/**
 * In-memory sliding-window rate limiter.
 * Tracks request counts per IP per minute bucket.
 * Auto-evicts expired entries to prevent memory leaks.
 *
 * ⚠️ NOTE: This is an in-memory store. In a multi-node or serverless
 * environment (Vercel, AWS Lambda, Docker Swarm), each instance will
 * have its own independent rate limit. For a global limit across
 * multiple nodes, a persistent store like Redis would be required.
 */

import { NextRequest } from 'next/server';
import { getConfig } from './config';

/** Bucket key used when the client cannot be identified with any confidence. */
const UNIDENTIFIED = 'unknown';

/**
 * Resolve the client IP to use as a rate-limit bucket key.
 *
 * Self-hosted Next.js cannot see the socket peer address (`request.ip` is only
 * populated by platforms like Vercel), so behind a reverse proxy the client IP
 * has to come from a header. Headers are attacker-controlled unless a proxy
 * overwrites them, which is what TRUSTED_PROXY_HOPS declares.
 *
 * With `TRUSTED_PROXY_HOPS=n`, the entry `n` from the *right* of
 * X-Forwarded-For is used. Proxies append the address they actually saw
 * (nginx: `proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for`), so
 * everything to the left is client-supplied and ignored. A client that sends
 * `X-Forwarded-For: 1.1.1.1` therefore cannot pick its own bucket.
 *
 * ⚠️ This holds only while the app is reachable *exclusively* through the
 * proxy. Bind the server to localhost — if clients can connect directly they
 * control the whole header and no parsing strategy can help.
 */
export function getClientIp(request: NextRequest): string {
  const hops = getConfig().trustedProxyHops;

  // @ts-expect-error - Next.js 15+ removed request.ip from types but hosting platforms still populate it
  const directIp = request.ip as string | undefined;
  // The real socket peer address, where available, is never spoofable.
  if (directIp) return directIp;

  const xForwardedFor = request.headers.get('x-forwarded-for');
  const xRealIp = request.headers.get('x-real-ip');

  if (hops > 0) {
    if (xForwardedFor) {
      const chain = xForwardedFor
        .split(',')
        .map((s) => s.trim())
        .filter(Boolean);
      // Chain shorter than the configured hop count means the request did not
      // traverse the full proxy chain — do not fall back to a spoofable entry.
      return chain[chain.length - hops] ?? UNIDENTIFIED;
    }
    // nginx's `proxy_set_header X-Real-IP $remote_addr` overwrites rather than
    // appends, so it is trustworthy — but only for a single hop, since an outer
    // proxy would leave an inner proxy's value in place.
    if (xRealIp && hops === 1) return xRealIp;
    return UNIDENTIFIED;
  }

  // No proxy declared: headers are best-effort only. This still separates
  // honest clients into their own buckets, but a deliberate attacker can spoof
  // them. Set TRUSTED_PROXY_HOPS if you run behind a reverse proxy.
  return xRealIp ?? xForwardedFor?.split(',')[0].trim() ?? UNIDENTIFIED;
}

interface RateLimitEntry {
  count: number;
  expiresAt: number;
}

const store = new Map<string, RateLimitEntry>();

// Hard ceiling to prevent unbounded memory growth under DDoS conditions
const MAX_STORE_ENTRIES = 10_000;

// Evict expired entries periodically (every 60s)
let lastCleanup = Date.now();
const CLEANUP_INTERVAL = 60_000;

function cleanup() {
  const now = Date.now();
  if (now - lastCleanup < CLEANUP_INTERVAL) return;
  lastCleanup = now;
  for (const [key, entry] of store) {
    if (now > entry.expiresAt) store.delete(key);
  }
}

/**
 * Check if a request should be allowed.
 * @param ip - Client IP address
 * @param maxRpm - Maximum requests per minute
 * @returns { success, remaining, resetAt (epoch ms) }
 */
export function checkRateLimit(
  ip: string,
  maxRpm: number,
): { success: boolean; remaining: number; resetAt: number } {
  cleanup();

  const now = Date.now();
  const windowMs = 60_000;
  const key = `rl:${ip}`;

  const entry = store.get(key);

  // New window or expired
  if (!entry || now > entry.expiresAt) {
    if (store.size >= MAX_STORE_ENTRIES) {
      // Store is full. Evict the oldest entry to prevent cache flooding
      // DOS attacks, which could otherwise block all legitimate users.
      const oldestKey = store.keys().next().value;
      if (oldestKey !== undefined) {
        store.delete(oldestKey);
      }
    }
    const resetAt = now + windowMs;
    store.set(key, { count: 1, expiresAt: resetAt });
    return { success: true, remaining: maxRpm - 1, resetAt };
  }

  // Within window
  entry.count++;
  if (entry.count > maxRpm) {
    return { success: false, remaining: 0, resetAt: entry.expiresAt };
  }

  return { success: true, remaining: maxRpm - entry.count, resetAt: entry.expiresAt };
}