converting CRC-ITU 16 c code to java

Question

I want to convert a C code algorithm to Java to calculate a CRC. I am trying to implement the BL10 Concox GPS lock in our project, but I am not sure which byte[] should I pass to the CRC calculation function. Also I am not sure my implementation for the CRC calculation is correct.

I am passing byte[] from Packet Length to Information Serial Number as mentioned below.

Error check (From “Packet Length” to“Information Serial Number”) , are values of CRC-ITU. CRC error occur when the received information is calculated, the receiver will ignore and discard the data packet.

Here is the communication protocol pdf:

https://drive.google.com/file/d/1AsBk3iPyLGk4QyuDevYx86lJlfUVYE1t/view

Below is the C code.

C code:

static const U16 crctab16[] ={
0X0000, 0X1189, 0X2312, 0X329B, 0X4624, 0X57AD, 0X6536, 0X74BF,
0X8C48, 0X9DC1, 0XAF5A, 0XBED3, 0XCA6C, 0XDBE5, 0XE97E, 0XF8F7,
 //remove some constant
0XF78F, 0XE606, 0XD49D, 0XC514, 0XB1AB, 0XA022, 0X92B9, 0X8330,
0X7BC7, 0X6A4E, 0X58D5, 0X495C, 0X3DE3, 0X2C6A, 0X1EF1, 0X0F78,
};

// calculate the 16-bit CRC of data with predetermined length.
U16 GetCrc16(const U8* pData, int nLength){
U16 fcs = 0xffff; // initialization
while(nLength>0){
 fcs = (fcs >> 8) ^ crctab16[(fcs ^ *pData) & 0xff];
 nLength--;
 pData++;
} 
 return ~fcs; // negated
}

So far i have tried:

Java Code:

private static int crctab16[] = { 0X0000, 0X1189, 0X2312, 0X329B, 0X4624, 0X57AD, 0X6536, 0X74BF, 0X8C48, 0X9DC1,
        0XAF5A, 0XBED3, 0XCA6C, 0XDBE5, 0XE97E, 0XF8F7, 0X1081, 0X0108, 0X6306, 0X728F, 0X4014, 0X519D, 0X2522,
        0X34AB, 0X0630, 0X17B9, 0XEF4E, 0XFEC7, 0XCC5C, 0XDDD5, 0XA96A, ....
        0X8B70, 0X8408, 0X9581, 0XA71A, 0XB693, 0XC22C, 0XD3A5, 0XE13E, 
        0XC60C, 0XD785, 0XE51E, 0XF497, 0X8028, 0X91A1, 0XA33A, 0XB2B3, 
        0XB1AB, 0XA022, 0X92B9, 0X8330, 0X7BC7, 0X6A4E, 0X58D5, 0X495C, 0X3DE3, 0X2C6A, 0X1EF1, 0X0F78, };


public static int GetCrc16(final byte[] pData) {
    int crc = 0xffff;// initialization
    for (byte b : pData) {
        crc = (crc >>> 8) ^ crctab16[(crc ^ b) & 0xff];
    }
    return crc; 
}

I checked the result using this, but not getting the correct result.

Answer 1

Your return crc; needs to be return crc ^ 0xffff;. Note the ~ in the return ~fcs; from the C code. You didn't negate the result. It even provides the helpful comment "negated".

You could use ~crc, but then you'd need to do ~crc & 0xffff since ints are 32 bits. So it's better to use crc ^ 0xffff.

You do not need to use >>> here. You can just use >> since crc will never be negative. Either one is fine.