using System.Runtime.InteropServices;
using System.Text.Json;

namespace NucleicProtocol.Interop;

/// Managed lifetime and callback barrier over one `np_handle`.
public interface IProtocolClient : IDisposable
{
    event Action<string>? MessageJsonReceived;
    event Action<string>? StateJsonReceived;
    ProtocolResult ConnectLocal(HostdRendezvous rendezvous);
    ProtocolResult ConnectRemote(string endpointJson);
    ProtocolResult Pair(string pairingCode);
    ProtocolResult SendIntent(string clientMessageJson);
}

public sealed class ProtocolClient : IProtocolClient
{
    private static readonly NativeProtocolCallback EventThunk = OnNativeEvent;
    private static readonly NativeProtocolCallback StateThunk = OnNativeState;

    private readonly object gate = new();
    private readonly INativeProtocol native;
    private nint handle;
    private GCHandle callbackContext;
    private bool disposed;

    public event Action<string>? MessageJsonReceived;
    public event Action<string>? StateJsonReceived;

    public ProtocolClient(
        ProtocolClientConfiguration configuration, INativeProtocol? native = null)
    {
        this.native = native ?? PInvokeNativeProtocol.Instance;
        var json = JsonSerializer.Serialize(
            configuration, ProtocolJsonContext.Default.ProtocolClientConfiguration);
        handle = this.native.ClientCreate(json);
        if (handle == 0) throw new InvalidOperationException("NucleicProtocolC rejected client configuration");

        callbackContext = GCHandle.Alloc(this, GCHandleType.Normal);
        try
        {
            this.native.ClientSetCallbacks(
                handle, EventThunk, StateThunk, GCHandle.ToIntPtr(callbackContext));
        }
        catch
        {
            this.native.ClientClose(handle);
            handle = 0;
            callbackContext.Free();
            throw;
        }
    }

    public ProtocolResult ConnectLocal(HostdRendezvous rendezvous)
    {
        rendezvous.Validate();
        return Invoke(handle => native.ClientConnectLocal(
            handle, JsonSerializer.Serialize(rendezvous, ProtocolJsonContext.Default.HostdRendezvous)));
    }

    public ProtocolResult ConnectRemote(string endpointJson) =>
        Invoke(handle => native.ClientConnectRemote(handle, endpointJson));

    public ProtocolResult Pair(string pairingCode) =>
        Invoke(handle => native.ClientPair(handle, pairingCode));

    public ProtocolResult SendIntent(string clientMessageJson)
    {
        // Fail malformed UI output before it reaches the ABI. The Swift decoder remains the
        // authority for the actual ClientMsg shape and returns DecodeError for a valid-but-wrong
        // document.
        using (JsonDocument.Parse(clientMessageJson)) { }
        return Invoke(handle => native.ClientSendIntent(handle, clientMessageJson));
    }

    private ProtocolResult Invoke(Func<nint, int> operation)
    {
        lock (gate)
        {
            ObjectDisposedException.ThrowIf(disposed, this);
            return (ProtocolResult)operation(handle);
        }
    }

    public void Dispose()
    {
        nint closing;
        lock (gate)
        {
            if (disposed) return;
            disposed = true;
            closing = handle;
            handle = 0;
        }

        // The Swift contract makes close a callback barrier. Only after it returns is it safe to
        // free the GCHandle that native callbacks carry as their opaque context.
        native.ClientClose(closing);
        MessageJsonReceived = null;
        StateJsonReceived = null;
        if (callbackContext.IsAllocated) callbackContext.Free();
    }

    private static void OnNativeEvent(nint context, nint utf8Json) =>
        Dispatch(context, utf8Json, static (client, json) => client.MessageJsonReceived?.Invoke(json));

    private static void OnNativeState(nint context, nint utf8Json) =>
        Dispatch(context, utf8Json, static (client, json) => client.StateJsonReceived?.Invoke(json));

    private static void Dispatch(
        nint context, nint utf8Json, Action<ProtocolClient, string> deliver)
    {
        if (context == 0 || utf8Json == 0) return;
        try
        {
            if (GCHandle.FromIntPtr(context).Target is ProtocolClient client)
            {
                var json = Marshal.PtrToStringUTF8(utf8Json);
                if (json is not null) deliver(client, json);
            }
        }
        catch
        {
            // Nothing may unwind through the C callback boundary. Renderer event subscribers own
            // their diagnostics; a bad observer must not tear down Swift's callback thread.
        }
    }
}

public interface IProtocolClientFactory
{
    IProtocolClient Create(ProtocolClientConfiguration configuration);
}

public sealed class ProtocolClientFactory : IProtocolClientFactory
{
    public IProtocolClient Create(ProtocolClientConfiguration configuration) => new ProtocolClient(configuration);
}