using System.Net.Sockets;
using System.Text;
using System.Text.Json;
using NATS.Server.Subscriptions;

namespace NATS.Server.Routes;

public sealed class RouteConnection(Socket socket) : IAsyncDisposable
{
    private readonly Socket _socket = socket;
    private readonly NetworkStream _stream = new(socket, ownsSocket: true);
    private readonly SemaphoreSlim _writeGate = new(1, 1);
    private readonly CancellationTokenSource _closedCts = new();
    private Task? _frameLoopTask;

    public string? RemoteServerId { get; private set; }
    public string RemoteEndpoint => _socket.RemoteEndPoint?.ToString() ?? Guid.NewGuid().ToString("N");

    /// <summary>
    /// The pool index assigned to this route connection. Used for account-based
    /// routing to deterministically select which pool connection handles traffic
    /// for a given account. See <see cref="RouteManager.ComputeRoutePoolIdx"/>.
    /// </summary>
    public int PoolIndex { get; set; }

    /// <summary>
    /// The pool size agreed upon during handshake negotiation with the remote peer.
    /// Defaults to 0 (no pooling / pre-negotiation state). Set after handshake completes.
    /// Go reference: server/route.go negotiateRoutePool.
    /// </summary>
    public int NegotiatedPoolSize { get; private set; }

    /// <summary>
    /// Negotiates the effective route pool size between local and remote peers.
    /// Returns <c>Math.Min(localPoolSize, remotePoolSize)</c>, but returns 0 if
    /// either side is 0 for backward compatibility with peers that do not support pooling.
    /// Go reference: server/route.go negotiateRoutePool.
    /// </summary>
    public static int NegotiatePoolSize(int localPoolSize, int remotePoolSize)
    {
        if (localPoolSize == 0 || remotePoolSize == 0)
            return 0;

        return Math.Min(localPoolSize, remotePoolSize);
    }

    /// <summary>
    /// Applies the result of pool size negotiation to this connection.
    /// </summary>
    internal void SetNegotiatedPoolSize(int negotiatedPoolSize)
    {
        NegotiatedPoolSize = negotiatedPoolSize;
    }

    public Func<RemoteSubscription, Task>? RemoteSubscriptionReceived { get; set; }
    public Func<RouteMessage, Task>? RoutedMessageReceived { get; set; }

    public async Task PerformOutboundHandshakeAsync(string serverId, CancellationToken ct)
    {
        await WriteLineAsync($"ROUTE {serverId}", ct);
        var line = await ReadLineAsync(ct);
        RemoteServerId = ParseHandshake(line);
    }

    public async Task PerformInboundHandshakeAsync(string serverId, CancellationToken ct)
    {
        var line = await ReadLineAsync(ct);
        RemoteServerId = ParseHandshake(line);
        await WriteLineAsync($"ROUTE {serverId}", ct);
    }

    public void StartFrameLoop(CancellationToken ct)
    {
        if (_frameLoopTask != null)
            return;

        var linked = CancellationTokenSource.CreateLinkedTokenSource(ct, _closedCts.Token);
        _frameLoopTask = Task.Run(() => ReadFramesAsync(linked.Token), linked.Token);
    }

    public async Task SendRsPlusAsync(string account, string subject, string? queue, CancellationToken ct)
    {
        var frame = queue is { Length: > 0 }
            ? $"RS+ {account} {subject} {queue}"
            : $"RS+ {account} {subject}";
        await WriteLineAsync(frame, ct);
    }

    public async Task SendRsMinusAsync(string account, string subject, string? queue, CancellationToken ct)
    {
        var frame = queue is { Length: > 0 }
            ? $"RS- {account} {subject} {queue}"
            : $"RS- {account} {subject}";
        await WriteLineAsync(frame, ct);
    }

    public async Task SendRmsgAsync(string account, string subject, string? replyTo, ReadOnlyMemory<byte> payload, CancellationToken ct)
    {
        var replyToken = string.IsNullOrEmpty(replyTo) ? "-" : replyTo;
        await _writeGate.WaitAsync(ct);
        try
        {
            var control = Encoding.ASCII.GetBytes($"RMSG {account} {subject} {replyToken} {payload.Length}\r\n");
            await _stream.WriteAsync(control, ct);
            if (!payload.IsEmpty)
                await _stream.WriteAsync(payload, ct);
            await _stream.WriteAsync("\r\n"u8.ToArray(), ct);
            await _stream.FlushAsync(ct);
        }
        finally
        {
            _writeGate.Release();
        }
    }

    public async Task WaitUntilClosedAsync(CancellationToken ct)
    {
        if (_frameLoopTask == null)
            return;

        using var linked = CancellationTokenSource.CreateLinkedTokenSource(ct, _closedCts.Token);
        await _frameLoopTask.WaitAsync(linked.Token);
    }

    public async ValueTask DisposeAsync()
    {
        await _closedCts.CancelAsync();
        if (_frameLoopTask != null)
            await _frameLoopTask.ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);
        _closedCts.Dispose();
        _writeGate.Dispose();
        await _stream.DisposeAsync();
    }

    private async Task ReadFramesAsync(CancellationToken ct)
    {
        while (!ct.IsCancellationRequested)
        {
            string line;
            try
            {
                line = await ReadLineAsync(ct);
            }
            catch (OperationCanceledException)
            {
                break;
            }
            catch (IOException)
            {
                break;
            }

            if (line.StartsWith("RS+ ", StringComparison.Ordinal))
            {
                var parts = line.Split(' ', StringSplitOptions.RemoveEmptyEntries);
                if (RemoteSubscriptionReceived != null && TryParseAccountScopedInterest(parts, out var parsedAccount, out var parsedSubject, out var queue))
                {
                    await RemoteSubscriptionReceived(new RemoteSubscription(parsedSubject, queue, RemoteServerId ?? string.Empty, parsedAccount));
                }
                continue;
            }

            if (line.StartsWith("RS- ", StringComparison.Ordinal))
            {
                var parts = line.Split(' ', StringSplitOptions.RemoveEmptyEntries);
                if (RemoteSubscriptionReceived != null && TryParseAccountScopedInterest(parts, out var parsedAccount, out var parsedSubject, out var queue))
                {
                    await RemoteSubscriptionReceived(RemoteSubscription.Removal(parsedSubject, queue, RemoteServerId ?? string.Empty, parsedAccount));
                }
                continue;
            }

            if (!line.StartsWith("RMSG ", StringComparison.Ordinal))
                continue;

            var args = line.Split(' ', StringSplitOptions.RemoveEmptyEntries);
            if (args.Length < 4)
                continue;

            var account = "$G";
            string subject;
            string replyToken;
            string sizeToken;

            // New format: RMSG <account> <subject> <reply> <size>
            // Legacy format: RMSG <subject> <reply> <size>
            if (args.Length >= 5 && !LooksLikeSubject(args[1]))
            {
                account = args[1];
                subject = args[2];
                replyToken = args[3];
                sizeToken = args[4];
            }
            else
            {
                subject = args[1];
                replyToken = args[2];
                sizeToken = args[3];
            }

            var reply = replyToken == "-" ? null : replyToken;
            if (!int.TryParse(sizeToken, out var size) || size < 0)
                continue;

            var payload = await ReadPayloadAsync(size, ct);
            if (RoutedMessageReceived != null)
                await RoutedMessageReceived(new RouteMessage(subject, reply, payload, account));
        }
    }

    private async Task<ReadOnlyMemory<byte>> ReadPayloadAsync(int size, CancellationToken ct)
    {
        var payload = new byte[size];
        var offset = 0;
        while (offset < size)
        {
            var read = await _stream.ReadAsync(payload.AsMemory(offset, size - offset), ct);
            if (read == 0)
                throw new IOException("Route connection closed during payload read");
            offset += read;
        }

        var trailer = new byte[2];
        var trailerRead = 0;
        while (trailerRead < 2)
        {
            var read = await _stream.ReadAsync(trailer.AsMemory(trailerRead, 2 - trailerRead), ct);
            if (read == 0)
                throw new IOException("Route connection closed during payload trailer read");
            trailerRead += read;
        }

        if (trailer[0] != (byte)'\r' || trailer[1] != (byte)'\n')
            throw new IOException("Invalid route payload trailer");

        return payload;
    }

    private async Task WriteLineAsync(string line, CancellationToken ct)
    {
        await _writeGate.WaitAsync(ct);
        try
        {
            var bytes = Encoding.ASCII.GetBytes($"{line}\r\n");
            await _stream.WriteAsync(bytes, ct);
            await _stream.FlushAsync(ct);
        }
        finally
        {
            _writeGate.Release();
        }
    }

    private async Task<string> ReadLineAsync(CancellationToken ct)
    {
        var bytes = new List<byte>(64);
        var single = new byte[1];
        while (true)
        {
            var read = await _stream.ReadAsync(single, ct);
            if (read == 0)
                throw new IOException("Route connection closed");

            if (single[0] == (byte)'\n')
                break;
            if (single[0] != (byte)'\r')
                bytes.Add(single[0]);
        }

        return Encoding.ASCII.GetString([.. bytes]);
    }

    private static string ParseHandshake(string line)
    {
        if (!line.StartsWith("ROUTE ", StringComparison.OrdinalIgnoreCase))
            throw new InvalidOperationException("Invalid route handshake");

        var id = line[6..].Trim();
        if (id.Length == 0)
            throw new InvalidOperationException("Route handshake missing server id");

        return id;
    }

    private static bool TryParseAccountScopedInterest(string[] parts, out string account, out string subject, out string? queue)
    {
        account = "$G";
        subject = string.Empty;
        queue = null;

        if (parts.Length < 2)
            return false;

        // New format: RS+ <account> <subject> [queue]
        // Legacy format: RS+ <subject> [queue]
        if (parts.Length >= 3 && !LooksLikeSubject(parts[1]))
        {
            account = parts[1];
            subject = parts[2];
            queue = parts.Length >= 4 ? parts[3] : null;
            return true;
        }

        subject = parts[1];
        queue = parts.Length >= 3 ? parts[2] : null;
        return true;
    }

    private static bool LooksLikeSubject(string token)
        => token.Contains('.', StringComparison.Ordinal)
            || token.Contains('*', StringComparison.Ordinal)
            || token.Contains('>', StringComparison.Ordinal);

    public static string BuildConnectInfoJson(string serverId, IEnumerable<string>? accounts, string? topologySnapshot)
    {
        var payload = new
        {
            server_id = serverId,
            accounts = (accounts ?? []).ToArray(),
            topology = topologySnapshot ?? string.Empty,
        };
        return JsonSerializer.Serialize(payload);
    }
}

public sealed record RouteMessage(string Subject, string? ReplyTo, ReadOnlyMemory<byte> Payload, string Account = "$G");