using System;
using System.Net;
using System.Net.Sockets;
using System.Threading;
using System.Threading.Tasks;
using XWorld.Framework.Protocol;

namespace XWorld.Server.Gateway
{
    public sealed class DevDiscoveryResponder
    {
        private readonly DevDiscoveryReply _template;
        private readonly string _token;
        private readonly UdpClient _udp;
        private CancellationTokenSource _cts;
        private Task _loop;

        public int Port { get; }

        public DevDiscoveryResponder(string serverName, string gatewayUrl, string resourceBaseUrl,
            string token, int listenPort = DevDiscovery.Port)
        {
            if (string.IsNullOrEmpty(token)) throw new ArgumentException("token cannot be empty", nameof(token));
            _token = token;
            _template = new DevDiscoveryReply
            {
                ProtoVersion = DevDiscovery.ProtoVersion,
                ServerName = serverName ?? "dev-server",
                GatewayUrl = gatewayUrl ?? "",
                ResourceBaseUrl = resourceBaseUrl ?? "",
                Token = token,
            };
            _udp = new UdpClient(AddressFamily.InterNetwork);
            _udp.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
            _udp.Client.Bind(new IPEndPoint(IPAddress.Any, listenPort));
            Port = ((IPEndPoint)_udp.Client.LocalEndPoint).Port;
        }

        public void Start()
        {
            if (_loop != null) return;
            _cts = new CancellationTokenSource();
            _loop = ReceiveLoopAsync(_cts.Token);
        }

        private async Task ReceiveLoopAsync(CancellationToken ct)
        {
            using (ct.Register(() => { try { _udp.Close(); } catch { } }))
            {
                while (!ct.IsCancellationRequested)
                {
                    UdpReceiveResult res;
                    try { res = await _udp.ReceiveAsync().ConfigureAwait(false); }
                    catch (ObjectDisposedException) { break; }
                    catch (SocketException) { break; }

                    if (!DevDiscoveryCodec.TryDecodeQuery(res.Buffer, out var q)) continue;
                    if (q.ProtoVersion != DevDiscovery.ProtoVersion) continue;
                    if (!string.Equals(q.Token, _token, StringComparison.Ordinal)) continue;

                    DevDiscoveryReply reply = _template;
                    reply.Nonce = q.Nonce;
                    byte[] bytes = DevDiscoveryCodec.EncodeReply(reply);
                    try { await _udp.SendAsync(bytes, bytes.Length, res.RemoteEndPoint).ConfigureAwait(false); }
                    catch (ObjectDisposedException) { break; }
                    catch (SocketException) { }
                }
            }
        }

        public async Task StopAsync()
        {
            if (_cts == null) return;
            _cts.Cancel();
            try { await _loop.ConfigureAwait(false); }
            catch { }
            _cts.Dispose();
            _cts = null;
            _loop = null;
        }
    }
}