Transforming C-source CRC16CITT function to Java

Question

I'm trying to port the following method to Java. So far I've been unsuccessful. I've been researching stackoverflow and found couple of implementations of the problem but I can't get any of them to work.

Convert C CRC16 to Java CRC16

This seems really promising but I'm missing something here. Low level operations aren't my strength. This is part of communications protocol implementation with Java.

Below is example C-source code from the specification.

Calculating CRC

The 16-bit CRC-CCITT Checksum is calculated from ID_H to the last databyte. See C-source example function as follows:

unsigned short crc;
crc = 0;
calc_crc( ID_H );
calc_crc( ID_L );
.
.
calc_crc( <last databyte>);
CRC_H = (unsigned char)(crc>>8);
CRC_L = (unsigned char)crc;
.
.
void calc_crc( unsigned char character )
{
unsigned short crc_tmp;
crc_tmp = (crc ^ character) & 0x0F;
crc = (crc >> 4) ^ (crc_tmp * 4225);
crc_tmp = (crc ^ (character >> 4)) & 0x0F;
crc = (crc >> 4) ^ (crc_tmp * 4225);
}

My current implementation:

public class CRC16CCITT {

    private static int crc = 0;

    private static void calc_crc(int character) {
        int crc_tmp;
        crc_tmp = ((crc ^ character) & 0x0F);
        crc = (crc >>> 4) ^ (crc_tmp * 4225);
        crc_tmp = ((crc ^ (character >>> 4)) & 0x0F);
        crc = (crc >>> 4) ^ (crc_tmp * 4225);
    }

    public static void update(byte b) {
        calc_crc(b);
    }

    public static void reset() {
        crc = 0;
    }

    public static int highByte() {
        return (( crc >> 8) & 0xFF);

    }

    public static int lowByte() {
        return crc & 0xFF;
    }

    public static int value() {
        return crc;
    }

    public static void main(string[] args) {

    // Refactoring test cases atm

    }
}

This works for some examples but I think it's failing when I use large numbers which surpass the signed byte interval. The server is C-made so this might be considerable factor. Help me out please.

Answer 1

It's working like a charm. Just used integer type in each variable. I presume it uses a lot more of memory but It will do for now. Imitating Sun's class design.

/*
 * Class for calculating CRC16CITT. 
 */
public class CRC16CCITT {

    private static int crc = 0;

    public static void calc_crc(int character) {
        int crc_tmp;
        crc_tmp = ((crc ^ character) & 0x0F);
        crc = (crc >>> 4) ^ (crc_tmp * 4225);
        crc_tmp = ((crc ^ (character >>> 4)) & 0x0F);
        crc = (crc >>> 4) ^ (crc_tmp * 4225);
    }

    public static void update(byte b) {
        calc_crc(b);
    }

    public static void reset() {
        crc = 0;
    }

    public static int highByte() {
        return (( crc >> 8) & 0xFF);

    }

    public static int lowByte() {
        return crc & 0xFF;
    }

     public static int value() {
        return crc;
    }
}

Answer 2

Whenever you need unsigned variables from C in Java, you have to use the larger java equivalent. uint8_t becomes short, uint16_t becomes int, and uint32_t becomes long.

You should also use >>> instead of >>. A short explanation why is here

As for CRC16 implementation -- I wrote this long ago, maybe it'll help:

public class CRC16 { 

    static int[] table = {
        0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241,
        0xC601, 0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440,
        0xCC01, 0x0CC0, 0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40,
        0x0A00, 0xCAC1, 0xCB81, 0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841,
        0xD801, 0x18C0, 0x1980, 0xD941, 0x1B00, 0xDBC1, 0xDA81, 0x1A40,
        0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01, 0x1DC0, 0x1C80, 0xDC41,
        0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0, 0x1680, 0xD641,
        0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081, 0x1040,
        0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,
        0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441,
        0x3C00, 0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41,
        0xFA01, 0x3AC0, 0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840,
        0x2800, 0xE8C1, 0xE981, 0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41,
        0xEE01, 0x2EC0, 0x2F80, 0xEF41, 0x2D00, 0xEDC1, 0xEC81, 0x2C40,
        0xE401, 0x24C0, 0x2580, 0xE541, 0x2700, 0xE7C1, 0xE681, 0x2640,
        0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0, 0x2080, 0xE041,
        0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281, 0x6240,
        0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,
        0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41,
        0xAA01, 0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840,
        0x7800, 0xB8C1, 0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41,
        0xBE01, 0x7EC0, 0x7F80, 0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40,
        0xB401, 0x74C0, 0x7580, 0xB541, 0x7700, 0xB7C1, 0xB681, 0x7640,
        0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101, 0x71C0, 0x7080, 0xB041,
        0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0, 0x5280, 0x9241,
        0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481, 0x5440,
        0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,
        0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841,
        0x8801, 0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40,
        0x4E00, 0x8EC1, 0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41,
        0x4400, 0x84C1, 0x8581, 0x4540, 0x8701, 0x47C0, 0x4680, 0x8641,
        0x8201, 0x42C0, 0x4380, 0x8341, 0x4100, 0x81C1, 0x8081, 0x4040,
    };

    public static int getCrc(byte[] bytes) {
        int crc = 0x0000;
        for (byte b : bytes) {
            crc = (crc >>> 8) ^ table[(crc ^ b) & 0xff];
        }
        return crc;
    }

}