using System.Net.Sockets;

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

/// <summary>
///     Owns the raw TCP socket lifecycle shared by every Modbus-over-TCP transport (MBAP and
///     RTU-over-TCP alike): the IPv4-preference connect, OS-level keep-alive, geometric-backoff
///     reconnect, idle-disconnect tracking, and teardown. It exposes the current
///     <see cref="NetworkStream"/> so the composing transport can drive its own framing on top —
///     the lifecycle knows nothing about MBAP / RTU wire layout.
/// </summary>
/// <remarks>
///     <para>
///         Extracted verbatim from <see cref="ModbusTcpTransport"/> so the connect / reconnect /
///         keep-alive / idle behaviour is written once and composed by both transports. The
///         constructor is connection-free — all socket I/O happens in <see cref="ConnectAsync"/> /
///         <see cref="ConnectWithBackoffAsync"/>.
///     </para>
///     <para>
///         Why keep-alive 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.
///         Enabling OS-level <c>SO_KEEPALIVE</c> lets the driver's own side detect a stuck socket
///         in reasonable time even when the application is mostly idle.
///     </para>
/// </remarks>
public sealed class ModbusSocketLifecycle : IAsyncDisposable
{
    private readonly string _host;
    private readonly int _port;
    private readonly TimeSpan _timeout;
    private readonly bool _autoReconnect;
    private readonly ModbusKeepAliveOptions _keepAlive;
    private readonly TimeSpan? _idleDisconnect;
    private readonly ModbusReconnectOptions _reconnect;
    private TcpClient? _client;
    private NetworkStream? _stream;
    private DateTime _lastSuccessUtc = DateTime.UtcNow;

    /// <summary>Initializes a new instance of the <see cref="ModbusSocketLifecycle"/> 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 ModbusSocketLifecycle(
        string host, int port, TimeSpan timeout, bool autoReconnect = true,
        ModbusKeepAliveOptions? keepAlive = null,
        TimeSpan? idleDisconnect = null,
        ModbusReconnectOptions? reconnect = null)
    {
        _host = host;
        _port = port;
        _timeout = timeout;
        _autoReconnect = autoReconnect;
        _keepAlive = keepAlive ?? new ModbusKeepAliveOptions();
        _idleDisconnect = idleDisconnect;
        _reconnect = reconnect ?? new ModbusReconnectOptions();
    }

    /// <summary>Gets the current live <see cref="NetworkStream"/>, or <see langword="null"/> when not connected.</summary>
    public NetworkStream? Stream => _stream;

    /// <summary>Gets a value indicating whether auto-reconnect on socket failures is enabled.</summary>
    public bool AutoReconnect => _autoReconnect;

    /// <summary>Establishes a connection to the Modbus server, preferring IPv4.</summary>
    /// <param name="ct">A cancellation token to observe for cancellation.</param>
    /// <returns>A task representing the asynchronous connection operation.</returns>
    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, _keepAlive);

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

    /// <summary>
    ///     Connect attempt with the configured geometric backoff. The first attempt fires after
    ///     <see cref="ModbusReconnectOptions.InitialDelay"/> (default zero — immediate); each
    ///     subsequent attempt sleeps for the previous delay times <c>BackoffMultiplier</c>,
    ///     capped at <c>MaxDelay</c>. Caller's cancellation token aborts the loop.
    /// </summary>
    /// <param name="ct">A cancellation token to observe for cancellation.</param>
    /// <returns>A task representing the asynchronous reconnect operation.</returns>
    public async Task ConnectWithBackoffAsync(CancellationToken ct)
    {
        var delay = _reconnect.InitialDelay;
        var attempt = 0;
        while (true)
        {
            if (delay > TimeSpan.Zero)
                await Task.Delay(delay, ct).ConfigureAwait(false);
            try
            {
                await ConnectAsync(ct).ConfigureAwait(false);
                return;
            }
            catch (Exception ex) when (IsSocketLevelFailure(ex) && _autoReconnect)
            {
                attempt++;
                // Geometric growth, capped. Use Math.Min on ticks so we don't overflow with
                // pathological multipliers / long deployments.
                var nextTicks = (long)(Math.Max(delay.Ticks, TimeSpan.FromMilliseconds(100).Ticks) * _reconnect.BackoffMultiplier);
                delay = TimeSpan.FromTicks(Math.Min(nextTicks, _reconnect.MaxDelay.Ticks));
                if (attempt >= 10)
                {
                    // Bail after 10 attempts to surface persistent failure to the caller. With
                    // the default backoff (1s base, 2.0x mult, 30s cap) this is roughly 4 minutes
                    // of attempts; with InitialDelay=0 it's immediate up to the same cap.
                    throw;
                }
            }
        }
    }

    /// <summary>
    ///     Reports whether the socket has been idle longer than the configured idle-disconnect
    ///     threshold and should be proactively torn down + reconnected before the next transaction.
    ///     Always <see langword="false"/> when no idle-disconnect timeout is configured.
    /// </summary>
    /// <returns><see langword="true"/> when the idle threshold has been exceeded.</returns>
    public bool ShouldReconnectForIdle() =>
        _idleDisconnect.HasValue && DateTime.UtcNow - _lastSuccessUtc > _idleDisconnect.Value;

    /// <summary>Records that a transaction just succeeded, resetting the idle-disconnect clock.</summary>
    public void MarkSuccess() => _lastSuccessUtc = DateTime.UtcNow;

    /// <summary>Tears down the current socket + stream, leaving the lifecycle ready to reconnect.</summary>
    /// <returns>A task representing the asynchronous teardown operation.</returns>
    public 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;
    }

    /// <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, ModbusKeepAliveOptions opts)
    {
        if (!opts.Enabled) return;
        try
        {
            client.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true);
            // A TimeSpan < 1s previously truncated to 0 via the int cast,
            // which Windows / Linux interpret as "use the default" — silently defeating the
            // configured keep-alive timing. Round up to at least 1 second so a sub-second
            // configuration still produces a real keep-alive cadence. Negative values are
            // also clamped to 1 to avoid surfacing as OS errors.
            client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveTime,
                ClampToWholeSeconds(opts.Time));
            client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveInterval,
                ClampToWholeSeconds(opts.Interval));
            client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveRetryCount, opts.RetryCount);
        }
        catch { /* best-effort; older OSes may not expose the granular knobs */ }
    }

    /// <summary>
    ///     Cast a <see cref="TimeSpan"/> to a whole number of seconds with a
    ///     minimum of 1 — protects callers from the int-cast truncation that turned 500 ms
    ///     keep-alive timing into "use the default" on most OSes.
    /// </summary>
    /// <param name="ts">The timespan to clamp to whole seconds.</param>
    /// <returns>The clamped duration expressed as a whole number of seconds, never less than 1.</returns>
    internal static int ClampToWholeSeconds(TimeSpan ts)
    {
        var seconds = (int)Math.Ceiling(ts.TotalSeconds);
        return seconds < 1 ? 1 : seconds;
    }

    /// <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>
    /// <param name="ex">The exception to classify.</param>
    /// <returns><see langword="true"/> when the exception is a socket-level failure.</returns>
    internal static bool IsSocketLevelFailure(Exception ex) =>
        ex is EndOfStreamException
        || ex is IOException
        || ex is SocketException
        || ex is ObjectDisposedException;

    /// <summary>Asynchronously disposes the underlying socket + stream resources.</summary>
    /// <returns>A task that represents the asynchronous operation.</returns>
    public async ValueTask DisposeAsync()
    {
        try
        {
            if (_stream is not null) await _stream.DisposeAsync().ConfigureAwait(false);
        }
        catch { /* best-effort */ }
        _client?.Dispose();
    }
}