Question

I need to check a given byte or series of bytes for a particular sequence of bits as follows:

• Can start with zero or more number of 0s.
• Can start with zero or more number of 1s.
• Must contain at least one 0 at the end.

In other words, if the value of bytes is not 0, then we are only interested in values that contain consecutive 1s followed by at least one 0 at the end.

I wrote the following code to do just that but wanted to make sure that it highly optimized. I feel that the multiple checks within the if branches could be optimized but am not sure how. Please advise.

// The parameter [number] will NEVER be negative.
public static bool ConformsToPattern (System.Numerics.BigInteger number)
{
    byte [] bytes = null;
    bool moreOnesPossible = true;

    if (number == 0) // 00000000
    {
        return (true); // All bits are zero.
    }
    else
    {
        bytes = number.ToByteArray();

        if ((bytes [bytes.Length - 1] & 1) == 1)
        {
            return (false);
        }
        else
        {
            for (byte b=0; b < bytes.Length; b++)
            {
                if (moreOnesPossible)
                {
                    if
                    (
                        (bytes [b] == 1) // 00000001
                        || (bytes [b] == 3) // 00000011
                        || (bytes [b] == 7) // 00000111
                        || (bytes [b] == 15) // 00001111
                        || (bytes [b] == 31) // 00011111
                        || (bytes [b] == 63) // 00111111
                        || (bytes [b] == 127) // 01111111
                        || (bytes [b] == 255) // 11111111
                    )
                    {
                        // So far so good. Continue to the next byte with
                        // a possibility of more consecutive 1s.
                    }
                    else if
                    (
                        (bytes [b] == 128) // 10000000
                        || (bytes [b] == 192) // 11000000
                        || (bytes [b] == 224) // 11100000
                        || (bytes [b] == 240) // 11110000
                        || (bytes [b] == 248) // 11111000
                        || (bytes [b] == 252) // 11111100
                        || (bytes [b] == 254) // 11111110
                    )
                    {
                        moreOnesPossible = false;
                    }
                    else
                    {
                        return (false);
                    }
                }
                else
                {
                    if (bytes [b] > 0)
                    {
                        return (false);
                    }
                }
            }
        }
    }

    return (true);
}

IMPORTANT: The argument [number] sent to the function will NEVER be negative so no need to check for the sign bit.

Answer 1

How about something like this? If you find a one, the only things after that can be 1s until a 0 is found. After that, only 0s. This looks like it’ll do the trick a little faster because it doesn’t do unnecessary or conditions.

// The parameter [number] will NEVER be negative.
public static bool ConformsToPattern (System.Numerics.BigInteger number)
{
    byte [] bytes = null;
    bool moreOnesPossible = true;
    bool foundFirstOne = false;

    if (number == 0) // 00000000
    {
        return (true); // All bits are zero.
    }
    else
    {
        bytes = number.ToByteArray();

        if ((bytes [bytes.Length - 1] & 1) == 1)
        {
            return (false);
        }
        else
        {
            for (byte b=0; b < bytes.Length; b++)
            {
                if (moreOnesPossible)
                {
                    if(!foundFirstOne)
                    {
                        if
                        (
                            (bytes [b] == 1) // 00000001
                            || (bytes [b] == 3) // 00000011
                            || (bytes [b] == 7) // 00000111
                            || (bytes [b] == 15) // 00001111
                            || (bytes [b] == 31) // 00011111
                            || (bytes [b] == 63) // 00111111
                            || (bytes [b] == 127) // 01111111
                            || (bytes [b] == 255) // 11111111
                        )
                        {
                            foundFirstOne = true;
                            // So far so good. Continue to the next byte with
                            // a possibility of more consecutive 1s.
                        }
                        else if
                        (
                            (bytes [b] == 128) // 10000000
                            || (bytes [b] == 192) // 11000000
                            || (bytes [b] == 224) // 11100000
                            || (bytes [b] == 240) // 11110000
                            || (bytes [b] == 248) // 11111000
                            || (bytes [b] == 252) // 11111100
                            || (bytes [b] == 254) // 11111110
                        )
                        {
                            moreOnesPossible = false;
                        }
                        else
                        {
                            return (false);
                        }
                    }
                    else
                    {
                        if(bytes [b] != 255) // 11111111
                        {
                            if
                            (
                                (bytes [b] == 128) // 10000000
                                || (bytes [b] == 192) // 11000000
                                    || (bytes [b] == 224) // 11100000
                                || (bytes [b] == 240) // 11110000
                                || (bytes [b] == 248) // 11111000
                                || (bytes [b] == 252) // 11111100
                                || (bytes [b] == 254) // 11111110
                            )
                            {
                                moreOnesPossible = false;
                            }
                        }                            
                    }
                }
                else
                {
                    if (bytes [b] > 0)
                    {
                        return (false);
                    }
                }
            }
        }
    }

    return (true);
}