Read a stream in reverse

Question

Is there a way to read a stream backwards? The stream is seekable.

Answer 1

You can reverse a stream by doing the following:

var reversedStream = new ReverseStream(stream);

To reverse a file:

using (var inputFile = File.OpenRead("file.dat"))
using (var inputFileReversed = new ReverseStream(inputFile))
using (var outputFile = File.Open("file.dat.reversed", FileMode.Create, FileAccess.Write))
{
    inputFileReversed.CopyTo(outputFile);
}

Which uses the ReverseStream class:

public class ReverseStream : Stream
{
    readonly Stream stream;
    public ReverseStream(Stream stream)
    {
        if (!stream.CanSeek) throw new Exception("Stream cannot seek");

        stream.Seek(stream.Position, SeekOrigin.End);
        this.stream = stream;
    }

    public override bool CanRead => true;

    public override bool CanSeek => true;

    public override bool CanWrite => false;

    public override long Length => stream.Length;

    public override long Position
    {
        get
        {
            var position = stream.Length - stream.Position;
            return position;
        }

        set
        {
            stream.Position = stream.Length - value;
        }
    }

    public override int Read(byte[] buffer, int offset, int count)
    {
        if (stream.Position == 0) return 0;

        var startReadFrom = stream.Position - count;
        if (startReadFrom < 0)
        {
            count += (int)startReadFrom;
            startReadFrom = 0;
        }

        stream.Seek(startReadFrom, SeekOrigin.Begin);
        var bytesRead = stream.Read(buffer, offset, count);
        stream.Seek(startReadFrom, SeekOrigin.Begin);

        Array.Reverse(buffer, offset, bytesRead);

        return bytesRead;
    }

    public override long Seek(long offset, SeekOrigin origin)
    {
        switch (origin)
        {
            case SeekOrigin.Begin:
                stream.Seek(offset, SeekOrigin.End);
                break;

            case SeekOrigin.End:
                stream.Seek(offset, SeekOrigin.Begin);
                break;

            case SeekOrigin.Current:
                stream.Seek(-offset, SeekOrigin.Current);
                break;
        }

        return Position;
    }

    public override void SetLength(long value)
    {
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
    }
    public override void Flush()
    {
    }
}