using System;
using System.Collections.Generic;
using System.Globalization;
using System.Threading;

using Renci.SshNet.Common;
using Renci.SshNet.Messages.Connection;

namespace Renci.SshNet.Channels
{
    /// <summary>
    /// Implements Session SSH channel.
    /// </summary>
    internal sealed class ChannelSession : ClientChannel, IChannelSession
    {
        /// <summary>
        /// Counts failed channel open attempts.
        /// </summary>
        private int _failedOpenAttempts;

        /// <summary>
        /// Holds a value indicating whether the session semaphore has been obtained by the current
        /// channel.
        /// </summary>
        /// <value>
        /// <c>0</c> when the session semaphore has not been obtained or has already been released,
        /// and <c>1</c> when the session has been obtained and still needs to be released.
        /// </value>
        private int _sessionSemaphoreObtained;

        /// <summary>
        /// Wait handle to signal when response was received to open the channel.
        /// </summary>
        private EventWaitHandle _channelOpenResponseWaitHandle = new AutoResetEvent(initialState: false);

        private EventWaitHandle _channelRequestResponse = new ManualResetEvent(initialState: false);

        private bool _channelRequestSucces;

        /// <summary>
        /// Initializes a new instance of the <see cref="ChannelSession"/> class.
        /// </summary>
        /// <param name="session">The session.</param>
        /// <param name="localChannelNumber">The local channel number.</param>
        /// <param name="localWindowSize">Size of the window.</param>
        /// <param name="localPacketSize">Size of the packet.</param>
        public ChannelSession(ISession session, uint localChannelNumber, uint localWindowSize, uint localPacketSize)
            : base(session, localChannelNumber, localWindowSize, localPacketSize)
        {
        }

        /// <summary>
        /// Gets the type of the channel.
        /// </summary>
        /// <value>
        /// The type of the channel.
        /// </value>
        public override ChannelTypes ChannelType
        {
            get { return ChannelTypes.Session; }
        }

        /// <summary>
        /// Opens the channel.
        /// </summary>
        public void Open()
        {
            // Try to open channel several times
            while (!IsOpen && _failedOpenAttempts < ConnectionInfo.RetryAttempts)
            {
                SendChannelOpenMessage();
                try
                {
                    WaitOnHandle(_channelOpenResponseWaitHandle);
                }
                catch (Exception)
                {
                    // avoid leaking session semaphore
                    ReleaseSemaphore();
                    throw;
                }
            }

            if (!IsOpen)
            {
                throw new SshException(string.Format(CultureInfo.CurrentCulture, "Failed to open a channel after {0} attempts.", _failedOpenAttempts));
            }
        }

        /// <summary>
        /// Called when channel is opened by the server.
        /// </summary>
        /// <param name="remoteChannelNumber">The remote channel number.</param>
        /// <param name="initialWindowSize">Initial size of the window.</param>
        /// <param name="maximumPacketSize">Maximum size of the packet.</param>
        protected override void OnOpenConfirmation(uint remoteChannelNumber, uint initialWindowSize, uint maximumPacketSize)
        {
            base.OnOpenConfirmation(remoteChannelNumber, initialWindowSize, maximumPacketSize);

            _ = _channelOpenResponseWaitHandle.Set();
        }

        /// <summary>
        /// Called when channel failed to open.
        /// </summary>
        /// <param name="reasonCode">The reason code.</param>
        /// <param name="description">The description.</param>
        /// <param name="language">The language.</param>
        protected override void OnOpenFailure(uint reasonCode, string description, string language)
        {
            _failedOpenAttempts++;
            ReleaseSemaphore();
            _ = _channelOpenResponseWaitHandle.Set();
        }

        protected override void Close()
        {
            base.Close();
            ReleaseSemaphore();
        }

        /// <summary>
        /// Sends the pseudo terminal request.
        /// </summary>
        /// <param name="environmentVariable">The environment variable.</param>
        /// <param name="columns">The columns.</param>
        /// <param name="rows">The rows.</param>
        /// <param name="width">The width.</param>
        /// <param name="height">The height.</param>
        /// <param name="terminalModeValues">The terminal mode values.</param>
        /// <returns>
        /// <see langword="true"/> if request was successful; otherwise <see langword="false"/>.
        /// </returns>
        public bool SendPseudoTerminalRequest(string environmentVariable, uint columns, uint rows, uint width, uint height, IDictionary<TerminalModes, uint> terminalModeValues)
        {
            _ = _channelRequestResponse.Reset();
            SendMessage(new ChannelRequestMessage(RemoteChannelNumber, new PseudoTerminalRequestInfo(environmentVariable, columns, rows, width, height, terminalModeValues)));
            WaitOnHandle(_channelRequestResponse);
            return _channelRequestSucces;
        }

        /// <summary>
        /// Sends the X11 forwarding request.
        /// </summary>
        /// <param name="isSingleConnection">if set to <see langword="true"/> the it is single connection.</param>
        /// <param name="protocol">The protocol.</param>
        /// <param name="cookie">The cookie.</param>
        /// <param name="screenNumber">The screen number.</param>
        /// <returns>
        /// <see langword="true"/> if request was successful; otherwise <see langword="false"/>.
        /// </returns>
        public bool SendX11ForwardingRequest(bool isSingleConnection, string protocol, byte[] cookie, uint screenNumber)
        {
            _ = _channelRequestResponse.Reset();
            SendMessage(new ChannelRequestMessage(RemoteChannelNumber, new X11ForwardingRequestInfo(isSingleConnection, protocol, cookie, screenNumber)));
            WaitOnHandle(_channelRequestResponse);
            return _channelRequestSucces;
        }

        /// <summary>
        /// Sends the environment variable request.
        /// </summary>
        /// <param name="variableName">Name of the variable.</param>
        /// <param name="variableValue">The variable value.</param>
        /// <returns>
        /// <see langword="true"/> if request was successful; otherwise <see langword="false"/>.
        /// </returns>
        public bool SendEnvironmentVariableRequest(string variableName, string variableValue)
        {
            _ = _channelRequestResponse.Reset();
            SendMessage(new ChannelRequestMessage(RemoteChannelNumber, new EnvironmentVariableRequestInfo(variableName, variableValue)));
            WaitOnHandle(_channelRequestResponse);
            return _channelRequestSucces;
        }

        /// <summary>
        /// Sends the shell request.
        /// </summary>
        /// <returns>
        /// <see langword="true"/> if request was successful; otherwise <see langword="false"/>.
        /// </returns>
        public bool SendShellRequest()
        {
            _ = _channelRequestResponse.Reset();
            SendMessage(new ChannelRequestMessage(RemoteChannelNumber, new ShellRequestInfo()));
            WaitOnHandle(_channelRequestResponse);
            return _channelRequestSucces;
        }

        /// <summary>
        /// Sends the exec request.
        /// </summary>
        /// <param name="command">The command.</param>
        /// <returns>
        /// <see langword="true"/> if request was successful; otherwise <see langword="false"/>.
        /// </returns>
        public bool SendExecRequest(string command)
        {
            _ = _channelRequestResponse.Reset();
            SendMessage(new ChannelRequestMessage(RemoteChannelNumber, new ExecRequestInfo(command, ConnectionInfo.Encoding)));
            WaitOnHandle(_channelRequestResponse);
            return _channelRequestSucces;
        }

        /// <summary>
        /// Sends the exec request.
        /// </summary>
        /// <param name="breakLength">Length of the break.</param>
        /// <returns>
        /// <see langword="true"/> if request was successful; otherwise <see langword="false"/>.
        /// </returns>
        public bool SendBreakRequest(uint breakLength)
        {
            _ = _channelRequestResponse.Reset();
            SendMessage(new ChannelRequestMessage(RemoteChannelNumber, new BreakRequestInfo(breakLength)));
            WaitOnHandle(_channelRequestResponse);
            return _channelRequestSucces;
        }

        /// <summary>
        /// Sends the subsystem request.
        /// </summary>
        /// <param name="subsystem">The subsystem.</param>
        /// <returns>
        /// <see langword="true"/> if request was successful; otherwise <see langword="false"/>.
        /// </returns>
        public bool SendSubsystemRequest(string subsystem)
        {
            _ = _channelRequestResponse.Reset();
            SendMessage(new ChannelRequestMessage(RemoteChannelNumber, new SubsystemRequestInfo(subsystem)));
            WaitOnHandle(_channelRequestResponse);
            return _channelRequestSucces;
        }

        /// <summary>
        /// Sends the window change request.
        /// </summary>
        /// <param name="columns">The columns.</param>
        /// <param name="rows">The rows.</param>
        /// <param name="width">The width.</param>
        /// <param name="height">The height.</param>
        /// <returns>
        /// <see langword="true"/> if request was successful; otherwise <see langword="false"/>.
        /// </returns>
        public bool SendWindowChangeRequest(uint columns, uint rows, uint width, uint height)
        {
            SendMessage(new ChannelRequestMessage(RemoteChannelNumber, new WindowChangeRequestInfo(columns, rows, width, height)));
            return true;
        }

        /// <summary>
        /// Sends the local flow request.
        /// </summary>
        /// <param name="clientCanDo">if set to <see langword="true"/> [client can do].</param>
        /// <returns>
        /// <see langword="true"/> if request was successful; otherwise <see langword="false"/>.
        /// </returns>
        public bool SendLocalFlowRequest(bool clientCanDo)
        {
            SendMessage(new ChannelRequestMessage(RemoteChannelNumber, new XonXoffRequestInfo(clientCanDo)));
            return true;
        }

        /// <summary>
        /// Sends the signal request.
        /// </summary>
        /// <param name="signalName">Name of the signal.</param>
        /// <returns>
        /// <see langword="true"/> if request was successful; otherwise <see langword="false"/>.
        /// </returns>
        public bool SendSignalRequest(string signalName)
        {
            SendMessage(new ChannelRequestMessage(RemoteChannelNumber, new SignalRequestInfo(signalName)));
            return true;
        }

        /// <summary>
        /// Sends eow@openssh.com request.
        /// </summary>
        /// <returns>
        /// <see langword="true"/> if request was successful; otherwise <see langword="false"/>.
        /// </returns>
        public bool SendEndOfWriteRequest()
        {
            _ = _channelRequestResponse.Reset();
            SendMessage(new ChannelRequestMessage(RemoteChannelNumber, new EndOfWriteRequestInfo()));
            WaitOnHandle(_channelRequestResponse);
            return _channelRequestSucces;
        }

        /// <summary>
        /// Sends keepalive@openssh.com request.
        /// </summary>
        /// <returns>
        /// <see langword="true"/> if request was successful; otherwise <see langword="false"/>.
        /// </returns>
        public bool SendKeepAliveRequest()
        {
            _ = _channelRequestResponse.Reset();
            SendMessage(new ChannelRequestMessage(RemoteChannelNumber, new KeepAliveRequestInfo()));
            WaitOnHandle(_channelRequestResponse);
            return _channelRequestSucces;
        }

        /// <summary>
        /// Called when channel request was successful.
        /// </summary>
        protected override void OnSuccess()
        {
            base.OnSuccess();

            _channelRequestSucces = true;
            _ = _channelRequestResponse?.Set();
        }

        /// <summary>
        /// Called when channel request failed.
        /// </summary>
        protected override void OnFailure()
        {
            base.OnFailure();

            _channelRequestSucces = false;
            _ = _channelRequestResponse?.Set();
        }

        /// <summary>
        /// Sends the channel open message.
        /// </summary>
        /// <exception cref="SshConnectionException">The client is not connected.</exception>
        /// <exception cref="SshOperationTimeoutException">The operation timed out.</exception>
        /// <exception cref="InvalidOperationException">The size of the packet exceeds the maximum size defined by the protocol.</exception>
        /// <remarks>
        /// <para>
        /// When a session semaphore for this instance has not yet been obtained by this or any other thread,
        /// the thread will block until such a semaphore is available and send a <see cref="ChannelOpenMessage"/>
        /// to the remote host.
        /// </para>
        /// <para>
        /// Note that the session semaphore is released in any of the following cases:
        /// <list type="bullet">
        ///   <item>
        ///     <description>A <see cref="ChannelOpenFailureMessage"/> is received for the channel being opened.</description>
        ///   </item>
        ///   <item>
        ///     <description>The remote host does not respond to the <see cref="ChannelOpenMessage"/> within the configured <see cref="ConnectionInfo.Timeout"/>.</description>
        ///   </item>
        ///   <item>
        ///     <description>The remote host closes the channel.</description>
        ///   </item>
        ///   <item>
        ///     <description>The <see cref="ChannelSession"/> is disposed.</description>
        ///   </item>
        ///   <item>
        ///     <description>A socket error occurs sending a message to the remote host.</description>
        ///   </item>
        /// </list>
        /// </para>
        /// <para>
        /// If the session semaphore was already obtained for this instance (and not released), then this method
        /// immediately returns control to the caller. This should only happen when another thread has obtain the
        /// session semaphore and already sent the <see cref="ChannelOpenMessage"/>, but the remote host did not
        /// confirmed or rejected attempt to open the channel.
        /// </para>
        /// </remarks>
        private void SendChannelOpenMessage()
        {
            // do not allow the ChannelOpenMessage to be sent again until we've
            // had a response on the previous attempt for the current channel
            if (Interlocked.CompareExchange(ref _sessionSemaphoreObtained, 1, 0) == 0)
            {
                SessionSemaphore.Wait();
                SendMessage(new ChannelOpenMessage(LocalChannelNumber,
                                                   LocalWindowSize,
                                                   LocalPacketSize,
                                                   new SessionChannelOpenInfo()));
            }
        }

        /// <summary>
        /// Releases unmanaged and - optionally - managed resources.
        /// </summary>
        /// <param name="disposing"><see langword="true"/> to release both managed and unmanaged resources; <see langword="false"/> to release only unmanaged resources.</param>
        protected override void Dispose(bool disposing)
        {
            base.Dispose(disposing);

            if (disposing)
            {
                var channelOpenResponseWaitHandle = _channelOpenResponseWaitHandle;
                if (channelOpenResponseWaitHandle != null)
                {
                    _channelOpenResponseWaitHandle = null;
                    channelOpenResponseWaitHandle.Dispose();
                }

                var channelRequestResponse = _channelRequestResponse;
                if (channelRequestResponse != null)
                {
                    _channelRequestResponse = null;
                    channelRequestResponse.Dispose();
                }
            }
        }

        /// <summary>
        /// Releases the session semaphore.
        /// </summary>
        /// <remarks>
        /// When the session semaphore has already been released, or was never obtained by
        /// this instance, then this method does nothing.
        /// </remarks>
        private void ReleaseSemaphore()
        {
            if (Interlocked.CompareExchange(ref _sessionSemaphoreObtained, 0, 1) == 1)
            {
                _ = SessionSemaphore.Release();
            }
        }
    }
}
