using System.Buffers;
using System.Diagnostics.CodeAnalysis;
using System.Net.WebSockets;
using System.Text.Json;
using Foxcord.Gateway.Events;
using Foxcord.Gateway.Events.Commands;
using Foxcord.Serialization;
using Serilog;

namespace Foxcord.Gateway;

public partial class DiscordGatewayClient
{
    private const int GatewayVersion = 10;
    private const int BufferSize = 64 * 1024;

    private readonly ILogger _logger;
    private ClientWebSocket? _ws;
    private readonly string _token;
    private readonly Uri _gatewayUri;
    private readonly GatewayIntent _intents;
    private readonly IdentifyProperties? _properties;
    private readonly int[]? _shardInfo;
    private readonly PresenceUpdateCommand? _initialPresence;
    private readonly JsonSerializerOptions _jsonSerializerOptions = JsonSerializerExtensions.CreateSerializer();

    private long? _lastSequence;
    private DateTimeOffset _lastHeartbeatSend = DateTimeOffset.UnixEpoch;
    private DateTimeOffset _lastHeartbeatAck = DateTimeOffset.UnixEpoch;
    public TimeSpan Latency => _lastHeartbeatAck - _lastHeartbeatSend;

    public DiscordGatewayClient(ILogger logger, DiscordGatewayClientOptions opts)
    {
        _logger = logger.ForContext<DiscordGatewayClient>();
        _token = $"Bot {opts.Token}";
        var uriBuilder = new UriBuilder(opts.Uri)
        {
            Query = $"v={GatewayVersion}&encoding=json"
        };
        _gatewayUri = uriBuilder.Uri;
        _intents = opts.Intents;
        _properties = opts.IdentifyProperties;
        _shardInfo = opts.Shards;
        _initialPresence = opts.InitialPresence;
    }

    public ConnectionStatus Status { get; private set; } = ConnectionStatus.Dead;

    /// <summary>
    /// Connects to the gateway. This method returns after a connection is established.
    /// <c>ct</c> is stored by the client and cancelling it will close the connection.
    /// The caller must pause indefinitely or the bot will shut down immediately.
    /// </summary>
    public async Task ConnectAsync(CancellationToken ct = default)
    {
        if (Status != ConnectionStatus.Dead)
            throw new DiscordGatewayRequestError(
                "Gateway is connecting or connected, only one concurrent connection allowed.");

        try
        {
            _ws = new ClientWebSocket();
            Status = ConnectionStatus.Connecting;
            await _ws.ConnectAsync(_gatewayUri, ct);

            using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
            cts.CancelAfter(TimeSpan.FromSeconds(30));
            var (rawHelloPacketType, rawHelloPacket) = await ReadPacketAsync(cts.Token);
            if (rawHelloPacketType == WebSocketMessageType.Close)
                throw new DiscordGatewayRequestError("First packet received was a close message");

            if (!TryDeserializeEvent(rawHelloPacket, out var rawHelloEvent))
                throw new DiscordGatewayRequestError("First packet was not a valid event");
            if (rawHelloEvent is not HelloEvent hello)
                throw new DiscordGatewayRequestError("First event was not a HELLO event");

            _logger.Debug("Received HELLO, heartbeat interval is {HeartbeatInterval}", hello.HeartbeatInterval);
            var _ = HeartbeatLoopAsync(hello.HeartbeatInterval, ct);
            var __ = ReceiveLoopAsync(ct);

            _logger.Debug("Sending IDENTIFY");
            await WritePacketAsync(new GatewayPacket
            {
                Opcode = GatewayOpcode.Identify,
                Payload = new IdentifyEvent
                {
                    Token = _token,
                    Intents = _intents,
                    Properties = _properties ?? new IdentifyProperties(),
                    Shards = _shardInfo,
                    Presence = _initialPresence?.ToPayload()
                }
            }, ct);
        }
        catch (Exception e)
        {
            _logger.Error(e, "Error connecting to gateway");
            Status = ConnectionStatus.Dead;
            _ws = null;
            throw;
        }
    }

    private async Task HeartbeatLoopAsync(int heartbeatInterval, CancellationToken ct = default)
    {
        var delay = TimeSpan.FromMilliseconds(heartbeatInterval * Random.Shared.NextDouble());
        _logger.Debug("Waiting {Delay} before sending first heartbeat", delay);

        using var timer = new PeriodicTimer(TimeSpan.FromMilliseconds(heartbeatInterval));
        while (await timer.WaitForNextTickAsync(ct))
        {
            _logger.Debug("Sending heartbeat with sequence {Sequence}", _lastSequence);
            _lastHeartbeatSend = DateTimeOffset.UtcNow;
            await SendCommandAsync(new HeartbeatCommand(_lastSequence), ct);
        }
    }

    private async Task ReceiveLoopAsync(CancellationToken ct = default)
    {
        while (!ct.IsCancellationRequested)
        {
            try
            {
                var (type, packet) = await ReadPacketAsync(ct);
                if (type == WebSocketMessageType.Close || packet == null)
                {
                    // TODO: close websocket
                    return;
                }

                _ = ReceiveAsync(packet, ct);
            }
            catch (Exception e)
            {
                _logger.Error(e, "Error while receiving data");
            }
        }

        async Task ReceiveAsync(GatewayPacket packet, CancellationToken ct2 = default)
        {
            if (!TryDeserializeEvent(packet, out var gatewayEvent))
            {
                _logger.Debug("Event {EventType} didn't have payload", packet.Opcode);
                return;
            }

            switch (gatewayEvent)
            {
                case HeartbeatEvent:
                    await HandleHeartbeatRequest(ct2);
                    break;
                case HeartbeatAckEvent:
                    HandleHeartbeatAck();
                    break;
                case DispatchEvent dispatch:
                    await HandleDispatch(dispatch.Payload, ct2);
                    break;
            }
        }
    }

    public async ValueTask SendCommandAsync(IGatewayCommand command, CancellationToken ct = default) =>
        await WritePacketAsync(command.ToGatewayPacket(), ct);

    private async ValueTask WritePacketAsync(GatewayPacket packet, CancellationToken ct = default)
    {
        using var buf = MemoryPool<byte>.Shared.Rent(BufferSize);
        var json = JsonSerializer.SerializeToUtf8Bytes(packet, _jsonSerializerOptions);
        await _ws!.SendAsync(json.AsMemory(), WebSocketMessageType.Text, true, ct);
    }

    private async ValueTask<(WebSocketMessageType type, GatewayPacket? packet)> ReadPacketAsync(
        CancellationToken ct = default)
    {
        using var buf = MemoryPool<byte>.Shared.Rent(BufferSize);
        var res = await _ws!.ReceiveAsync(buf.Memory, ct);
        if (res.MessageType == WebSocketMessageType.Close) return (res.MessageType, null);

        if (res.EndOfMessage)
            return DeserializePacket(res, buf.Memory.Span[..res.Count]);

        return await DeserializeMultipleBufferAsync(res, buf);
    }

    private async Task<(WebSocketMessageType type, GatewayPacket packet)> DeserializeMultipleBufferAsync(
        ValueWebSocketReceiveResult res, IMemoryOwner<byte> buf)
    {
        await using var stream = new MemoryStream(BufferSize * 4);
        stream.Write(buf.Memory.Span.Slice(0, res.Count));

        while (!res.EndOfMessage)
        {
            res = await _ws!.ReceiveAsync(buf.Memory, default);
            stream.Write(buf.Memory.Span.Slice(0, res.Count));
        }

        return DeserializePacket(res, stream.GetBuffer().AsSpan(0, (int)stream.Length));
    }

    private (WebSocketMessageType type, GatewayPacket packet) DeserializePacket(
        ValueWebSocketReceiveResult res, Span<byte> span) => (res.MessageType,
        JsonSerializer.Deserialize<GatewayPacket>(span, _jsonSerializerOptions)!);

    private bool TryDeserializeEvent(GatewayPacket? packet, [NotNullWhen(true)] out IGatewayEvent? gatewayEvent)
    {
        gatewayEvent = null;
        if (packet == null) return false;
        var payload = packet.Payload is JsonElement element ? element : default;
        switch (packet.Opcode)
        {
            case GatewayOpcode.Hello:
                gatewayEvent = payload.Deserialize<HelloEvent>(_jsonSerializerOptions)!;
                break;
            case GatewayOpcode.Dispatch:
                _lastSequence = packet.Sequence;
                gatewayEvent = new DispatchEvent { Payload = ParseDispatchEvent(packet.EventType!, payload) };
                break;
            case GatewayOpcode.Heartbeat:
                gatewayEvent = new HeartbeatEvent();
                break;
            case GatewayOpcode.Reconnect:
            case GatewayOpcode.InvalidSession:
                throw new NotImplementedException();
            case GatewayOpcode.HeartbeatAck:
                gatewayEvent = new HeartbeatAckEvent();
                break;
            default:
                throw new ArgumentOutOfRangeException();
        }

        return true;
    }

    public enum ConnectionStatus
    {
        Dead,
        Connecting,
        Connected
    }
}

public class DiscordGatewayClientOptions
{
    public required string Token { get; init; }
    public required string Uri { get; init; }
    public required GatewayIntent Intents { get; init; }
    public IdentifyProperties? IdentifyProperties { get; init; }
    public int? ShardId { get; init; }
    public int? ShardCount { get; init; }
    public PresenceUpdateCommand? InitialPresence { get; init; }

    internal int[]? Shards => ShardId != null && ShardCount != null ? [ShardId.Value, ShardCount.Value] : null;
}