// A DNS resolver that queries through a specific tunnel.
//
// ---------------------------------------------------------------------------
// Why not lwIP's resolver
// ---------------------------------------------------------------------------
// lwIP ships one (LWIP_DNS), and it is disabled in lwip_port/lwipopts.h. It
// keeps a single global server list, a single global cache and a fixed table of
// in-flight queries -- all file-scope state, exactly like the rest of lwIP. That
// is fine with one interface and wrong with two: during a make-before-break
// switch both tunnels are up, each pushed its own resolvers, and a name looked
// up for a session on the old tunnel must be answered by the old tunnel's
// servers over the old tunnel's socket. A per-netif resolver is the only way to
// keep that straight, so this is roughly 400 lines we own instead of a global
// we would have to serialize access to and still get wrong.
//
// It also buys three things lwIP's does not offer: a TTL-clamped cache sized
// from config, coalescing of concurrent lookups for the same name, and failover
// across the pushed servers plus the configured fallbacks.
//
// ---------------------------------------------------------------------------
// Scope
// ---------------------------------------------------------------------------
// A records only. The stack is IPv4-only (see lwip_stack.h), so an AAAA answer
// could not be connected to even if we asked for it. No EDNS0, no DNSSEC, no
// TCP fallback: a truncated A-record answer that still carries one address is
// usable, and one that carries none fails over to the next server.
//
// ---------------------------------------------------------------------------
// Threading
// ---------------------------------------------------------------------------
// Its own strand, not the lwIP one. Parsing responses and walking the cache has
// no business running where every TCP segment in the process is also processed.
// Public methods are safe to call from any thread; handlers run on the strand.
#pragma once

#include <asio.hpp>

#include <chrono>
#include <cstdint>
#include <deque>
#include <memory>
#include <mutex>
#include <random>
#include <string>
#include <unordered_map>
#include <vector>

#include "common/config.h"
#include "netstack/lwip_stack.h"
#include "netstack/stream.h"

namespace ovg::netstack {

class DnsResolver final : public Resolver,
                          public std::enable_shared_from_this<DnsResolver> {
 public:
  // The server list is the netif's pushed resolvers followed by
  // cfg.fallback_servers. Both are queried *through the tunnel*: a fallback is
  // a different address, not a different path.
  static std::shared_ptr<DnsResolver> create(std::shared_ptr<Netif> netif,
                                             DnsConfig cfg);
  ~DnsResolver() override;

  DnsResolver(const DnsResolver &) = delete;
  DnsResolver &operator=(const DnsResolver &) = delete;

  // Resolver
  void async_resolve(const std::string &host, Handler h) override;
  void clear_cache() override;

  struct Stats {
    uint64_t queries = 0;
    uint64_t cache_hits = 0;
    uint64_t coalesced = 0;
    uint64_t timeouts = 0;
    uint64_t failures = 0;
    size_t cached = 0;
    size_t in_flight = 0;
  };
  Stats stats() const;

 private:
  DnsResolver(std::shared_ptr<Netif> netif, DnsConfig cfg);

  using Clock = std::chrono::steady_clock;

  struct Query {
    explicit Query(const Strand &s) : timer(s) {}
    std::string name;                 // lowercased
    uint16_t id = 0;
    std::vector<Handler> waiters;
    size_t server_idx = 0;
    asio::steady_timer timer;
    Clock::time_point deadline;
    std::vector<uint8_t> wire;        // kept so a retry does not rebuild it
    int last_rcode = -1;              // for the error message when all fail
    bool done = false;
  };

  struct CacheEntry {
    std::vector<IpAddress> addrs;
    Clock::time_point expires;
  };

  // Strand-only.
  void start(const std::string &name, Handler h);
  void open_socket();
  void arm_receive();
  void on_datagram(const std::error_code &ec, size_t n, const Endpoint &from);
  void send_query(const std::shared_ptr<Query> &q);
  void on_query_timeout(const std::shared_ptr<Query> &q);
  void finish(const std::shared_ptr<Query> &q, const std::error_code &ec,
              std::vector<IpAddress> addrs, uint32_t ttl_seconds);
  void fail_all(const std::error_code &ec);
  uint16_t allocate_id();
  void cache_put(const std::string &name, const std::vector<IpAddress> &addrs,
                 uint32_t ttl_seconds);
  bool cache_get(const std::string &name, std::vector<IpAddress> *out);

  std::shared_ptr<Netif> netif_;
  DnsConfig cfg_;
  Strand strand_;

  std::vector<IpAddress> servers_;
  UdpSocketPtr sock_;
  bool opening_ = false;
  std::vector<std::shared_ptr<Query>> pending_open_;
  std::vector<uint8_t> rx_buf_;
  bool receiving_ = false;

  std::unordered_map<uint16_t, std::shared_ptr<Query>> by_id_;
  std::unordered_map<std::string, std::shared_ptr<Query>> by_name_;

  std::unordered_map<std::string, CacheEntry> cache_;
  std::deque<std::string> cache_order_;  // insertion order, for eviction

  std::mt19937 rng_;

  mutable std::mutex stats_mu_;
  Stats stats_;
};

// Exposed for tests: build an A-record query and parse a response. Pure
// functions over byte buffers, which is the only part of DNS worth unit-testing
// in isolation.
namespace dns {

// Returns false if `name` is not a legal DNS name (label > 63, total > 255).
bool build_query(const std::string &name, uint16_t id,
                 std::vector<uint8_t> *out);

struct ParseResult {
  uint16_t id = 0;
  int rcode = 0;
  bool truncated = false;
  std::vector<IpAddress> addrs;
  uint32_t min_ttl = 0;
};

// Returns false only for a response that is malformed at the wire level. An
// answer that is well-formed but empty or an error comes back true with rcode
// and addrs telling the caller what happened.
bool parse_response(const uint8_t *data, size_t len, ParseResult *out);

}  // namespace dns

}  // namespace ovg::netstack