://127.0.0.1:3001';
  return cachedBase;
}

export function resetApiBase() {
  cachedBase = null;
}