Чтение текстовых файлов в обратном направлении действительно сложно, если вы не используете кодировку фиксированного размера (например, ASCII). Когда у вас есть кодировка переменного размера (например, UTF-8), вам придется проверять, находитесь ли вы в середине символа или нет, когда выбираете данные.
Там нет ничего встроенного в структуру, и я подозреваю, что вам придется делать отдельное жесткое кодирование для каждой кодировки переменной ширины.
РЕДАКТИРОВАТЬ: Это было в некоторой степени проверено - но это не значит, что в нем все еще нет каких-то тонких ошибок. Он использует StreamUtil из MiscUtil, но я добавил только необходимый (новый) метод оттуда внизу. О, и это требует рефакторинга - есть один довольно здоровенный метод, как вы увидите:
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;
}
}
}
Обратная связь очень приветствуется. Это было весело:)