Best way to find position in the Stream where given byte sequence starts

Question

How do you think what is the best way to find position in the System.Stream where given byte sequence starts (first occurence):

public static long FindPositi         


        

Answer 1

I've reached this solution.

I did some benchmarks with an ASCII file that was 3.050 KB and 38803 lines. With a search byte array of 22 bytes in the last line of the file I've got the result in about 2.28 seconds (in a slow/old machine).

public static long FindPosition(Stream stream, byte[] byteSequence)
{
    if (byteSequence.Length > stream.Length)
        return -1;

    byte[] buffer = new byte[byteSequence.Length];

    using (BufferedStream bufStream = new BufferedStream(stream, byteSequence.Length))
    {
        int i;
        while ((i = bufStream.Read(buffer, 0, byteSequence.Length)) == byteSequence.Length)
        {
            if (byteSequence.SequenceEqual(buffer))
                return bufStream.Position - byteSequence.Length;
            else
                bufStream.Position -= byteSequence.Length - PadLeftSequence(buffer, byteSequence);
        }
    }

    return -1;
}

private static int PadLeftSequence(byte[] bytes, byte[] seqBytes)
{
    int i = 1;
    while (i < bytes.Length)
    {
        int n = bytes.Length - i;
        byte[] aux1 = new byte[n];
        byte[] aux2 = new byte[n];
        Array.Copy(bytes, i, aux1, 0, n);
        Array.Copy(seqBytes, aux2, n);
        if (aux1.SequenceEqual(aux2))
            return i;
        i++;
    }
    return i;
}