using System.Security.Cryptography;
using Google.Protobuf.WellKnownTypes;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Logging.Abstractions;
using Microsoft.Extensions.Options;
using MxGateway.Contracts;
using MxGateway.Contracts.Proto;
using MxGateway.Server.Configuration;
using MxGateway.Server.Metrics;
using MxGateway.Server.Workers;

namespace MxGateway.Server.Sessions;

public sealed class SessionManager : ISessionManager
{
    public const string DefaultCloseReason = "client-close";
    public const string GatewayShutdownReason = "gateway-shutdown";
    public const string LeaseExpiredReason = "lease-expired";

    private readonly ISessionRegistry _registry;
    private readonly ISessionWorkerClientFactory _workerClientFactory;
    private readonly GatewayMetrics _metrics;
    private readonly TimeProvider _timeProvider;
    private readonly ILogger<SessionManager> _logger;
    private readonly GatewayOptions _options;

    public SessionManager(
        ISessionRegistry registry,
        ISessionWorkerClientFactory workerClientFactory,
        IOptions<GatewayOptions> options,
        GatewayMetrics metrics,
        TimeProvider? timeProvider = null,
        ILogger<SessionManager>? logger = null)
    {
        _registry = registry ?? throw new ArgumentNullException(nameof(registry));
        _workerClientFactory = workerClientFactory ?? throw new ArgumentNullException(nameof(workerClientFactory));
        ArgumentNullException.ThrowIfNull(options);
        _metrics = metrics ?? throw new ArgumentNullException(nameof(metrics));
        _timeProvider = timeProvider ?? TimeProvider.System;
        _logger = logger ?? NullLogger<SessionManager>.Instance;
        _options = options.Value;
    }

    public async Task<GatewaySession> OpenSessionAsync(
        SessionOpenRequest request,
        string? clientIdentity,
        CancellationToken cancellationToken)
    {
        ArgumentNullException.ThrowIfNull(request);
        EnsureSessionCapacity();

        GatewaySession session = CreateSession(request, clientIdentity);
        if (!_registry.TryAdd(session))
        {
            throw new SessionManagerException(
                SessionManagerErrorCode.OpenFailed,
                $"Session id collision while opening session {session.SessionId}.");
        }

        try
        {
            session.TransitionTo(SessionState.StartingWorker);
            IWorkerClient workerClient = await _workerClientFactory
                .CreateAsync(session, cancellationToken)
                .ConfigureAwait(false);

            session.AttachWorkerClient(workerClient);
            session.MarkReady();
            _metrics.SessionOpened();

            return session;
        }
        catch (Exception exception)
        {
            session.MarkFaulted(exception.Message);
            _registry.TryRemove(session.SessionId, out _);
            await session.DisposeAsync().ConfigureAwait(false);
            _metrics.Fault(SessionManagerErrorCode.OpenFailed.ToString());
            _logger.LogWarning(
                exception,
                "Failed to open gateway session {SessionId}.",
                session.SessionId);

            throw new SessionManagerException(
                SessionManagerErrorCode.OpenFailed,
                $"Failed to open session {session.SessionId}.",
                exception);
        }
    }

    public bool TryGetSession(
        string sessionId,
        out GatewaySession session)
    {
        return _registry.TryGet(sessionId, out session);
    }

    public async Task<WorkerCommandReply> InvokeAsync(
        string sessionId,
        WorkerCommand command,
        CancellationToken cancellationToken)
    {
        GatewaySession session = GetRequiredSession(sessionId);

        try
        {
            return await session.InvokeAsync(command, cancellationToken).ConfigureAwait(false);
        }
        catch (SessionManagerException)
        {
            throw;
        }
        catch (Exception exception)
        {
            if (session.WorkerClient?.State == WorkerClientState.Faulted)
            {
                session.MarkFaulted(exception.Message);
            }

            throw;
        }
    }

    public IAsyncEnumerable<WorkerEvent> ReadEventsAsync(
        string sessionId,
        CancellationToken cancellationToken)
    {
        GatewaySession session = GetRequiredSession(sessionId);

        return session.ReadEventsAsync(cancellationToken);
    }

    public async Task<SessionCloseResult> CloseSessionAsync(
        string sessionId,
        CancellationToken cancellationToken)
    {
        GatewaySession session = GetRequiredSession(sessionId);
        SessionCloseResult result = await CloseSessionCoreAsync(
            session,
            DefaultCloseReason,
            cancellationToken).ConfigureAwait(false);

        return result;
    }

    public async Task<int> CloseExpiredLeasesAsync(
        DateTimeOffset now,
        CancellationToken cancellationToken)
    {
        int closedCount = 0;
        foreach (GatewaySession session in _registry.Snapshot())
        {
            if (!session.IsLeaseExpired(now))
            {
                continue;
            }

            await CloseSessionCoreAsync(session, LeaseExpiredReason, cancellationToken).ConfigureAwait(false);
            closedCount++;
        }

        return closedCount;
    }

    public async Task ShutdownAsync(CancellationToken cancellationToken)
    {
        foreach (GatewaySession session in _registry.Snapshot())
        {
            try
            {
                await CloseSessionCoreAsync(session, GatewayShutdownReason, cancellationToken).ConfigureAwait(false);
            }
            catch (Exception exception)
            {
                _logger.LogWarning(
                    exception,
                    "Graceful shutdown failed for session {SessionId}; killing worker.",
                    session.SessionId);
                session.KillWorker(GatewayShutdownReason);
            }
        }
    }

    private async Task<SessionCloseResult> CloseSessionCoreAsync(
        GatewaySession session,
        string reason,
        CancellationToken cancellationToken)
    {
        bool wasClosed = session.State == SessionState.Closed;
        try
        {
            SessionCloseResult result = await session.CloseAsync(reason, cancellationToken).ConfigureAwait(false);
            if (!wasClosed && !result.AlreadyClosed)
            {
                _metrics.SessionClosed();
            }

            return result;
        }
        catch (Exception exception)
        {
            session.MarkFaulted(exception.Message);
            _metrics.Fault(SessionManagerErrorCode.CloseFailed.ToString());
            throw new SessionManagerException(
                SessionManagerErrorCode.CloseFailed,
                $"Failed to close session {session.SessionId}.",
                exception);
        }
    }

    private GatewaySession GetRequiredSession(string sessionId)
    {
        if (!_registry.TryGet(sessionId, out GatewaySession session))
        {
            throw new SessionManagerException(
                SessionManagerErrorCode.SessionNotFound,
                $"Session {sessionId} was not found.");
        }

        return session;
    }

    private void EnsureSessionCapacity()
    {
        if (_registry.ActiveCount >= _options.Sessions.MaxSessions)
        {
            throw new SessionManagerException(
                SessionManagerErrorCode.SessionLimitExceeded,
                $"Gateway session limit {_options.Sessions.MaxSessions} has been reached.");
        }
    }

    private GatewaySession CreateSession(
        SessionOpenRequest request,
        string? clientIdentity)
    {
        string sessionId = CreateSessionId();
        string backendName = string.IsNullOrWhiteSpace(request.RequestedBackend)
            ? GatewayContractInfo.DefaultBackendName
            : request.RequestedBackend!;
        TimeSpan commandTimeout = ResolveCommandTimeout(request.CommandTimeout);
        TimeSpan startupTimeout = TimeSpan.FromSeconds(_options.Worker.StartupTimeoutSeconds);
        TimeSpan shutdownTimeout = TimeSpan.FromSeconds(_options.Worker.ShutdownTimeoutSeconds);
        string pipeName = $"mxaccess-gateway-{Environment.ProcessId}-{sessionId}";
        string nonce = CreateNonce();
        DateTimeOffset openedAt = _timeProvider.GetUtcNow();

        return new GatewaySession(
            sessionId,
            backendName,
            pipeName,
            nonce,
            clientIdentity,
            request.ClientSessionName,
            request.ClientCorrelationId,
            commandTimeout,
            startupTimeout,
            shutdownTimeout,
            openedAt);
    }

    private TimeSpan ResolveCommandTimeout(Duration? requestedTimeout)
    {
        if (requestedTimeout is null)
        {
            return TimeSpan.FromSeconds(_options.Sessions.DefaultCommandTimeoutSeconds);
        }

        TimeSpan timeout = requestedTimeout.ToTimeSpan();
        return timeout <= TimeSpan.Zero
            ? TimeSpan.FromSeconds(_options.Sessions.DefaultCommandTimeoutSeconds)
            : timeout;
    }

    private static string CreateSessionId()
    {
        return $"session-{Guid.NewGuid():N}";
    }

    private static string CreateNonce()
    {
        Span<byte> bytes = stackalloc byte[32];
        RandomNumberGenerator.Fill(bytes);

        return Convert.ToBase64String(bytes);
    }
}