using System.Net.Sockets;

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

/// <summary>
///     Concrete Modbus TCP transport. Wraps a single socket (owned by <see cref="ModbusSocketLifecycle"/>)
///     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>
///         Owns the MBAP framing (<see cref="SendOnceAsync"/>: 7-byte header + transaction-id
///         pairing) and composes <see cref="ModbusSocketLifecycle"/> for the connect / reconnect /
///         keep-alive / idle-disconnect machinery — the same lifecycle the RTU-over-TCP transport
///         reuses with different framing.
///     </para>
///     <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 ModbusSocketLifecycle _life;
    private readonly TimeSpan _timeout;
    private readonly SemaphoreSlim _gate = new(1, 1);
    private ushort _nextTx;
    private bool _disposed;

    /// <summary>Initializes a new instance of the <see cref="ModbusTcpTransport"/> class.</summary>
    /// <param name="host">The host address or hostname of the Modbus server.</param>
    /// <param name="port">The TCP port of the Modbus server.</param>
    /// <param name="timeout">The timeout for socket operations.</param>
    /// <param name="autoReconnect">Whether to automatically reconnect on socket failures.</param>
    /// <param name="keepAlive">Optional keep-alive configuration for the socket.</param>
    /// <param name="idleDisconnect">Optional duration after which an idle socket is disconnected.</param>
    /// <param name="reconnect">Optional reconnect backoff configuration.</param>
    public ModbusTcpTransport(
        string host, int port, TimeSpan timeout, bool autoReconnect = true,
        ModbusKeepAliveOptions? keepAlive = null,
        TimeSpan? idleDisconnect = null,
        ModbusReconnectOptions? reconnect = null)
    {
        _timeout = timeout;
        _life = new ModbusSocketLifecycle(host, port, timeout, autoReconnect, keepAlive, idleDisconnect, reconnect);
    }

    /// <inheritdoc />
    public Task ConnectAsync(CancellationToken ct) => _life.ConnectAsync(ct);

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

        await _gate.WaitAsync(ct).ConfigureAwait(false);
        try
        {
            // Proactive idle-disconnect: if the socket has been quiet longer than the configured
            // threshold, tear it down + reconnect before this PDU lands. Defends against silent
            // NAT / firewall reaping where the socket looks alive locally but the upstream side
            // dropped it minutes ago.
            if (_life.ShouldReconnectForIdle())
            {
                await _life.TearDownAsync().ConfigureAwait(false);
                await _life.ConnectWithBackoffAsync(ct).ConfigureAwait(false);
            }

            try
            {
                var result = await SendOnceAsync(unitId, pdu, ct).ConfigureAwait(false);
                _life.MarkSuccess();
                return result;
            }
            catch (Exception ex) when (_life.AutoReconnect && ModbusSocketLifecycle.IsSocketLevelFailure(ex))
            {
                // Mid-transaction drop: tear down the dead socket, reconnect (with backoff if
                // configured), resend. Single retry — if it fails again, let it propagate so
                // health/status reflect reality.
                await _life.TearDownAsync().ConfigureAwait(false);
                await _life.ConnectWithBackoffAsync(ct).ConfigureAwait(false);
                var result = await SendOnceAsync(unitId, pdu, ct).ConfigureAwait(false);
                _life.MarkSuccess();
                return result;
            }
        }
        finally
        {
            _gate.Release();
        }
    }

    /// <summary>
    ///     Executes exactly one Modbus transaction on the current socket.
    /// </summary>
    /// <remarks>
    ///     <para>
    ///         A per-op deadline (linked <see cref="CancellationTokenSource.CancelAfter(TimeSpan)"/>)
    ///         and framing violations (TxId mismatch / truncated MBAP length) are treated as
    ///         <b>connection-fatal</b>: the socket may have a half-read or still-in-flight response,
    ///         so reusing it would read stale bytes and desynchronize the stream permanently. Such
    ///         failures tear the socket down here (before propagating) and surface as
    ///         <see cref="ModbusTransportDesyncException"/> — which, being an
    ///         <see cref="IOException"/>, feeds <see cref="SendAsync"/>'s existing single reconnect
    ///         retry with no extra plumbing.
    ///     </para>
    ///     <para>
    ///         The timeout-vs-caller-cancel distinction is load-bearing: a <b>caller</b> cancellation
    ///         (<c>ct.IsCancellationRequested</c>) is a legitimate shutdown and propagates as a plain
    ///         <see cref="OperationCanceledException"/> with <b>no</b> teardown — only the linked
    ///         per-op timeout (the caller token is NOT cancelled) is a desync.
    ///     </para>
    /// </remarks>
    private async Task<byte[]> SendOnceAsync(byte unitId, byte[] pdu, CancellationToken ct)
    {
        var stream = _life.Stream;
        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);
        try
        {
            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 ModbusTransportDesyncException(
                    ModbusTransportDesyncException.DesyncReason.TxIdMismatch,
                    $"Modbus TxId mismatch: expected {txId} got {respTxId}");
            var respLen = (ushort)((header[4] << 8) | header[5]);
            if (respLen < 1)
                throw new ModbusTransportDesyncException(
                    ModbusTransportDesyncException.DesyncReason.TruncatedFrame,
                    $"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. This is a well-formed protocol-level
            // error — the socket is still coherent, so it MUST propagate without teardown.
            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;
        }
        catch (ModbusTransportDesyncException)
        {
            // Framing violation: the socket is desynchronized — never reuse it.
            await _life.TearDownAsync().ConfigureAwait(false);
            throw;
        }
        catch (OperationCanceledException) when (!ct.IsCancellationRequested)
        {
            // Per-op timeout fired (NOT the caller). The response is unknown / may still land later
            // and corrupt the next read — tear the socket down and normalize as a desync so the
            // reconnect retry / status mapping engages.
            await _life.TearDownAsync().ConfigureAwait(false);
            throw new ModbusTransportDesyncException(
                ModbusTransportDesyncException.DesyncReason.Timeout,
                $"Modbus per-op response timeout after {_timeout.TotalMilliseconds:0}ms");
        }
    }

    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;
        }
    }

    /// <summary>Asynchronously disposes the transport and underlying socket resources.</summary>
    /// <returns>A task that represents the asynchronous operation.</returns>
    public async ValueTask DisposeAsync()
    {
        if (_disposed) return;
        _disposed = true;
        await _life.DisposeAsync().ConfigureAwait(false);
        _gate.Dispose();
    }
}