using System;
using System.Diagnostics.CodeAnalysis;
using System.Globalization;
using System.IO;
using System.Threading;
using System.Threading.Tasks;

using Renci.SshNet.Common;

namespace Renci.SshNet.Sftp
{
    /// <summary>
    /// Exposes a <see cref="Stream"/> around a remote SFTP file, supporting both synchronous and asynchronous read and write operations.
    /// </summary>
    /// <threadsafety static="true" instance="false"/>
#pragma warning disable IDE0079 // We intentionally want to suppress the below warning.
    [SuppressMessage("Performance", "CA1844: Provide memory-based overrides of async methods when subclassing 'Stream'", Justification = "TODO: This should be addressed in the future.")]
#pragma warning restore IDE0079
    public class SftpFileStream : Stream
    {
        private readonly Lock _lock = new Lock();
        private readonly int _readBufferSize;
        private readonly int _writeBufferSize;

        // Internal state.
        private byte[] _handle;
        private ISftpSession _session;

        // Buffer information.
        private byte[] _readBuffer;
        private byte[] _writeBuffer;
        private int _bufferPosition;
        private int _bufferLen;
        private long _position;
        private bool _bufferOwnedByWrite;
        private bool _canRead;
        private bool _canSeek;
        private bool _canWrite;
        private TimeSpan _timeout;

        /// <summary>
        /// Gets a value indicating whether the current stream supports reading.
        /// </summary>
        /// <value>
        /// <see langword="true"/> if the stream supports reading; otherwise, <see langword="false"/>.
        /// </value>
        public override bool CanRead
        {
            get { return _canRead; }
        }

        /// <summary>
        /// Gets a value indicating whether the current stream supports seeking.
        /// </summary>
        /// <value>
        /// <see langword="true"/> if the stream supports seeking; otherwise, <see langword="false"/>.
        /// </value>
        public override bool CanSeek
        {
            get { return _canSeek; }
        }

        /// <summary>
        /// Gets a value indicating whether the current stream supports writing.
        /// </summary>
        /// <value>
        /// <see langword="true"/> if the stream supports writing; otherwise, <see langword="false"/>.
        /// </value>
        public override bool CanWrite
        {
            get { return _canWrite; }
        }

        /// <summary>
        /// Gets a value indicating whether timeout properties are usable for <see cref="SftpFileStream"/>.
        /// </summary>
        /// <value>
        /// <see langword="true"/> in all cases.
        /// </value>
        public override bool CanTimeout
        {
            get { return true; }
        }

        /// <summary>
        /// Gets the length in bytes of the stream.
        /// </summary>
        /// <value>A long value representing the length of the stream in bytes.</value>
        /// <exception cref="NotSupportedException">A class derived from Stream does not support seeking. </exception>
        /// <exception cref="ObjectDisposedException">Methods were called after the stream was closed. </exception>
        /// <exception cref="IOException">IO operation failed. </exception>
        public override long Length
        {
            get
            {
                // Lock down the file stream while we do this.
                lock (_lock)
                {
                    CheckSessionIsOpen();

                    if (!CanSeek)
                    {
                        throw new NotSupportedException("Seek operation is not supported.");
                    }

                    // Flush the write buffer, because it may
                    // affect the length of the stream.
                    if (_bufferOwnedByWrite)
                    {
                        FlushWriteBuffer();
                    }

                    // obtain file attributes
                    var attributes = _session.RequestFStat(_handle, nullOnError: true);
                    if (attributes != null)
                    {
                        return attributes.Size;
                    }

                    throw new IOException("Seek operation failed.");
                }
            }
        }

        /// <summary>
        /// Gets or sets the position within the current stream.
        /// </summary>
        /// <value>The current position within the stream.</value>
        /// <exception cref="IOException">An I/O error occurs. </exception>
        /// <exception cref="NotSupportedException">The stream does not support seeking. </exception>
        /// <exception cref="ObjectDisposedException">Methods were called after the stream was closed. </exception>
        public override long Position
        {
            get
            {
                CheckSessionIsOpen();

                if (!CanSeek)
                {
                    throw new NotSupportedException("Seek operation not supported.");
                }

                return _position;
            }
            set
            {
                _ = Seek(value, SeekOrigin.Begin);
            }
        }

        /// <summary>
        /// Gets the name of the path that was used to construct the current <see cref="SftpFileStream"/>.
        /// </summary>
        /// <value>
        /// The name of the path that was used to construct the current <see cref="SftpFileStream"/>.
        /// </value>
        public string Name { get; private set; }

        /// <summary>
        /// Gets the operating system file handle for the file that the current <see cref="SftpFileStream"/> encapsulates.
        /// </summary>
        /// <value>
        /// The operating system file handle for the file that the current <see cref="SftpFileStream"/> encapsulates.
        /// </value>
        public virtual byte[] Handle
        {
            get
            {
                Flush();
                return _handle;
            }
        }

        /// <summary>
        /// Gets or sets the operation timeout.
        /// </summary>
        /// <value>
        /// The timeout.
        /// </value>
        public TimeSpan Timeout
        {
            get
            {
                return _timeout;
            }
            set
            {
                value.EnsureValidTimeout(nameof(Timeout));

                _timeout = value;
            }
        }

        private SftpFileStream(ISftpSession session, string path, FileAccess access, int bufferSize, byte[] handle, long position)
        {
            Timeout = TimeSpan.FromSeconds(30);
            Name = path;

            _session = session;
            _canRead = (access & FileAccess.Read) == FileAccess.Read;
            _canSeek = true;
            _canWrite = (access & FileAccess.Write) == FileAccess.Write;

            _handle = handle;

            /*
             * Instead of using the specified buffer size as is, we use it to calculate a buffer size
             * that ensures we always receive or send the max. number of bytes in a single SSH_FXP_READ
             * or SSH_FXP_WRITE message.
             */

            _readBufferSize = (int)session.CalculateOptimalReadLength((uint)bufferSize);
            _writeBufferSize = (int)session.CalculateOptimalWriteLength((uint)bufferSize, _handle);

            _position = position;
        }

        internal SftpFileStream(ISftpSession session, string path, FileMode mode, FileAccess access, int bufferSize)
        {
            if (session is null)
            {
                throw new SshConnectionException("Client not connected.");
            }

            ThrowHelper.ThrowIfNull(path);

            if (bufferSize <= 0)
            {
                throw new ArgumentOutOfRangeException(nameof(bufferSize), "Cannot be less than or equal to zero.");
            }

            Timeout = TimeSpan.FromSeconds(30);
            Name = path;

            // Initialize the object state.
            _session = session;
            _canRead = (access & FileAccess.Read) == FileAccess.Read;
            _canSeek = true;
            _canWrite = (access & FileAccess.Write) == FileAccess.Write;

            var flags = Flags.None;

            switch (access)
            {
                case FileAccess.Read:
                    flags |= Flags.Read;
                    break;
                case FileAccess.Write:
                    flags |= Flags.Write;
                    break;
                case FileAccess.ReadWrite:
                    flags |= Flags.Read;
                    flags |= Flags.Write;
                    break;
                default:
                    throw new ArgumentOutOfRangeException(nameof(access));
            }

            if ((access & FileAccess.Read) == FileAccess.Read && mode == FileMode.Append)
            {
                throw new ArgumentException(string.Format(CultureInfo.InvariantCulture,
                                                          "{0} mode can be requested only when combined with write-only access.",
                                                          mode.ToString("G")),
                                            nameof(mode));
            }

            if ((access & FileAccess.Write) != FileAccess.Write)
            {
                if (mode is FileMode.Create or FileMode.CreateNew or FileMode.Truncate or FileMode.Append)
                {
                    throw new ArgumentException(string.Format(CultureInfo.InvariantCulture,
                                                              "Combining {0}: {1} with {2}: {3} is invalid.",
                                                              nameof(FileMode),
                                                              mode,
                                                              nameof(FileAccess),
                                                              access),
                                                nameof(mode));
                }
            }

            switch (mode)
            {
                case FileMode.Append:
                    flags |= Flags.Append | Flags.CreateNewOrOpen;
                    break;
                case FileMode.Create:
                    _handle = _session.RequestOpen(path, flags | Flags.Truncate, nullOnError: true);
                    if (_handle is null)
                    {
                        flags |= Flags.CreateNew;
                    }
                    else
                    {
                        flags |= Flags.Truncate;
                    }

                    break;
                case FileMode.CreateNew:
                    flags |= Flags.CreateNew;
                    break;
                case FileMode.Open:
                    break;
                case FileMode.OpenOrCreate:
                    flags |= Flags.CreateNewOrOpen;
                    break;
                case FileMode.Truncate:
                    flags |= Flags.Truncate;
                    break;
                default:
                    throw new ArgumentOutOfRangeException(nameof(mode));
            }

            _handle ??= _session.RequestOpen(path, flags);

            /*
             * Instead of using the specified buffer size as is, we use it to calculate a buffer size
             * that ensures we always receive or send the max. number of bytes in a single SSH_FXP_READ
             * or SSH_FXP_WRITE message.
             */

            _readBufferSize = (int)session.CalculateOptimalReadLength((uint)bufferSize);
            _writeBufferSize = (int)session.CalculateOptimalWriteLength((uint)bufferSize, _handle);

            if (mode == FileMode.Append)
            {
                var attributes = _session.RequestFStat(_handle, nullOnError: false);
                _position = attributes.Size;
            }
        }

        internal static async Task<SftpFileStream> OpenAsync(ISftpSession session, string path, FileMode mode, FileAccess access, int bufferSize, CancellationToken cancellationToken)
        {
            if (session is null)
            {
                throw new SshConnectionException("Client not connected.");
            }

            ThrowHelper.ThrowIfNull(path);

            if (bufferSize <= 0)
            {
                throw new ArgumentOutOfRangeException(nameof(bufferSize), "Cannot be less than or equal to zero.");
            }

            var flags = Flags.None;

            switch (access)
            {
                case FileAccess.Read:
                    flags |= Flags.Read;
                    break;
                case FileAccess.Write:
                    flags |= Flags.Write;
                    break;
                case FileAccess.ReadWrite:
                    flags |= Flags.Read;
                    flags |= Flags.Write;
                    break;
                default:
                    throw new ArgumentOutOfRangeException(nameof(access));
            }

            if ((access & FileAccess.Read) == FileAccess.Read && mode == FileMode.Append)
            {
                throw new ArgumentException(string.Format(CultureInfo.InvariantCulture,
                                                          "{0} mode can be requested only when combined with write-only access.",
                                                          mode.ToString("G")),
                                            nameof(mode));
            }

            if ((access & FileAccess.Write) != FileAccess.Write)
            {
                if (mode is FileMode.Create or FileMode.CreateNew or FileMode.Truncate or FileMode.Append)
                {
                    throw new ArgumentException(string.Format(CultureInfo.InvariantCulture,
                                                              "Combining {0}: {1} with {2}: {3} is invalid.",
                                                              nameof(FileMode),
                                                              mode,
                                                              nameof(FileAccess),
                                                              access),
                                                nameof(mode));
                }
            }

            switch (mode)
            {
                case FileMode.Append:
                    flags |= Flags.Append | Flags.CreateNewOrOpen;
                    break;
                case FileMode.Create:
                    flags |= Flags.CreateNewOrOpen | Flags.Truncate;
                    break;
                case FileMode.CreateNew:
                    flags |= Flags.CreateNew;
                    break;
                case FileMode.Open:
                    break;
                case FileMode.OpenOrCreate:
                    flags |= Flags.CreateNewOrOpen;
                    break;
                case FileMode.Truncate:
                    flags |= Flags.Truncate;
                    break;
                default:
                    throw new ArgumentOutOfRangeException(nameof(mode));
            }

            var handle = await session.RequestOpenAsync(path, flags, cancellationToken).ConfigureAwait(false);

            long position = 0;
            if (mode == FileMode.Append)
            {
                try
                {
                    var attributes = await session.RequestFStatAsync(handle, cancellationToken).ConfigureAwait(false);
                    position = attributes.Size;
                }
                catch
                {
                    try
                    {
                        await session.RequestCloseAsync(handle, cancellationToken).ConfigureAwait(false);
                    }
                    catch
                    {
                        // The original exception is presumably more informative, so we just ignore this one.
                    }

                    throw;
                }
            }

            return new SftpFileStream(session, path, access, bufferSize, handle, position);
        }

        /// <summary>
        /// Clears all buffers for this stream and causes any buffered data to be written to the file.
        /// </summary>
        /// <exception cref="IOException">An I/O error occurs. </exception>
        /// <exception cref="ObjectDisposedException">Stream is closed.</exception>
        public override void Flush()
        {
            lock (_lock)
            {
                CheckSessionIsOpen();

                if (_bufferOwnedByWrite)
                {
                    FlushWriteBuffer();
                }
                else
                {
                    FlushReadBuffer();
                }
            }
        }

        /// <summary>
        /// Asynchronously clears all buffers for this stream and causes any buffered data to be written to the file.
        /// </summary>
        /// <param name="cancellationToken">The <see cref="CancellationToken"/> to observe.</param>
        /// <returns>A <see cref="Task"/> that represents the asynchronous flush operation.</returns>
        /// <exception cref="IOException">An I/O error occurs. </exception>
        /// <exception cref="ObjectDisposedException">Stream is closed.</exception>
        public override Task FlushAsync(CancellationToken cancellationToken)
        {
            CheckSessionIsOpen();

            if (_bufferOwnedByWrite)
            {
                return FlushWriteBufferAsync(cancellationToken);
            }

            FlushReadBuffer();

            return Task.CompletedTask;
        }

        /// <summary>
        /// Reads a sequence of bytes from the current stream and advances the position within the stream by the
        /// number of bytes read.
        /// </summary>
        /// <param name="buffer">An array of bytes. When this method returns, the buffer contains the specified byte array with the values between <paramref name="offset"/> and (<paramref name="offset"/> + <paramref name="count"/> - 1) replaced by the bytes read from the current source.</param>
        /// <param name="offset">The zero-based byte offset in <paramref name="buffer"/> at which to begin storing the data read from the current stream.</param>
        /// <param name="count">The maximum number of bytes to be read from the current stream.</param>
        /// <returns>
        /// The total number of bytes read into the buffer. This can be less than the number of bytes requested
        /// if that many bytes are not currently available, or zero (0) if the end of the stream has been reached.
        /// </returns>
        /// <exception cref="ArgumentException">The sum of <paramref name="offset"/> and <paramref name="count"/> is larger than the buffer length.</exception>
        /// <exception cref="ArgumentNullException"><paramref name="buffer"/> is <see langword="null"/>. </exception>
        /// <exception cref="ArgumentOutOfRangeException"><paramref name="offset"/> or <paramref name="count"/> is negative.</exception>
        /// <exception cref="IOException">An I/O error occurs. </exception>
        /// <exception cref="NotSupportedException">The stream does not support reading. </exception>
        /// <exception cref="ObjectDisposedException">Methods were called after the stream was closed. </exception>
        /// <remarks>
        /// <para>
        /// This method attempts to read up to <paramref name="count"/> bytes. This either from the buffer, from the
        /// server (using one or more <c>SSH_FXP_READ</c> requests) or using a combination of both.
        /// </para>
        /// <para>
        /// The read loop is interrupted when either <paramref name="count"/> bytes are read, the server returns zero
        /// bytes (EOF) or less bytes than the read buffer size.
        /// </para>
        /// <para>
        /// When a server returns less number of bytes than the read buffer size, this <c>may</c> indicate that EOF has
        /// been reached. A subsequent (<c>SSH_FXP_READ</c>) server request is necessary to make sure EOF has effectively
        /// been reached.  Breaking out of the read loop avoids reading from the server twice to determine EOF: once in
        /// the read loop, and once upon the next <see cref="Read"/> or <see cref="ReadByte"/> invocation.
        /// </para>
        /// </remarks>
        public override int Read(byte[] buffer, int offset, int count)
        {
            var readLen = 0;

            ThrowHelper.ThrowIfNull(buffer);

#if NET8_0_OR_GREATER
            ArgumentOutOfRangeException.ThrowIfNegative(offset);
            ArgumentOutOfRangeException.ThrowIfNegative(count);
#else
            if (offset < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(offset));
            }

            if (count < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(count));
            }
#endif
            if ((buffer.Length - offset) < count)
            {
                throw new ArgumentException("Invalid array range.");
            }

            // Lock down the file stream while we do this.
            lock (_lock)
            {
                CheckSessionIsOpen();

                // Set up for the read operation.
                SetupRead();

                // Read data into the caller's buffer.
                while (count > 0)
                {
                    // How much data do we have available in the buffer?
                    var bytesAvailableInBuffer = _bufferLen - _bufferPosition;
                    if (bytesAvailableInBuffer <= 0)
                    {
                        var data = _session.RequestRead(_handle, (ulong)_position, (uint)_readBufferSize);

                        if (data.Length == 0)
                        {
                            _bufferPosition = 0;
                            _bufferLen = 0;

                            break;
                        }

                        var bytesToWriteToCallerBuffer = count;
                        if (bytesToWriteToCallerBuffer >= data.Length)
                        {
                            // write all data read to caller-provided buffer
                            bytesToWriteToCallerBuffer = data.Length;

                            // reset buffer since we will skip buffering
                            _bufferPosition = 0;
                            _bufferLen = 0;
                        }
                        else
                        {
                            // determine number of bytes that we should write into read buffer
                            var bytesToWriteToReadBuffer = data.Length - bytesToWriteToCallerBuffer;

                            // write remaining bytes to read buffer
                            Buffer.BlockCopy(data, count, GetOrCreateReadBuffer(), 0, bytesToWriteToReadBuffer);

                            // update position in read buffer
                            _bufferPosition = 0;

                            // update number of bytes in read buffer
                            _bufferLen = bytesToWriteToReadBuffer;
                        }

                        // write bytes to caller-provided buffer
                        Buffer.BlockCopy(data, 0, buffer, offset, bytesToWriteToCallerBuffer);

                        // update stream position
                        _position += bytesToWriteToCallerBuffer;

                        // record total number of bytes read into caller-provided buffer
                        readLen += bytesToWriteToCallerBuffer;

                        // break out of the read loop when the server returned less than the request number of bytes
                        // as that *may* indicate that we've reached EOF
                        //
                        // doing this avoids reading from server twice to determine EOF: once in the read loop, and
                        // once upon the next Read or ReadByte invocation by the caller
                        if (data.Length < _readBufferSize)
                        {
                            break;
                        }

                        // advance offset to start writing bytes into caller-provided buffer
                        offset += bytesToWriteToCallerBuffer;

                        // update number of bytes left to read into caller-provided buffer
                        count -= bytesToWriteToCallerBuffer;
                    }
                    else
                    {
                        // limit the number of bytes to use from read buffer to the caller-request number of bytes
                        if (bytesAvailableInBuffer > count)
                        {
                            bytesAvailableInBuffer = count;
                        }

                        // copy data from read buffer to the caller-provided buffer
                        Buffer.BlockCopy(GetOrCreateReadBuffer(), _bufferPosition, buffer, offset, bytesAvailableInBuffer);

                        // update position in read buffer
                        _bufferPosition += bytesAvailableInBuffer;

                        // update stream position
                        _position += bytesAvailableInBuffer;

                        // record total number of bytes read into caller-provided buffer
                        readLen += bytesAvailableInBuffer;

                        // advance offset to start writing bytes into caller-provided buffer
                        offset += bytesAvailableInBuffer;

                        // update number of bytes left to read
                        count -= bytesAvailableInBuffer;
                    }
                }
            }

            // return the number of bytes that were read to the caller.
            return readLen;
        }

        /// <summary>
        /// Asynchronously reads a sequence of bytes from the current stream and advances the position within the stream by the
        /// number of bytes read.
        /// </summary>
        /// <param name="buffer">An array of bytes. When this method returns, the buffer contains the specified byte array with the values between <paramref name="offset"/> and (<paramref name="offset"/> + <paramref name="count"/> - 1) replaced by the bytes read from the current source.</param>
        /// <param name="offset">The zero-based byte offset in <paramref name="buffer"/> at which to begin storing the data read from the current stream.</param>
        /// <param name="count">The maximum number of bytes to be read from the current stream.</param>
        /// <param name="cancellationToken">The <see cref="CancellationToken" /> to observe.</param>
        /// <returns>A <see cref="Task" /> that represents the asynchronous read operation.</returns>
        public override async Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
        {
            var readLen = 0;

            ThrowHelper.ThrowIfNull(buffer);

#if NET8_0_OR_GREATER
            ArgumentOutOfRangeException.ThrowIfNegative(offset);
            ArgumentOutOfRangeException.ThrowIfNegative(count);
#else
            if (offset < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(offset));
            }

            if (count < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(count));
            }
#endif
            if ((buffer.Length - offset) < count)
            {
                throw new ArgumentException("Invalid array range.");
            }

            CheckSessionIsOpen();

            // Set up for the read operation.
            SetupRead();

            // Read data into the caller's buffer.
            while (count > 0)
            {
                // How much data do we have available in the buffer?
                var bytesAvailableInBuffer = _bufferLen - _bufferPosition;
                if (bytesAvailableInBuffer <= 0)
                {
                    var data = await _session.RequestReadAsync(_handle, (ulong)_position, (uint)_readBufferSize, cancellationToken).ConfigureAwait(false);

                    if (data.Length == 0)
                    {
                        _bufferPosition = 0;
                        _bufferLen = 0;

                        break;
                    }

                    var bytesToWriteToCallerBuffer = count;
                    if (bytesToWriteToCallerBuffer >= data.Length)
                    {
                        // write all data read to caller-provided buffer
                        bytesToWriteToCallerBuffer = data.Length;

                        // reset buffer since we will skip buffering
                        _bufferPosition = 0;
                        _bufferLen = 0;
                    }
                    else
                    {
                        // determine number of bytes that we should write into read buffer
                        var bytesToWriteToReadBuffer = data.Length - bytesToWriteToCallerBuffer;

                        // write remaining bytes to read buffer
                        Buffer.BlockCopy(data, count, GetOrCreateReadBuffer(), 0, bytesToWriteToReadBuffer);

                        // update position in read buffer
                        _bufferPosition = 0;

                        // update number of bytes in read buffer
                        _bufferLen = bytesToWriteToReadBuffer;
                    }

                    // write bytes to caller-provided buffer
                    Buffer.BlockCopy(data, 0, buffer, offset, bytesToWriteToCallerBuffer);

                    // update stream position
                    _position += bytesToWriteToCallerBuffer;

                    // record total number of bytes read into caller-provided buffer
                    readLen += bytesToWriteToCallerBuffer;

                    // break out of the read loop when the server returned less than the request number of bytes
                    // as that *may* indicate that we've reached EOF
                    //
                    // doing this avoids reading from server twice to determine EOF: once in the read loop, and
                    // once upon the next Read or ReadByte invocation by the caller
                    if (data.Length < _readBufferSize)
                    {
                        break;
                    }

                    // advance offset to start writing bytes into caller-provided buffer
                    offset += bytesToWriteToCallerBuffer;

                    // update number of bytes left to read into caller-provided buffer
                    count -= bytesToWriteToCallerBuffer;
                }
                else
                {
                    // limit the number of bytes to use from read buffer to the caller-request number of bytes
                    if (bytesAvailableInBuffer > count)
                    {
                        bytesAvailableInBuffer = count;
                    }

                    // copy data from read buffer to the caller-provided buffer
                    Buffer.BlockCopy(GetOrCreateReadBuffer(), _bufferPosition, buffer, offset, bytesAvailableInBuffer);

                    // update position in read buffer
                    _bufferPosition += bytesAvailableInBuffer;

                    // update stream position
                    _position += bytesAvailableInBuffer;

                    // record total number of bytes read into caller-provided buffer
                    readLen += bytesAvailableInBuffer;

                    // advance offset to start writing bytes into caller-provided buffer
                    offset += bytesAvailableInBuffer;

                    // update number of bytes left to read
                    count -= bytesAvailableInBuffer;
                }
            }

            // return the number of bytes that were read to the caller.
            return readLen;
        }

        /// <summary>
        /// Reads a byte from the stream and advances the position within the stream by one byte, or returns -1 if at the end of the stream.
        /// </summary>
        /// <returns>
        /// The unsigned byte cast to an <see cref="int"/>, or -1 if at the end of the stream.
        /// </returns>
        /// <exception cref="NotSupportedException">The stream does not support reading. </exception>
        /// <exception cref="ObjectDisposedException">Methods were called after the stream was closed. </exception>
        /// <exception cref="IOException">Read operation failed.</exception>
        public override int ReadByte()
        {
            // Lock down the file stream while we do this.
            lock (_lock)
            {
                CheckSessionIsOpen();

                // Setup the object for reading.
                SetupRead();

                byte[] readBuffer;

                // Read more data into the internal buffer if necessary.
                if (_bufferPosition >= _bufferLen)
                {
                    var data = _session.RequestRead(_handle, (ulong)_position, (uint)_readBufferSize);
                    if (data.Length == 0)
                    {
                        // We've reached EOF.
                        return -1;
                    }

                    readBuffer = GetOrCreateReadBuffer();
                    Buffer.BlockCopy(data, 0, readBuffer, 0, data.Length);

                    _bufferPosition = 0;
                    _bufferLen = data.Length;
                }
                else
                {
                    readBuffer = GetOrCreateReadBuffer();
                }

                // Extract the next byte from the buffer.
                ++_position;

                return readBuffer[_bufferPosition++];
            }
        }

        /// <summary>
        /// Sets the position within the current stream.
        /// </summary>
        /// <param name="offset">A byte offset relative to the <paramref name="origin"/> parameter.</param>
        /// <param name="origin">A value of type <see cref="SeekOrigin"/> indicating the reference point used to obtain the new position.</param>
        /// <returns>
        /// The new position within the current stream.
        /// </returns>
        /// <exception cref="IOException">An I/O error occurs. </exception>
        /// <exception cref="NotSupportedException">The stream does not support seeking, such as if the stream is constructed from a pipe or console output. </exception>
        /// <exception cref="ObjectDisposedException">Methods were called after the stream was closed. </exception>
        public override long Seek(long offset, SeekOrigin origin)
        {
            long newPosn;

            // Lock down the file stream while we do this.
            lock (_lock)
            {
                CheckSessionIsOpen();

                if (!CanSeek)
                {
                    throw new NotSupportedException("Seek is not supported.");
                }

                // Don't do anything if the position won't be moving.
                if (origin == SeekOrigin.Begin && offset == _position)
                {
                    return offset;
                }

                if (origin == SeekOrigin.Current && offset == 0)
                {
                    return _position;
                }

                // The behaviour depends upon the read/write mode.
                if (_bufferOwnedByWrite)
                {
                    // Flush the write buffer and then seek.
                    FlushWriteBuffer();
                }
                else
                {
                    // Determine if the seek is to somewhere inside
                    // the current read buffer bounds.
                    if (origin == SeekOrigin.Begin)
                    {
                        newPosn = _position - _bufferPosition;
                        if (offset >= newPosn && offset < (newPosn + _bufferLen))
                        {
                            _bufferPosition = (int)(offset - newPosn);
                            _position = offset;
                            return _position;
                        }
                    }
                    else if (origin == SeekOrigin.Current)
                    {
                        newPosn = _position + offset;
                        if (newPosn >= (_position - _bufferPosition) &&
                           newPosn < (_position - _bufferPosition + _bufferLen))
                        {
                            _bufferPosition = (int)(newPosn - (_position - _bufferPosition));
                            _position = newPosn;
                            return _position;
                        }
                    }

                    // Abandon the read buffer.
                    _bufferPosition = 0;
                    _bufferLen = 0;
                }

                // Seek to the new position.
                switch (origin)
                {
                    case SeekOrigin.Begin:
                        newPosn = offset;
                        break;
                    case SeekOrigin.Current:
                        newPosn = _position + offset;
                        break;
                    case SeekOrigin.End:
                        var attributes = _session.RequestFStat(_handle, nullOnError: false);
                        newPosn = attributes.Size + offset;
                        break;
                    default:
                        throw new ArgumentException("Invalid seek origin.", nameof(origin));
                }

                if (newPosn < 0)
                {
                    throw new EndOfStreamException();
                }

                _position = newPosn;
                return _position;
            }
        }

        /// <summary>
        /// Sets the length of the current stream.
        /// </summary>
        /// <param name="value">The desired length of the current stream in bytes.</param>
        /// <exception cref="IOException">An I/O error occurs.</exception>
        /// <exception cref="NotSupportedException">The stream does not support both writing and seeking.</exception>
        /// <exception cref="ObjectDisposedException">Methods were called after the stream was closed.</exception>
        /// <exception cref="ArgumentOutOfRangeException"><paramref name="value"/> must be greater than zero.</exception>
        /// <remarks>
        /// <para>
        /// Buffers are first flushed.
        /// </para>
        /// <para>
        /// If the specified value is less than the current length of the stream, the stream is truncated and - if the
        /// current position is greater than the new length - the current position is moved to the last byte of the stream.
        /// </para>
        /// <para>
        /// If the given value is greater than the current length of the stream, the stream is expanded and the current
        /// position remains the same.
        /// </para>
        /// </remarks>
        public override void SetLength(long value)
        {
#if NET8_0_OR_GREATER
            ArgumentOutOfRangeException.ThrowIfNegative(value);
#else
            if (value < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(value));
            }
#endif

            // Lock down the file stream while we do this.
            lock (_lock)
            {
                CheckSessionIsOpen();

                if (!CanSeek)
                {
                    throw new NotSupportedException("Seek is not supported.");
                }

                if (_bufferOwnedByWrite)
                {
                    FlushWriteBuffer();
                }
                else
                {
                    SetupWrite();
                }

                var attributes = _session.RequestFStat(_handle, nullOnError: false);
                attributes.Size = value;
                _session.RequestFSetStat(_handle, attributes);

                if (_position > value)
                {
                    _position = value;
                }
            }
        }

        /// <summary>
        /// Writes a sequence of bytes to the current stream and advances the current position within this stream by the number of bytes written.
        /// </summary>
        /// <param name="buffer">An array of bytes. This method copies <paramref name="count"/> bytes from <paramref name="buffer"/> to the current stream.</param>
        /// <param name="offset">The zero-based byte offset in <paramref name="buffer"/> at which to begin copying bytes to the current stream.</param>
        /// <param name="count">The number of bytes to be written to the current stream.</param>
        /// <exception cref="ArgumentException">The sum of <paramref name="offset"/> and <paramref name="count"/> is greater than the buffer length.</exception>
        /// <exception cref="ArgumentNullException"><paramref name="buffer"/> is <see langword="null"/>.</exception>
        /// <exception cref="ArgumentOutOfRangeException"><paramref name="offset"/> or <paramref name="count"/> is negative.</exception>
        /// <exception cref="IOException">An I/O error occurs.</exception>
        /// <exception cref="NotSupportedException">The stream does not support writing.</exception>
        /// <exception cref="ObjectDisposedException">Methods were called after the stream was closed.</exception>
        public override void Write(byte[] buffer, int offset, int count)
        {
            ThrowHelper.ThrowIfNull(buffer);

#if NET8_0_OR_GREATER
            ArgumentOutOfRangeException.ThrowIfNegative(offset);
            ArgumentOutOfRangeException.ThrowIfNegative(count);
#else
            if (offset < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(offset));
            }

            if (count < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(count));
            }
#endif
            if ((buffer.Length - offset) < count)
            {
                throw new ArgumentException("Invalid array range.");
            }

            // Lock down the file stream while we do this.
            lock (_lock)
            {
                CheckSessionIsOpen();

                // Setup this object for writing.
                SetupWrite();

                // Write data to the file stream.
                while (count > 0)
                {
                    // Determine how many bytes we can write to the buffer.
                    var tempLen = _writeBufferSize - _bufferPosition;
                    if (tempLen <= 0)
                    {
                        // flush write buffer, and mark it empty
                        FlushWriteBuffer();

                        // we can now write or buffer the full buffer size
                        tempLen = _writeBufferSize;
                    }

                    // limit the number of bytes to write to the actual number of bytes requested
                    if (tempLen > count)
                    {
                        tempLen = count;
                    }

                    // Can we short-cut the internal buffer?
                    if (_bufferPosition == 0 && tempLen == _writeBufferSize)
                    {
                        using (var wait = new AutoResetEvent(initialState: false))
                        {
                            _session.RequestWrite(_handle, (ulong)_position, buffer, offset, tempLen, wait);
                        }
                    }
                    else
                    {
                        // No: copy the data to the write buffer first.
                        Buffer.BlockCopy(buffer, offset, GetOrCreateWriteBuffer(), _bufferPosition, tempLen);
                        _bufferPosition += tempLen;
                    }

                    // Advance the buffer and stream positions.
                    _position += tempLen;
                    offset += tempLen;
                    count -= tempLen;
                }

                // If the buffer is full, then do a speculative flush now,
                // rather than waiting for the next call to this method.
                if (_bufferPosition >= _writeBufferSize)
                {
                    using (var wait = new AutoResetEvent(initialState: false))
                    {
                        _session.RequestWrite(_handle, (ulong)(_position - _bufferPosition), GetOrCreateWriteBuffer(), 0, _bufferPosition, wait);
                    }

                    _bufferPosition = 0;
                }
            }
        }

        /// <summary>
        /// Asynchronously writes a sequence of bytes to the current stream and advances the current position within this stream by the number of bytes written.
        /// </summary>
        /// <param name="buffer">An array of bytes. This method copies <paramref name="count"/> bytes from <paramref name="buffer"/> to the current stream.</param>
        /// <param name="offset">The zero-based byte offset in <paramref name="buffer"/> at which to begin copying bytes to the current stream.</param>
        /// <param name="count">The number of bytes to be written to the current stream.</param>
        /// <param name="cancellationToken">The <see cref="CancellationToken"/> to observe.</param>
        /// <returns>A <see cref="Task"/> that represents the asynchronous write operation.</returns>
        /// <exception cref="ArgumentException">The sum of <paramref name="offset"/> and <paramref name="count"/> is greater than the buffer length.</exception>
        /// <exception cref="ArgumentNullException"><paramref name="buffer"/> is <see langword="null"/>.</exception>
        /// <exception cref="ArgumentOutOfRangeException"><paramref name="offset"/> or <paramref name="count"/> is negative.</exception>
        /// <exception cref="IOException">An I/O error occurs.</exception>
        /// <exception cref="NotSupportedException">The stream does not support writing.</exception>
        /// <exception cref="ObjectDisposedException">Methods were called after the stream was closed.</exception>
        public override async Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
        {
            ThrowHelper.ThrowIfNull(buffer);

#if NET8_0_OR_GREATER
            ArgumentOutOfRangeException.ThrowIfNegative(offset);
            ArgumentOutOfRangeException.ThrowIfNegative(count);
#else
            if (offset < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(offset));
            }

            if (count < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(count));
            }
#endif
            if ((buffer.Length - offset) < count)
            {
                throw new ArgumentException("Invalid array range.");
            }

            CheckSessionIsOpen();

            // Setup this object for writing.
            SetupWrite();

            // Write data to the file stream.
            while (count > 0)
            {
                // Determine how many bytes we can write to the buffer.
                var tempLen = _writeBufferSize - _bufferPosition;
                if (tempLen <= 0)
                {
                    // flush write buffer, and mark it empty
                    await FlushWriteBufferAsync(cancellationToken).ConfigureAwait(false);

                    // we can now write or buffer the full buffer size
                    tempLen = _writeBufferSize;
                }

                // limit the number of bytes to write to the actual number of bytes requested
                if (tempLen > count)
                {
                    tempLen = count;
                }

                // Can we short-cut the internal buffer?
                if (_bufferPosition == 0 && tempLen == _writeBufferSize)
                {
                    await _session.RequestWriteAsync(_handle, (ulong)_position, buffer, offset, tempLen, cancellationToken).ConfigureAwait(false);
                }
                else
                {
                    // No: copy the data to the write buffer first.
                    Buffer.BlockCopy(buffer, offset, GetOrCreateWriteBuffer(), _bufferPosition, tempLen);
                    _bufferPosition += tempLen;
                }

                // Advance the buffer and stream positions.
                _position += tempLen;
                offset += tempLen;
                count -= tempLen;
            }

            // If the buffer is full, then do a speculative flush now,
            // rather than waiting for the next call to this method.
            if (_bufferPosition >= _writeBufferSize)
            {
                await _session.RequestWriteAsync(_handle, (ulong)(_position - _bufferPosition), GetOrCreateWriteBuffer(), 0, _bufferPosition, cancellationToken).ConfigureAwait(false);
                _bufferPosition = 0;
            }
        }

        /// <summary>
        /// Writes a byte to the current position in the stream and advances the position within the stream by one byte.
        /// </summary>
        /// <param name="value">The byte to write to the stream.</param>
        /// <exception cref="IOException">An I/O error occurs. </exception>
        /// <exception cref="NotSupportedException">The stream does not support writing, or the stream is already closed. </exception>
        /// <exception cref="ObjectDisposedException">Methods were called after the stream was closed. </exception>
        public override void WriteByte(byte value)
        {
            // Lock down the file stream while we do this.
            lock (_lock)
            {
                CheckSessionIsOpen();

                // Setup the object for writing.
                SetupWrite();

                var writeBuffer = GetOrCreateWriteBuffer();

                // Flush the current buffer if it is full.
                if (_bufferPosition >= _writeBufferSize)
                {
                    using (var wait = new AutoResetEvent(initialState: false))
                    {
                        _session.RequestWrite(_handle, (ulong)(_position - _bufferPosition), writeBuffer, 0, _bufferPosition, wait);
                    }

                    _bufferPosition = 0;
                }

                // Write the byte into the buffer and advance the posn.
                writeBuffer[_bufferPosition++] = value;
                ++_position;
            }
        }

        /// <summary>
        /// Releases the unmanaged resources used by the <see cref="Stream"/> and optionally releases the 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 (_session != null)
            {
                if (disposing)
                {
                    lock (_lock)
                    {
                        if (_session != null)
                        {
                            _canRead = false;
                            _canSeek = false;
                            _canWrite = false;

                            if (_handle != null)
                            {
                                if (_session.IsOpen)
                                {
                                    if (_bufferOwnedByWrite)
                                    {
                                        FlushWriteBuffer();
                                    }

                                    _session.RequestClose(_handle);
                                }

                                _handle = null;
                            }

                            _session = null;
                        }
                    }
                }
            }
        }

        private byte[] GetOrCreateReadBuffer()
        {
            _readBuffer ??= new byte[_readBufferSize];
            return _readBuffer;
        }

        private byte[] GetOrCreateWriteBuffer()
        {
            _writeBuffer ??= new byte[_writeBufferSize];
            return _writeBuffer;
        }

        /// <summary>
        /// Flushes the read data from the buffer.
        /// </summary>
        private void FlushReadBuffer()
        {
            _bufferPosition = 0;
            _bufferLen = 0;
        }

        /// <summary>
        /// Flush any buffered write data to the file.
        /// </summary>
        private void FlushWriteBuffer()
        {
            if (_bufferPosition > 0)
            {
                using (var wait = new AutoResetEvent(initialState: false))
                {
                    _session.RequestWrite(_handle, (ulong)(_position - _bufferPosition), _writeBuffer, 0, _bufferPosition, wait);
                }

                _bufferPosition = 0;
            }
        }

        private async Task FlushWriteBufferAsync(CancellationToken cancellationToken)
        {
            if (_bufferPosition > 0)
            {
                await _session.RequestWriteAsync(_handle, (ulong)(_position - _bufferPosition), _writeBuffer, 0, _bufferPosition, cancellationToken).ConfigureAwait(false);
                _bufferPosition = 0;
            }
        }

        /// <summary>
        /// Setups the read.
        /// </summary>
        private void SetupRead()
        {
            if (!CanRead)
            {
                throw new NotSupportedException("Read not supported.");
            }

            if (_bufferOwnedByWrite)
            {
                FlushWriteBuffer();
                _bufferOwnedByWrite = false;
            }
        }

        /// <summary>
        /// Setups the write.
        /// </summary>
        private void SetupWrite()
        {
            if (!CanWrite)
            {
                throw new NotSupportedException("Write not supported.");
            }

            if (!_bufferOwnedByWrite)
            {
                FlushReadBuffer();
                _bufferOwnedByWrite = true;
            }
        }

        private void CheckSessionIsOpen()
        {
            ThrowHelper.ThrowObjectDisposedIf(_session is null, this);

            if (!_session.IsOpen)
            {
                throw new ObjectDisposedException(GetType().FullName, "Cannot access a closed SFTP session.");
            }
        }
    }
}