Cómo leer un archivo de texto a la inversa con iterador en C #

Necesito procesar un archivo grande, alrededor de 400K líneas y 200 M. Pero a veces tengo que procesar de abajo hacia arriba. ¿Cómo puedo usar el iterador (rendimiento de retorno) aquí? Básicamente no me gusta cargar todo en la memoria. Sé que es más eficiente usar iterador en .NET.

Leer archivos de texto al revés es realmente complicado a menos que esté utilizando una codificación de tamaño fijo (por ejemplo, ASCII). Cuando tenga codificación de tamaño variable (como UTF-8), tendrá que verificar si está en medio de un carácter o no cuando obtenga datos.

No hay nada integrado en el marco, y sospecho que tendría que hacer una codificación rígida separada para cada codificación de ancho variable.

EDITAR: Esto se ha probado un poco , pero eso no quiere decir que todavía no tenga algunos errores sutiles. Utiliza StreamUtil de MiscUtil, pero he incluido solo el método necesario (nuevo) desde allí en la parte inferior. Ah, y necesita refactorización; hay un método bastante considerable, como verá:

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Text;

namespace MiscUtil.IO
{
    /// <summary>
    /// Takes an encoding (defaulting to UTF-8) and a function which produces a seekable stream
    /// (or a filename for convenience) and yields lines from the end of the stream backwards.
    /// Only single byte encodings, and UTF-8 and Unicode, are supported. The stream
    /// returned by the function must be seekable.
    /// </summary>
    public sealed class ReverseLineReader : IEnumerable<string>
    {
        /// <summary>
        /// Buffer size to use by default. Classes with internal access can specify
        /// a different buffer size - this is useful for testing.
        /// </summary>
        private const int DefaultBufferSize = 4096;

        /// <summary>
        /// Means of creating a Stream to read from.
        /// </summary>
        private readonly Func<Stream> streamSource;

        /// <summary>
        /// Encoding to use when converting bytes to text
        /// </summary>
        private readonly Encoding encoding;

        /// <summary>
        /// Size of buffer (in bytes) to read each time we read from the
        /// stream. This must be at least as big as the maximum number of
        /// bytes for a single character.
        /// </summary>
        private readonly int bufferSize;

        /// <summary>
        /// Function which, when given a position within a file and a byte, states whether
        /// or not the byte represents the start of a character.
        /// </summary>
        private Func<long,byte,bool> characterStartDetector;

        /// <summary>
        /// Creates a LineReader from a stream source. The delegate is only
        /// called when the enumerator is fetched. UTF-8 is used to decode
        /// the stream into text.
        /// </summary>
        /// <param name="streamSource">Data source</param>
        public ReverseLineReader(Func<Stream> streamSource)
            : this(streamSource, Encoding.UTF8)
        {
        }

        /// <summary>
        /// Creates a LineReader from a filename. The file is only opened
        /// (or even checked for existence) when the enumerator is fetched.
        /// UTF8 is used to decode the file into text.
        /// </summary>
        /// <param name="filename">File to read from</param>
        public ReverseLineReader(string filename)
            : this(filename, Encoding.UTF8)
        {
        }

        /// <summary>
        /// Creates a LineReader from a filename. The file is only opened
        /// (or even checked for existence) when the enumerator is fetched.
        /// </summary>
        /// <param name="filename">File to read from</param>
        /// <param name="encoding">Encoding to use to decode the file into text</param>
        public ReverseLineReader(string filename, Encoding encoding)
            : this(() => File.OpenRead(filename), encoding)
        {
        }

        /// <summary>
        /// Creates a LineReader from a stream source. The delegate is only
        /// called when the enumerator is fetched.
        /// </summary>
        /// <param name="streamSource">Data source</param>
        /// <param name="encoding">Encoding to use to decode the stream into text</param>
        public ReverseLineReader(Func<Stream> streamSource, Encoding encoding)
            : this(streamSource, encoding, DefaultBufferSize)
        {
        }

        internal ReverseLineReader(Func<Stream> streamSource, Encoding encoding, int bufferSize)
        {
            this.streamSource = streamSource;
            this.encoding = encoding;
            this.bufferSize = bufferSize;
            if (encoding.IsSingleByte)
            {
                // For a single byte encoding, every byte is the start (and end) of a character
                characterStartDetector = (pos, data) => true;
            }
            else if (encoding is UnicodeEncoding)
            {
                // For UTF-16, even-numbered positions are the start of a character.
                // TODO: This assumes no surrogate pairs. More work required
                // to handle that.
                characterStartDetector = (pos, data) => (pos & 1) == 0;
            }
            else if (encoding is UTF8Encoding)
            {
                // For UTF-8, bytes with the top bit clear or the second bit set are the start of a character
                // See http://www.cl.cam.ac.uk/~mgk25/unicode.html
                characterStartDetector = (pos, data) => (data & 0x80) == 0 || (data & 0x40) != 0;
            }
            else
            {
                throw new ArgumentException("Only single byte, UTF-8 and Unicode encodings are permitted");
            }
        }

        /// <summary>
        /// Returns the enumerator reading strings backwards. If this method discovers that
        /// the returned stream is either unreadable or unseekable, a NotSupportedException is thrown.
        /// </summary>
        public IEnumerator<string> GetEnumerator()
        {
            Stream stream = streamSource();
            if (!stream.CanSeek)
            {
                stream.Dispose();
                throw new NotSupportedException("Unable to seek within stream");
            }
            if (!stream.CanRead)
            {
                stream.Dispose();
                throw new NotSupportedException("Unable to read within stream");
            }
            return GetEnumeratorImpl(stream);
        }

        private IEnumerator<string> GetEnumeratorImpl(Stream stream)
        {
            try
            {
                long position = stream.Length;

                if (encoding is UnicodeEncoding && (position & 1) != 0)
                {
                    throw new InvalidDataException("UTF-16 encoding provided, but stream has odd length.");
                }

                // Allow up to two bytes for data from the start of the previous
                // read which didn't quite make it as full characters
                byte[] buffer = new byte[bufferSize + 2];
                char[] charBuffer = new char[encoding.GetMaxCharCount(buffer.Length)];
                int leftOverData = 0;
                String previousEnd = null;
                // TextReader doesn't return an empty string if there's line break at the end
                // of the data. Therefore we don't return an empty string if it's our *first*
                // return.
                bool firstYield = true;

                // A line-feed at the start of the previous buffer means we need to swallow
                // the carriage-return at the end of this buffer - hence this needs declaring
                // way up here!
                bool swallowCarriageReturn = false;

                while (position > 0)
                {
                    int bytesToRead = Math.Min(position > int.MaxValue ? bufferSize : (int)position, bufferSize);

                    position -= bytesToRead;
                    stream.Position = position;
                    StreamUtil.ReadExactly(stream, buffer, bytesToRead);
                    // If we haven't read a full buffer, but we had bytes left
                    // over from before, copy them to the end of the buffer
                    if (leftOverData > 0 && bytesToRead != bufferSize)
                    {
                        // Buffer.BlockCopy doesn't document its behaviour with respect
                        // to overlapping data: we *might* just have read 7 bytes instead of
                        // 8, and have two bytes to copy...
                        Array.Copy(buffer, bufferSize, buffer, bytesToRead, leftOverData);
                    }
                    // We've now *effectively* read this much data.
                    bytesToRead += leftOverData;

                    int firstCharPosition = 0;
                    while (!characterStartDetector(position + firstCharPosition, buffer[firstCharPosition]))
                    {
                        firstCharPosition++;
                        // Bad UTF-8 sequences could trigger this. For UTF-8 we should always
                        // see a valid character start in every 3 bytes, and if this is the start of the file
                        // so we've done a short read, we should have the character start
                        // somewhere in the usable buffer.
                        if (firstCharPosition == 3 || firstCharPosition == bytesToRead)
                        {
                            throw new InvalidDataException("Invalid UTF-8 data");
                        }
                    }
                    leftOverData = firstCharPosition;

                    int charsRead = encoding.GetChars(buffer, firstCharPosition, bytesToRead - firstCharPosition, charBuffer, 0);
                    int endExclusive = charsRead;

                    for (int i = charsRead - 1; i >= 0; i--)
                    {
                        char lookingAt = charBuffer[i];
                        if (swallowCarriageReturn)
                        {
                            swallowCarriageReturn = false;
                            if (lookingAt == '\r')
                            {
                                endExclusive--;
                                continue;
                            }
                        }
                        // Anything non-line-breaking, just keep looking backwards
                        if (lookingAt != '\n' && lookingAt != '\r')
                        {
                            continue;
                        }
                        // End of CRLF? Swallow the preceding CR
                        if (lookingAt == '\n')
                        {
                            swallowCarriageReturn = true;
                        }
                        int start = i + 1;
                        string bufferContents = new string(charBuffer, start, endExclusive - start);
                        endExclusive = i;
                        string stringToYield = previousEnd == null ? bufferContents : bufferContents + previousEnd;
                        if (!firstYield || stringToYield.Length != 0)
                        {
                            yield return stringToYield;
                        }
                        firstYield = false;
                        previousEnd = null;
                    }

                    previousEnd = endExclusive == 0 ? null : (new string(charBuffer, 0, endExclusive) + previousEnd);

                    // If we didn't decode the start of the array, put it at the end for next time
                    if (leftOverData != 0)
                    {
                        Buffer.BlockCopy(buffer, 0, buffer, bufferSize, leftOverData);
                    }
                }
                if (leftOverData != 0)
                {
                    // At the start of the final buffer, we had the end of another character.
                    throw new InvalidDataException("Invalid UTF-8 data at start of stream");
                }
                if (firstYield && string.IsNullOrEmpty(previousEnd))
                {
                    yield break;
                }
                yield return previousEnd ?? "";
            }
            finally
            {
                stream.Dispose();
            }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }
}


// StreamUtil.cs:
public static class StreamUtil
{
    public static void ReadExactly(Stream input, byte[] buffer, int bytesToRead)
    {
        int index = 0;
        while (index < bytesToRead)
        {
            int read = input.Read(buffer, index, bytesToRead - index);
            if (read == 0)
            {
                throw new EndOfStreamException
                    (String.Format("End of stream reached with {0} byte{1} left to read.",
                                   bytesToRead - index,
                                   bytesToRead - index == 1 ? "s" : ""));
            }
            index += read;
        }
    }
}

Comentarios muy bienvenidos. Esto fue divertido :)

Atención: este enfoque no funciona (explicado en EDITAR)

Puede usar File.ReadLines para obtener líneas iterador

foreach (var line in File.ReadLines(@"C:\temp\ReverseRead.txt").Reverse())
{
    if (noNeedToReadFurther)
        break;

    // process line here
    Console.WriteLine(line);
}

EDITAR:

Después de leer el comentario de applejacks01 , ejecuté algunas pruebas y parece que .Reverse() realmente carga un archivo completo.

Solía File.ReadLines()imprimir la primera línea de un archivo de 40 MB ; el uso de memoria de la aplicación de consola era de 5 MB . Luego, se utiliza File.ReadLines().Reverse()para imprimir la última línea del mismo archivo; el uso de memoria era de 95 MB .

Conclusión

Independientemente de lo que esté haciendo 'Reverse ()', no es una buena opción para leer la parte inferior de un archivo grande.

Para crear un iterador de archivos, puede hacer esto:

EDITAR:

Esta es mi versión fija de un lector de archivos inverso de ancho fijo:

public static IEnumerable<string> readFile()
{
    using (FileStream reader = new FileStream(@"c:\test.txt",FileMode.Open,FileAccess.Read))
    {
        int i=0;
        StringBuilder lineBuffer = new StringBuilder();
        int byteRead;
        while (-i < reader.Length)
        {
            reader.Seek(--i, SeekOrigin.End);
            byteRead = reader.ReadByte();
            if (byteRead == 10 && lineBuffer.Length > 0)
            {
                yield return Reverse(lineBuffer.ToString());
                lineBuffer.Remove(0, lineBuffer.Length);
            }
            lineBuffer.Append((char)byteRead);
        }
        yield return Reverse(lineBuffer.ToString());
        reader.Close();
    }
}

public static string Reverse(string str)
{
    char[] arr = new char[str.Length];
    for (int i = 0; i < str.Length; i++)
        arr[i] = str[str.Length - 1 - i];
    return new string(arr);
}

Puse el archivo en una lista línea por línea, luego usé List.Reverse ();

        StreamReader objReader = new StreamReader(filename);
        string sLine = "";
        ArrayList arrText = new ArrayList();

        while (sLine != null)
        {
            sLine = objReader.ReadLine();
            if (sLine != null)
                arrText.Add(sLine);
        }
        objReader.Close();


        arrText.Reverse();

        foreach (string sOutput in arrText)
        {

...

Puede leer el archivo un carácter a la vez hacia atrás y almacenar en caché todos los caracteres hasta llegar a un retorno de carro y / o salto de línea.

Luego, invierte la cuerda recolectada y grita como una línea.

Sé que esta publicación es muy antigua, pero como no pude encontrar cómo usar la solución más votada, finalmente encontré esto: aquí está la mejor respuesta que encontré con un bajo costo de memoria en VB y C #

http://www.blakepell.com/2010-11-29-backward-file-reader-vb-csharp-source

¡Espero ayudar a otros con eso porque me tomó horas encontrar finalmente esta publicación!

[Editar]

Aquí está el código de c #:

//*********************************************************************************************************************************
//
//             Class:  BackwardReader
//      Initial Date:  11/29/2010
//     Last Modified:  11/29/2010
//     Programmer(s):  Original C# Source - the_real_herminator
//                     http://social.msdn.microsoft.com/forums/en-US/csharpgeneral/thread/9acdde1a-03cd-4018-9f87-6e201d8f5d09
//                     VB Converstion - Blake Pell
//
//*********************************************************************************************************************************

using System.Text;
using System.IO;
public class BackwardReader
{
    private string path;
    private FileStream fs = null;
    public BackwardReader(string path)
    {
        this.path = path;
        fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
        fs.Seek(0, SeekOrigin.End);
    }
    public string Readline()
    {
        byte[] line;
        byte[] text = new byte[1];
        long position = 0;
        int count;
        fs.Seek(0, SeekOrigin.Current);
        position = fs.Position;
        //do we have trailing rn?
        if (fs.Length > 1)
        {
            byte[] vagnretur = new byte[2];
            fs.Seek(-2, SeekOrigin.Current);
            fs.Read(vagnretur, 0, 2);
            if (ASCIIEncoding.ASCII.GetString(vagnretur).Equals("rn"))
            {
                //move it back
                fs.Seek(-2, SeekOrigin.Current);
                position = fs.Position;
            }
        }
        while (fs.Position > 0)
        {
            text.Initialize();
            //read one char
            fs.Read(text, 0, 1);
            string asciiText = ASCIIEncoding.ASCII.GetString(text);
            //moveback to the charachter before
            fs.Seek(-2, SeekOrigin.Current);
            if (asciiText.Equals("n"))
            {
                fs.Read(text, 0, 1);
                asciiText = ASCIIEncoding.ASCII.GetString(text);
                if (asciiText.Equals("r"))
                {
                    fs.Seek(1, SeekOrigin.Current);
                    break;
                }
            }
        }
        count = int.Parse((position - fs.Position).ToString());
        line = new byte[count];
        fs.Read(line, 0, count);
        fs.Seek(-count, SeekOrigin.Current);
        return ASCIIEncoding.ASCII.GetString(line);
    }
    public bool SOF
    {
        get
        {
            return fs.Position == 0;
        }
    }
    public void Close()
    {
        fs.Close();
    }
}

Solución muy rápida para archivos grandes . Utilice el cmdlet Get-Content de powershell con la opción Tail. Llamar a powershell dará un poco de sobrecarga, pero para archivos grandes no tiene valor

using System.Management.Automation;

const string FILE_PATH = @"d:\temp\b_media_27_34_0000_25393.txt";
var ps = PowerShell.Create();
ps.AddCommand("Get-Content")
    .AddParameter("Path", FILE_PATH)
    .AddParameter("Tail", 1);
var psResults = ps.Invoke();
var lastLine = psResults.FirstOrDefault()?.BaseObject.ToString();

ps.Dispose();

Referencia de powershell requerida

C: \ Archivos de programa (x86) \ Reference Assemblies \ Microsoft \ WindowsPowerShell \ 3.0 \ System.Management.Automation.dll

También agrego mi solución. Después de leer algunas respuestas, nada encajaba realmente en mi caso. Estoy leyendo byte a byte desde atrás hasta que encuentro un LineFeed, luego estoy recuperando los bytes recopilados como una cadena, sin usar el almacenamiento en búfer .

Uso:

var reader = new ReverseTextReader(path);
while (!reader.EndOfStream)
{
    Console.WriteLine(reader.ReadLine());  
}

Implementación:

public class ReverseTextReader
{
    private const int LineFeedLf = 10;
    private const int LineFeedCr = 13;
    private readonly Stream _stream;
    private readonly Encoding _encoding;

    public bool EndOfStream => _stream.Position == 0;

    public ReverseTextReader(Stream stream, Encoding encoding)
    {
        _stream = stream;
        _encoding = encoding;
        _stream.Position = _stream.Length;
    }

    public string ReadLine()
    {
        if (_stream.Position == 0) return null;

        var line = new List<byte>();
        var endOfLine = false;
        while (!endOfLine)
        {
            var b = _stream.ReadByteFromBehind();

            if (b == -1 || b == LineFeedLf)
            {
                endOfLine = true;
            } 
            line.Add(Convert.ToByte(b));
        }

        line.Reverse();
        return _encoding.GetString(line.ToArray());
    }
}

public static class StreamExtensions
{
    public static int ReadByteFromBehind(this Stream stream)
    {
        if (stream.Position == 0) return -1;

        stream.Position = stream.Position - 1;
        var value = stream.ReadByte();
        stream.Position = stream.Position - 1;
        return value;
    }
}

Quería hacer algo similar. Aquí está mi código. Esta clase creará archivos temporales que contienen fragmentos del archivo grande. Esto evitará la hinchazón de la memoria. El usuario puede especificar si desea invertir el archivo. En consecuencia, devolverá el contenido de manera inversa.

Esta clase también se puede utilizar para escribir macrodatos en un solo archivo sin sobrecargar la memoria.

Envíe sus comentarios.

        using System;
        using System.Collections.Generic;
        using System.Diagnostics;
        using System.IO;
        using System.Linq;
        using System.Text;
        using System.Threading.Tasks;

        namespace BigFileService
        {    
            public class BigFileDumper
            {
                /// <summary>
                /// Buffer that will store the lines until it is full.
                /// Then it will dump it to temp files.
                /// </summary>
                public int CHUNK_SIZE = 1000;
                public bool ReverseIt { get; set; }
                public long TotalLineCount { get { return totalLineCount; } }
                private long totalLineCount;
                private int BufferCount = 0;
                private StreamWriter Writer;
                /// <summary>
                /// List of files that would store the chunks.
                /// </summary>
                private List<string> LstTempFiles;
                private string ParentDirectory;
                private char[] trimchars = { '/', '\\'};


                public BigFileDumper(string FolderPathToWrite)
                {
                    this.LstTempFiles = new List<string>();
                    this.ParentDirectory = FolderPathToWrite.TrimEnd(trimchars) + "\\" + "BIG_FILE_DUMP";
                    this.totalLineCount = 0;
                    this.BufferCount = 0;
                    this.Initialize();
                }

                private void Initialize()
                {
                    // Delete existing directory.
                    if (Directory.Exists(this.ParentDirectory))
                    {
                        Directory.Delete(this.ParentDirectory, true);
                    }

                    // Create a new directory.
                    Directory.CreateDirectory(this.ParentDirectory);
                }

                public void WriteLine(string line)
                {
                    if (this.BufferCount == 0)
                    {
                        string newFile = "DumpFile_" + LstTempFiles.Count();
                        LstTempFiles.Add(newFile);
                        Writer = new StreamWriter(this.ParentDirectory + "\\" + newFile);
                    }
                    // Keep on adding in the buffer as long as size is okay.
                    if (this.BufferCount < this.CHUNK_SIZE)
                    {
                        this.totalLineCount++; // main count
                        this.BufferCount++; // Chunk count.
                        Writer.WriteLine(line);
                    }
                    else
                    {
                        // Buffer is full, time to create a new file.
                        // Close the existing file first.
                        Writer.Close();
                        // Make buffer count 0 again.
                        this.BufferCount = 0;
                        this.WriteLine(line);
                    }
                }

                public void Close()
                {
                    if (Writer != null)
                        Writer.Close();
                }

                public string GetFullFile()
                {
                    if (LstTempFiles.Count <= 0)
                    {
                        Debug.Assert(false, "There are no files created.");
                        return "";
                    }
                    string returnFilename = this.ParentDirectory + "\\" + "FullFile";
                    if (File.Exists(returnFilename) == false)
                    {
                        // Create a consolidated file from the existing small dump files.
                        // Now this is interesting. We will open the small dump files one by one.
                        // Depending on whether the user require inverted file, we will read them in descending order & reverted, 
                        // or ascending order in normal way.

                        if (this.ReverseIt)
                            this.LstTempFiles.Reverse();

                        foreach (var fileName in LstTempFiles)
                        {
                            string fullFileName = this.ParentDirectory + "\\" + fileName;
// FileLines will use small memory depending on size of CHUNK. User has control.
                            var fileLines = File.ReadAllLines(fullFileName);

                            // Time to write in the writer.
                            if (this.ReverseIt)
                                fileLines = fileLines.Reverse().ToArray();

                            // Write the lines 
                            File.AppendAllLines(returnFilename, fileLines);
                        }
                    }

                    return returnFilename;
                }
            }
        }

Este servicio se puede utilizar de la siguiente manera:

void TestBigFileDump_File(string BIG_FILE, string FOLDER_PATH_FOR_CHUNK_FILES)
        {
            // Start processing the input Big file.
            StreamReader reader = new StreamReader(BIG_FILE);
            // Create a dump file class object to handle efficient memory management.
            var bigFileDumper = new BigFileDumper(FOLDER_PATH_FOR_CHUNK_FILES);
            // Set to reverse the output file.
            bigFileDumper.ReverseIt = true;
            bigFileDumper.CHUNK_SIZE = 100; // How much at a time to keep in RAM before dumping to local file.

            while (reader.EndOfStream == false)
            {
                string line = reader.ReadLine();
                bigFileDumper.WriteLine(line);
            }
            bigFileDumper.Close();
            reader.Close();

            // Get back full reversed file.
            var reversedFilename = bigFileDumper.GetFullFile();
            Console.WriteLine("Check output file - " + reversedFilename);
        }

Aquí ya hay buenas respuestas, y aquí hay otra clase compatible con LINQ que puede usar y que se centra en el rendimiento y la compatibilidad con archivos grandes. Asume un terminador de línea "\ r \ n".

Uso :

var reader = new ReverseTextReader(@"C:\Temp\ReverseTest.txt");
while (!reader.EndOfStream)
    Console.WriteLine(reader.ReadLine());

Clase ReverseTextReader :

/// <summary>
/// Reads a text file backwards, line-by-line.
/// </summary>
/// <remarks>This class uses file seeking to read a text file of any size in reverse order.  This
/// is useful for needs such as reading a log file newest-entries first.</remarks>
public sealed class ReverseTextReader : IEnumerable<string>
{
    private const int BufferSize = 16384;   // The number of bytes read from the uderlying stream.
    private readonly Stream _stream;        // Stores the stream feeding data into this reader
    private readonly Encoding _encoding;    // Stores the encoding used to process the file
    private byte[] _leftoverBuffer;         // Stores the leftover partial line after processing a buffer
    private readonly Queue<string> _lines;  // Stores the lines parsed from the buffer

    #region Constructors

    /// <summary>
    /// Creates a reader for the specified file.
    /// </summary>
    /// <param name="filePath"></param>
    public ReverseTextReader(string filePath)
        : this(new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read), Encoding.Default)
    { }

    /// <summary>
    /// Creates a reader using the specified stream.
    /// </summary>
    /// <param name="stream"></param>
    public ReverseTextReader(Stream stream)
        : this(stream, Encoding.Default)
    { }

    /// <summary>
    /// Creates a reader using the specified path and encoding.
    /// </summary>
    /// <param name="filePath"></param>
    /// <param name="encoding"></param>
    public ReverseTextReader(string filePath, Encoding encoding)
        : this(new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read), encoding)
    { }

    /// <summary>
    /// Creates a reader using the specified stream and encoding.
    /// </summary>
    /// <param name="stream"></param>
    /// <param name="encoding"></param>
    public ReverseTextReader(Stream stream, Encoding encoding)
    {          
        _stream = stream;
        _encoding = encoding;
        _lines = new Queue<string>(128);            
        // The stream needs to support seeking for this to work
        if(!_stream.CanSeek)
            throw new InvalidOperationException("The specified stream needs to support seeking to be read backwards.");
        if (!_stream.CanRead)
            throw new InvalidOperationException("The specified stream needs to support reading to be read backwards.");
        // Set the current position to the end of the file
        _stream.Position = _stream.Length;
        _leftoverBuffer = new byte[0];
    }

    #endregion

    #region Overrides

    /// <summary>
    /// Reads the next previous line from the underlying stream.
    /// </summary>
    /// <returns></returns>
    public string ReadLine()
    {
        // Are there lines left to read? If so, return the next one
        if (_lines.Count != 0) return _lines.Dequeue();
        // Are we at the beginning of the stream? If so, we're done
        if (_stream.Position == 0) return null;

        #region Read and Process the Next Chunk

        // Remember the current position
        var currentPosition = _stream.Position;
        var newPosition = currentPosition - BufferSize;
        // Are we before the beginning of the stream?
        if (newPosition < 0) newPosition = 0;
        // Calculate the buffer size to read
        var count = (int)(currentPosition - newPosition);
        // Set the new position
        _stream.Position = newPosition;
        // Make a new buffer but append the previous leftovers
        var buffer = new byte[count + _leftoverBuffer.Length];
        // Read the next buffer
        _stream.Read(buffer, 0, count);
        // Move the position of the stream back
        _stream.Position = newPosition;
        // And copy in the leftovers from the last buffer
        if (_leftoverBuffer.Length != 0)
            Array.Copy(_leftoverBuffer, 0, buffer, count, _leftoverBuffer.Length);
        // Look for CrLf delimiters
        var end = buffer.Length - 1;
        var start = buffer.Length - 2;
        // Search backwards for a line feed
        while (start >= 0)
        {
            // Is it a line feed?
            if (buffer[start] == 10)
            {
                // Yes.  Extract a line and queue it (but exclude the \r\n)
                _lines.Enqueue(_encoding.GetString(buffer, start + 1, end - start - 2));
                // And reset the end
                end = start;
            }
            // Move to the previous character
            start--;
        }
        // What's left over is a portion of a line. Save it for later.
        _leftoverBuffer = new byte[end + 1];
        Array.Copy(buffer, 0, _leftoverBuffer, 0, end + 1);
        // Are we at the beginning of the stream?
        if (_stream.Position == 0)
            // Yes.  Add the last line.
            _lines.Enqueue(_encoding.GetString(_leftoverBuffer, 0, end - 1));

        #endregion

        // If we have something in the queue, return it
        return _lines.Count == 0 ? null : _lines.Dequeue();
    }

    #endregion

    #region IEnumerator<string> Interface

    public IEnumerator<string> GetEnumerator()
    {
        string line;
        // So long as the next line isn't null...
        while ((line = ReadLine()) != null)
            // Read and return it.
            yield return line;
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        throw new NotImplementedException();
    }

    #endregion
}

En caso de que alguien más se encuentre con esto, lo resolví con el siguiente script de PowerShell que se puede modificar fácilmente en un script de C # con un pequeño esfuerzo.

[System.IO.FileStream]$fileStream = [System.IO.File]::Open("C:\Name_of_very_large_file.log", [System.IO.FileMode]::Open, [System.IO.FileAccess]::Read, [System.IO.FileShare]::ReadWrite)
[System.IO.BufferedStream]$bs = New-Object System.IO.BufferedStream $fileStream;
[System.IO.StreamReader]$sr = New-Object System.IO.StreamReader $bs;


$buff = New-Object char[] 20;
$seek = $bs.Seek($fileStream.Length - 10000, [System.IO.SeekOrigin]::Begin);

while(($line = $sr.ReadLine()) -ne $null)
{
     $line;
}

Básicamente, esto comienza a leer desde los últimos 10,000 caracteres de un archivo, generando cada línea.