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>
public sealed class ModbusTcpTransport : IModbusTransport
{
    private readonly string _host;
    private readonly int _port;
    private readonly TimeSpan _timeout;
    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)
    {
        _host = host;
        _port = port;
        _timeout = timeout;
    }

    public async Task ConnectAsync(CancellationToken ct)
    {
        _client = new TcpClient();
        using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
        cts.CancelAfter(_timeout);
        await _client.ConnectAsync(_host, _port, cts.Token).ConfigureAwait(false);
        _stream = _client.GetStream();
    }

    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
        {
            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;
        }
        finally
        {
            _gate.Release();
        }
    }

    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();
    }
}