using S7.Net;
using ZB.MOM.WW.OtOpcUa.Core.Abstractions;

namespace ZB.MOM.WW.OtOpcUa.Driver.S7;

/// <summary>
///     Siemens S7 native driver — speaks S7comm over ISO-on-TCP (port 102) via the S7netplus
///     library. First implementation of <see cref="IDriver"/> for an in-process .NET Standard
///     PLC protocol that is NOT Modbus, validating that the v2 driver-capability interfaces
///     generalize beyond Modbus + Galaxy.
/// </summary>
/// <remarks>
///     <para>
///         PR 62 ships the scaffold: <see cref="IDriver"/> only (Initialize / Reinitialize /
///         Shutdown / GetHealth). <see cref="ITagDiscovery"/>, <see cref="IReadable"/>,
///         <see cref="IWritable"/>, <see cref="ISubscribable"/>, <see cref="IHostConnectivityProbe"/>
///         land in PRs 63-65 once the address parser (PR 63) is in place.
///     </para>
///     <para>
///         <b>Single-connection policy</b>: S7netplus documented pattern is one
///         <c>Plc</c> instance per PLC, serialized with a <see cref="SemaphoreSlim"/>.
///         Parallelising reads against a single S7 CPU doesn't help — the CPU scans the
///         communication mailbox at most once per cycle (2-10 ms) and queues concurrent
///         requests wire-side anyway. Multiple client-side connections just waste the CPU's
///         8-64 connection-resource budget.
///     </para>
/// </remarks>
public sealed class S7Driver(S7DriverOptions options, string driverInstanceId)
    : IDriver, IDisposable, IAsyncDisposable
{
    private readonly S7DriverOptions _options = options;
    private readonly SemaphoreSlim _gate = new(1, 1);

    /// <summary>
    ///     Per-connection gate. Internal so PRs 63-65 (read/write/subscribe) can serialize on
    ///     the same semaphore without exposing it publicly. Single-connection-per-PLC is a
    ///     hard requirement of S7netplus — see class remarks.
    /// </summary>
    internal SemaphoreSlim Gate => _gate;

    /// <summary>
    ///     Active S7.Net PLC connection. Null until <see cref="InitializeAsync"/> returns; null
    ///     after <see cref="ShutdownAsync"/>. Read-only outside this class; PR 64's Read/Write
    ///     will take the <see cref="_gate"/> before touching it.
    /// </summary>
    internal Plc? Plc { get; private set; }

    private DriverHealth _health = new(DriverState.Unknown, null, null);
    private bool _disposed;

    public string DriverInstanceId => driverInstanceId;
    public string DriverType => "S7";

    public async Task InitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
    {
        _health = new DriverHealth(DriverState.Initializing, null, null);
        try
        {
            var plc = new Plc(_options.CpuType, _options.Host, _options.Rack, _options.Slot);
            // S7netplus writes timeouts into the underlying TcpClient via Plc.WriteTimeout /
            // Plc.ReadTimeout (milliseconds). Set before OpenAsync so the handshake itself
            // honours the bound.
            plc.WriteTimeout = (int)_options.Timeout.TotalMilliseconds;
            plc.ReadTimeout = (int)_options.Timeout.TotalMilliseconds;

            using var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
            cts.CancelAfter(_options.Timeout);
            await plc.OpenAsync(cts.Token).ConfigureAwait(false);

            Plc = plc;
            _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
        }
        catch (Exception ex)
        {
            // Clean up a partially-constructed Plc so a retry from the caller doesn't leak
            // the TcpClient. S7netplus's Close() is best-effort and idempotent.
            try { Plc?.Close(); } catch { }
            Plc = null;
            _health = new DriverHealth(DriverState.Faulted, null, ex.Message);
            throw;
        }
    }

    public async Task ReinitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
    {
        await ShutdownAsync(cancellationToken).ConfigureAwait(false);
        await InitializeAsync(driverConfigJson, cancellationToken).ConfigureAwait(false);
    }

    public Task ShutdownAsync(CancellationToken cancellationToken)
    {
        try { Plc?.Close(); } catch { /* best-effort — tearing down anyway */ }
        Plc = null;
        _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
        return Task.CompletedTask;
    }

    public DriverHealth GetHealth() => _health;

    /// <summary>
    ///     Approximate memory footprint. The Plc instance + one 240-960 byte PDU buffer is
    ///     under 4 KB; return 0 because the <see cref="IDriver"/> contract asks for a
    ///     driver-attributable growth number and S7.Net doesn't expose one.
    /// </summary>
    public long GetMemoryFootprint() => 0;

    public Task FlushOptionalCachesAsync(CancellationToken cancellationToken) => Task.CompletedTask;

    public void Dispose() => DisposeAsync().AsTask().GetAwaiter().GetResult();

    public async ValueTask DisposeAsync()
    {
        if (_disposed) return;
        _disposed = true;
        try { await ShutdownAsync(CancellationToken.None).ConfigureAwait(false); }
        catch { /* disposal is best-effort */ }
        _gate.Dispose();
    }
}