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javaobjective-ccrc

Negative of UInt8

I have this code in Objective-C and I want to write it in Java. However there is the statement (-(crc & 1)) that gives me problems. After some googeling it seems that the negative of an unsigned is generally not well defined. I didn't quite understand it though. Maybe somebody who knows CRC can tell me what the equivalent in Java would be.

I need exactly an equivalent method as a device with this Objective-C code checks my calculated crc against its self calculated.

+ (UInt32) crc32:(UInt32) crc ofByte: (UInt8) byte
{
    int8_t i;
    crc = crc ^ byte;
    for (i = 7; i >= 0; i--)
    {
        crc = (crc >> 1) ^ (0xedb88320ul & (-(crc & 1)));
    }
    return(crc);
}

The solved code with help of comments. Seems like I searched the mistake in the wrong place.

private void addByteToCRC(byte oneByte) {
    crc ^= oneByte & 0xFF;
    for (int i = 7; i >= 0; i--)
    {
        crc = (crc >>> 1) ^ (0xedb88320 & (-(crc & 1)));
    }
}

Both checksums start of with a crc value of 0xFFFFFFFF.

Solution

  • The -(crc & 1) works fine, and is not your problem. What it does is to copy the low bit of crc to all of the bits, since in two's complement representation, –1 is all ones.

    There are two things you need to fix for Java, which does not have unsigned integers. First is that the right shift needs to be a logical right shift, >>>. That avoids copying the sign bit when shifting down, and instead shifts in a zero bit. Second, you need to guard against a character being signed-extended to an int, so byte (which will need a different name in Java) needs to be anded with 0xff. So:

    crc ^= octet & 0xff;

    and

    crc = (crc >>> 1) ^ (0xedb88320ul & (-(crc & 1)));