Reading text files line by line, with exact offset/position reporting

Question

My simple requirement: Reading a huge (> a million) line test file (For this example assume it's a CSV of some sorts) and keeping a reference to the beginning of that line for faster lookup in the future (read a line, starting at X).

I tried the naive and easy way first, using a StreamWriter and accessing the underlying BaseStream.Position. Unfortunately that doesn't work as I intended:

Given a file containing the following

Foo
Bar
Baz
Bla
Fasel

and this very simple code

using (var sr = new StreamReader(@"C:\Temp\LineTest.txt")) {
  string line;
  long pos = sr.BaseStream.Position;
  while ((line = sr.ReadLine()) != null) {
    Console.Write("{0:d3} ", pos);
    Console.WriteLine(line);
    pos = sr.BaseStream.Position;
  }
}

the output is:

000 Foo
025 Bar
025 Baz
025 Bla
025 Fasel

I can imagine that the stream is trying to be helpful/efficient and probably reads in (big) chunks whenever new data is necessary. For me this is bad..

The question, finally: Any way to get the (byte, char) offset while reading a file line by line without using a basic Stream and messing with \r \n \r\n and string encoding etc. manually? Not a big deal, really, I just don't like to build things that might exist already..

Answer 1

You could create a TextReader wrapper, which would track the current position in the base TextReader :

public class TrackingTextReader : TextReader
{
    private TextReader _baseReader;
    private int _position;

    public TrackingTextReader(TextReader baseReader)
    {
        _baseReader = baseReader;
    }

    public override int Read()
    {
        _position++;
        return _baseReader.Read();
    }

    public override int Peek()
    {
        return _baseReader.Peek();
    }

    public int Position
    {
        get { return _position; }
    }
}

You could then use it as follows :

string text = @"Foo
Bar
Baz
Bla
Fasel";

using (var reader = new StringReader(text))
using (var trackingReader = new TrackingTextReader(reader))
{
    string line;
    while ((line = trackingReader.ReadLine()) != null)
    {
        Console.WriteLine("{0:d3} {1}", trackingReader.Position, line);
    }
}

Answer 2

After searching, testing and do something crazy, there is my code to solve (I'm currently using this code in my product).

public sealed class TextFileReader : IDisposable
{

    FileStream _fileStream = null;
    BinaryReader _binReader = null;
    StreamReader _streamReader = null;
    List<string> _lines = null;
    long _length = -1;

    /// <summary>
    /// Initializes a new instance of the <see cref="TextFileReader"/> class with default encoding (UTF8).
    /// </summary>
    /// <param name="filePath">The path to text file.</param>
    public TextFileReader(string filePath) : this(filePath, Encoding.UTF8) { }

    /// <summary>
    /// Initializes a new instance of the <see cref="TextFileReader"/> class.
    /// </summary>
    /// <param name="filePath">The path to text file.</param>
    /// <param name="encoding">The encoding of text file.</param>
    public TextFileReader(string filePath, Encoding encoding)
    {
        if (!File.Exists(filePath))
            throw new FileNotFoundException("File (" + filePath + ") is not found.");

        _fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read);
        _length = _fileStream.Length;
        _binReader = new BinaryReader(_fileStream, encoding);
    }

    /// <summary>
    /// Reads a line of characters from the current stream at the current position and returns the data as a string.
    /// </summary>
    /// <returns>The next line from the input stream, or null if the end of the input stream is reached</returns>
    public string ReadLine()
    {
        if (_binReader.PeekChar() == -1)
            return null;

        string line = "";
        int nextChar = _binReader.Read();
        while (nextChar != -1)
        {
            char current = (char)nextChar;
            if (current.Equals('\n'))
                break;
            else if (current.Equals('\r'))
            {
                int pickChar = _binReader.PeekChar();
                if (pickChar != -1 && ((char)pickChar).Equals('\n'))
                    nextChar = _binReader.Read();
                break;
            }
            else
                line += current;
            nextChar = _binReader.Read();
        }
        return line;
    }

    /// <summary>
    /// Reads some lines of characters from the current stream at the current position and returns the data as a collection of string.
    /// </summary>
    /// <param name="totalLines">The total number of lines to read (set as 0 to read from current position to end of file).</param>
    /// <returns>The next lines from the input stream, or empty collectoin if the end of the input stream is reached</returns>
    public List<string> ReadLines(int totalLines)
    {
        if (totalLines < 1 && this.Position == 0)
            return this.ReadAllLines();

        _lines = new List<string>();
        int counter = 0;
        string line = this.ReadLine();
        while (line != null)
        {
            _lines.Add(line);
            counter++;
            if (totalLines > 0 && counter >= totalLines)
                break;
            line = this.ReadLine();
        }
        return _lines;
    }

    /// <summary>
    /// Reads all lines of characters from the current stream (from the begin to end) and returns the data as a collection of string.
    /// </summary>
    /// <returns>The next lines from the input stream, or empty collectoin if the end of the input stream is reached</returns>
    public List<string> ReadAllLines()
    {
        if (_streamReader == null)
            _streamReader = new StreamReader(_fileStream);
        _streamReader.BaseStream.Seek(0, SeekOrigin.Begin);
        _lines = new List<string>();
        string line = _streamReader.ReadLine();
        while (line != null)
        {
            _lines.Add(line);
            line = _streamReader.ReadLine();
        }
        return _lines;
    }

    /// <summary>
    /// Gets the length of text file (in bytes).
    /// </summary>
    public long Length
    {
        get { return _length; }
    }

    /// <summary>
    /// Gets or sets the current reading position.
    /// </summary>
    public long Position
    {
        get
        {
            if (_binReader == null)
                return -1;
            else
                return _binReader.BaseStream.Position;
        }
        set
        {
            if (_binReader == null)
                return;
            else if (value >= this.Length)
                this.SetPosition(this.Length);
            else
                this.SetPosition(value);
        }
    }

    void SetPosition(long position)
    {
        _binReader.BaseStream.Seek(position, SeekOrigin.Begin);
    }

    /// <summary>
    /// Gets the lines after reading.
    /// </summary>
    public List<string> Lines
    {
        get
        {
            return _lines;
        }
    }

    /// <summary>
    /// Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
    /// </summary>
    public void Dispose()
    {
        if (_binReader != null)
            _binReader.Close();
        if (_streamReader != null)
        {
            _streamReader.Close();
            _streamReader.Dispose();
        }
        if (_fileStream != null)
        {
            _fileStream.Close();
            _fileStream.Dispose();
        }
    }

    ~TextFileReader()
    {
        this.Dispose();
    }
}

Answer 3

This is really tough issue. After very long and exhausting enumeration of different solutions in the internet (including solutions from this thread, thank you!) I had to create my own bicycle.

I had following requirements:

• Performance - reading must be very fast, so reading one char at the time or using reflection are not acceptable, so buffering is required
• Streaming - file can be huge, so it is not acceptable to read it to memory entirely
• Tailing - file tailing should be available
• Long lines - lines can be very long, so buffer can't be limited

• Stable - single byte error was immediately visible during usage. Unfortunately for me, several implementations I found were with stability problems

  public class OffsetStreamReader
  {
      private const int InitialBufferSize = 4096;    
      private readonly char _bom;
      private readonly byte _end;
      private readonly Encoding _encoding;
      private readonly Stream _stream;
      private readonly bool _tail;
  
      private byte[] _buffer;
      private int _processedInBuffer;
      private int _informationInBuffer;
  
      public OffsetStreamReader(Stream stream, bool tail)
      {
          _buffer = new byte[InitialBufferSize];
          _processedInBuffer = InitialBufferSize;
  
          if (stream == null || !stream.CanRead)
              throw new ArgumentException("stream");
  
          _stream = stream;
          _tail = tail;
          _encoding = Encoding.UTF8;
  
          _bom = '\uFEFF';
          _end = _encoding.GetBytes(new [] {'\n'})[0];
      }
  
      public long Offset { get; private set; }
  
      public string ReadLine()
      {
          // Underlying stream closed
          if (!_stream.CanRead)
              return null;
  
          // EOF
          if (_processedInBuffer == _informationInBuffer)
          {
              if (_tail)
              {
                  _processedInBuffer = _buffer.Length;
                  _informationInBuffer = 0;
                  ReadBuffer();
              }
  
              return null;
          }
  
          var lineEnd = Search(_buffer, _end, _processedInBuffer);
          var haveEnd = true;
  
          // File ended but no finalizing newline character
          if (lineEnd.HasValue == false && _informationInBuffer + _processedInBuffer < _buffer.Length)
          {
              if (_tail)
                  return null;
              else
              {
                  lineEnd = _informationInBuffer;
                  haveEnd = false;
              }
          }
  
          // No end in current buffer
          if (!lineEnd.HasValue)
          {
              ReadBuffer();
              if (_informationInBuffer != 0)
                  return ReadLine();
  
              return null;
          }
  
          var arr = new byte[lineEnd.Value - _processedInBuffer];
          Array.Copy(_buffer, _processedInBuffer, arr, 0, arr.Length);
  
          Offset = Offset + lineEnd.Value - _processedInBuffer + (haveEnd ? 1 : 0);
          _processedInBuffer = lineEnd.Value + (haveEnd ? 1 : 0);
  
          return _encoding.GetString(arr).TrimStart(_bom).TrimEnd('\r', '\n');
      }
  
      private void ReadBuffer()
      {
          var notProcessedPartLength = _buffer.Length - _processedInBuffer;
  
          // Extend buffer to be able to fit whole line to the buffer
          // Was     [NOT_PROCESSED]
          // Become  [NOT_PROCESSED        ]
          if (notProcessedPartLength == _buffer.Length)
          {
              var extendedBuffer = new byte[_buffer.Length + _buffer.Length/2];
              Array.Copy(_buffer, extendedBuffer, _buffer.Length);
              _buffer = extendedBuffer;
          }
  
          // Copy not processed information to the begining
          // Was    [PROCESSED NOT_PROCESSED]
          // Become [NOT_PROCESSED          ]
          Array.Copy(_buffer, (long) _processedInBuffer, _buffer, 0, notProcessedPartLength);
  
          // Read more information to the empty part of buffer
          // Was    [ NOT_PROCESSED                   ]
          // Become [ NOT_PROCESSED NEW_NOT_PROCESSED ]
          _informationInBuffer = notProcessedPartLength + _stream.Read(_buffer, notProcessedPartLength, _buffer.Length - notProcessedPartLength);
  
          _processedInBuffer = 0;
      }
  
      private int? Search(byte[] buffer, byte byteToSearch, int bufferOffset)
      {
          for (int i = bufferOffset; i < buffer.Length - 1; i++)
          {
              if (buffer[i] == byteToSearch)
                  return i;
          }
          return null;
      }
  }
  

Answer 4

Though Thomas Levesque's solution works well, here's mine. It uses reflection so it will be slower, but it's encoding-independent. Plus I added Seek extension too.

/// <summary>Useful <see cref="StreamReader"/> extentions.</summary>
public static class StreamReaderExtentions
{
    /// <summary>Gets the position within the <see cref="StreamReader.BaseStream"/> of the <see cref="StreamReader"/>.</summary>
    /// <remarks><para>This method is quite slow. It uses reflection to access private <see cref="StreamReader"/> fields. Don't use it too often.</para></remarks>
    /// <param name="streamReader">Source <see cref="StreamReader"/>.</param>
    /// <exception cref="ArgumentNullException">Occurs when passed <see cref="StreamReader"/> is null.</exception>
    /// <returns>The current position of this stream.</returns>
    public static long GetPosition(this StreamReader streamReader)
    {
        if (streamReader == null)
            throw new ArgumentNullException("streamReader");

        var charBuffer = (char[])streamReader.GetType().InvokeMember("charBuffer", BindingFlags.DeclaredOnly | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.GetField, null, streamReader, null);
        var charPos = (int)streamReader.GetType().InvokeMember("charPos", BindingFlags.DeclaredOnly | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.GetField, null, streamReader, null);
        var charLen = (int)streamReader.GetType().InvokeMember("charLen", BindingFlags.DeclaredOnly | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.GetField, null, streamReader, null);

        var offsetLength = streamReader.CurrentEncoding.GetByteCount(charBuffer, charPos, charLen - charPos);

        return streamReader.BaseStream.Position - offsetLength;
    }

    /// <summary>Sets the position within the <see cref="StreamReader.BaseStream"/> of the <see cref="StreamReader"/>.</summary>
    /// <remarks>
    /// <para><see cref="StreamReader.BaseStream"/> should be seekable.</para>
    /// <para>This method is quite slow. It uses reflection and flushes the charBuffer of the <see cref="StreamReader.BaseStream"/>. Don't use it too often.</para>
    /// </remarks>
    /// <param name="streamReader">Source <see cref="StreamReader"/>.</param>
    /// <param name="position">The point relative to origin from which to begin seeking.</param>
    /// <param name="origin">Specifies the beginning, the end, or the current position as a reference point for origin, using a value of type <see cref="SeekOrigin"/>. </param>
    /// <exception cref="ArgumentNullException">Occurs when passed <see cref="StreamReader"/> is null.</exception>
    /// <exception cref="ArgumentException">Occurs when <see cref="StreamReader.BaseStream"/> is not seekable.</exception>
    /// <returns>The new position in the stream. This position can be different to the <see cref="position"/> because of the preamble.</returns>
    public static long Seek(this StreamReader streamReader, long position, SeekOrigin origin)
    {
        if (streamReader == null)
            throw new ArgumentNullException("streamReader");

        if (!streamReader.BaseStream.CanSeek)
            throw new ArgumentException("Underlying stream should be seekable.", "streamReader");

        var preamble = (byte[])streamReader.GetType().InvokeMember("_preamble", BindingFlags.DeclaredOnly | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.GetField, null, streamReader, null);
        if (preamble.Length > 0 && position < preamble.Length) // preamble or BOM must be skipped
            position += preamble.Length;

        var newPosition = streamReader.BaseStream.Seek(position, origin); // seek
        streamReader.DiscardBufferedData(); // this updates the buffer

        return newPosition;
    }
}

Answer 5

Would this work:

using (var sr = new StreamReader(@"C:\Temp\LineTest.txt")) {
  string line;
  long pos = 0;
  while ((line = sr.ReadLine()) != null) {
    Console.Write("{0:d3} ", pos);
    Console.WriteLine(line);
    pos += line.Length;
  }
}