using System.Net.Sockets;

namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus;

/// <summary>
///     Concrete Modbus TCP transport. Wraps a single <see cref="TcpClient"/> and serializes
///     requests so at most one transaction is in-flight at a time — Modbus servers typically
///     support concurrent transactions, but the single-flight model keeps the wire trace
///     easy to diagnose and avoids interleaved-response correlation bugs.
/// </summary>
/// <remarks>
///     <para>
///         Survives mid-transaction socket drops: when a send/read fails with a socket-level
///         error (<see cref="IOException"/>, <see cref="SocketException"/>, <see cref="EndOfStreamException"/>)
///         the transport disposes the dead socket, reconnects, and retries the PDU exactly
///         once. Deliberately limited to a single retry — further failures bubble up so the
///         driver's health surface reflects the real state instead of masking a dead PLC.
///     </para>
///     <para>
///         Why this matters for DL205/DL260: the AutomationDirect H2-ECOM100 does NOT send
///         TCP keepalives per <c>docs/v2/dl205.md</c> §behavioral-oddities, so any NAT/firewall
///         between the gateway and PLC can silently close an idle socket after 2-5 minutes.
///         Also enables OS-level <c>SO_KEEPALIVE</c> so the driver's own side detects a stuck
///         socket in reasonable time even when the application is mostly idle.
///     </para>
/// </remarks>
public sealed class ModbusTcpTransport : IModbusTransport
{
    private readonly string _host;
    private readonly int _port;
    private readonly TimeSpan _timeout;
    private readonly bool _autoReconnect;
    private readonly SemaphoreSlim _gate = new(1, 1);
    private TcpClient? _client;
    private NetworkStream? _stream;
    private ushort _nextTx;
    private bool _disposed;

    public ModbusTcpTransport(string host, int port, TimeSpan timeout, bool autoReconnect = true)
    {
        _host = host;
        _port = port;
        _timeout = timeout;
        _autoReconnect = autoReconnect;
    }

    public async Task ConnectAsync(CancellationToken ct)
    {
        // Resolve the host explicitly + prefer IPv4. .NET's TcpClient default-constructor is
        // dual-stack (IPv6 first, fallback to IPv4) — but most Modbus TCP devices (PLCs and
        // simulators like pymodbus) bind 0.0.0.0 only, so the IPv6 attempt times out and we
        // burn the entire ConnectAsync budget before even trying IPv4. Resolving first +
        // dialing the IPv4 address directly sidesteps that.
        var addresses = await System.Net.Dns.GetHostAddressesAsync(_host, ct).ConfigureAwait(false);
        var ipv4 = System.Linq.Enumerable.FirstOrDefault(addresses,
            a => a.AddressFamily == System.Net.Sockets.AddressFamily.InterNetwork);
        var target = ipv4 ?? (addresses.Length > 0 ? addresses[0] : System.Net.IPAddress.Loopback);

        _client = new TcpClient(target.AddressFamily);
        EnableKeepAlive(_client);

        using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
        cts.CancelAfter(_timeout);
        await _client.ConnectAsync(target, _port, cts.Token).ConfigureAwait(false);
        _stream = _client.GetStream();
    }

    /// <summary>
    ///     Enable SO_KEEPALIVE with aggressive probe timing. DL205/DL260 doesn't send keepalives
    ///     itself; having the OS probe the socket every ~30s lets the driver notice a dead PLC
    ///     or broken NAT path long before the default 2-hour Windows idle timeout fires.
    ///     Non-fatal if the underlying OS rejects the option (some older Linux / container
    ///     sandboxes don't expose the fine-grained timing levers — the driver still works,
    ///     application-level probe still detects problems).
    /// </summary>
    private static void EnableKeepAlive(TcpClient client)
    {
        try
        {
            client.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true);
            client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveTime, 30);
            client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveInterval, 10);
            client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveRetryCount, 3);
        }
        catch { /* best-effort; older OSes may not expose the granular knobs */ }
    }

    public async Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct)
    {
        if (_disposed) throw new ObjectDisposedException(nameof(ModbusTcpTransport));
        if (_stream is null) throw new InvalidOperationException("Transport not connected");

        await _gate.WaitAsync(ct).ConfigureAwait(false);
        try
        {
            try
            {
                return await SendOnceAsync(unitId, pdu, ct).ConfigureAwait(false);
            }
            catch (Exception ex) when (_autoReconnect && IsSocketLevelFailure(ex))
            {
                // Mid-transaction drop: tear down the dead socket, reconnect, resend. Single
                // retry — if it fails again, let it propagate so health/status reflect reality.
                await TearDownAsync().ConfigureAwait(false);
                await ConnectAsync(ct).ConfigureAwait(false);
                return await SendOnceAsync(unitId, pdu, ct).ConfigureAwait(false);
            }
        }
        finally
        {
            _gate.Release();
        }
    }

    private async Task<byte[]> SendOnceAsync(byte unitId, byte[] pdu, CancellationToken ct)
    {
        if (_stream is null) throw new InvalidOperationException("Transport not connected");
        var txId = ++_nextTx;

        // MBAP: [TxId(2)][Proto=0(2)][Length(2)][UnitId(1)] + PDU
        var adu = new byte[7 + pdu.Length];
        adu[0] = (byte)(txId >> 8);
        adu[1] = (byte)(txId & 0xFF);
        // protocol id already zero
        var len = (ushort)(1 + pdu.Length); // unit id + pdu
        adu[4] = (byte)(len >> 8);
        adu[5] = (byte)(len & 0xFF);
        adu[6] = unitId;
        Buffer.BlockCopy(pdu, 0, adu, 7, pdu.Length);

        using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
        cts.CancelAfter(_timeout);
        await _stream.WriteAsync(adu.AsMemory(), cts.Token).ConfigureAwait(false);
        await _stream.FlushAsync(cts.Token).ConfigureAwait(false);

        var header = new byte[7];
        await ReadExactlyAsync(_stream, header, cts.Token).ConfigureAwait(false);
        var respTxId = (ushort)((header[0] << 8) | header[1]);
        if (respTxId != txId)
            throw new InvalidDataException($"Modbus TxId mismatch: expected {txId} got {respTxId}");
        var respLen = (ushort)((header[4] << 8) | header[5]);
        if (respLen < 1) throw new InvalidDataException($"Modbus response length too small: {respLen}");
        var respPdu = new byte[respLen - 1];
        await ReadExactlyAsync(_stream, respPdu, cts.Token).ConfigureAwait(false);

        // Exception PDU: function code has high bit set.
        if ((respPdu[0] & 0x80) != 0)
        {
            var fc = (byte)(respPdu[0] & 0x7F);
            var ex = respPdu[1];
            throw new ModbusException(fc, ex, $"Modbus exception fc={fc} code={ex}");
        }

        return respPdu;
    }

    /// <summary>
    ///     Distinguish socket-layer failures (eligible for reconnect-and-retry) from
    ///     protocol-layer failures (must propagate — retrying the same PDU won't help if the
    ///     PLC just returned exception 02 Illegal Data Address).
    /// </summary>
    private static bool IsSocketLevelFailure(Exception ex) =>
        ex is EndOfStreamException
        || ex is IOException
        || ex is SocketException
        || ex is ObjectDisposedException;

    private async Task TearDownAsync()
    {
        try { if (_stream is not null) await _stream.DisposeAsync().ConfigureAwait(false); }
        catch { /* best-effort */ }
        _stream = null;
        try { _client?.Dispose(); } catch { }
        _client = null;
    }

    private static async Task ReadExactlyAsync(Stream s, byte[] buf, CancellationToken ct)
    {
        var read = 0;
        while (read < buf.Length)
        {
            var n = await s.ReadAsync(buf.AsMemory(read), ct).ConfigureAwait(false);
            if (n == 0) throw new EndOfStreamException("Modbus socket closed mid-response");
            read += n;
        }
    }

    public async ValueTask DisposeAsync()
    {
        if (_disposed) return;
        _disposed = true;
        try
        {
            if (_stream is not null) await _stream.DisposeAsync().ConfigureAwait(false);
        }
        catch { /* best-effort */ }
        _client?.Dispose();
        _gate.Dispose();
    }
}