Question

Does this seem correct? I m trying to port this checksum method from C to Java but the values returned from my Java function are too high. I suspect the way I m fudging the unsigned int is not correct?

char *Generate( char * buffer, long length ) {

     static char tBuf[4];
     long index;
     unsigned int checksum;

     for( index = 0L, checksum = 0; index < length; checksum += (unsigned int) buffer[index++] );
     sprintf( tBuf, "%03d", (unsigned int) ( checksum % 256 ) );
     return( tBuf );

}

My Java port:

public String generateCheckSum( String value ) {        

    char[] chars = value.toCharArray();        
    long checksum = 0L;

    for( int i = 0; i < chars.length; i++ ) {                 
        checksum += (checksum + (long) chars[ i ]) & 0xFF;
    }

    return String.valueOf( checksum % 256 );
}

Any insight is very much appreciated, thanks

Answer 1

C++

checksum += (unsigned int) buffer[index++]

Java

checksum += (checksum + (long) chars[ i ]) & 0xFF

There s an extra checksum in there

Answer 2

A shorter version

public String generateCheckSum(String value) {        
    int checksum = 0;
    for(char ch : value.toCharArray()) checksum += ch;
    return "" + (checksum & 0xFF);
}

Note: checksum % 256 and checksum & 0xFF don t do the same thing. The first is signed and can return a negative result, the second will not.

Answer 3

Thank you for all of the feedback, it allowed me to get to my final solution. I m sure it could be tweaked further but it does produce the desired checksum values.

public Integer generateCheckSum(String value) throws UnsupportedEncodingException {

    byte[] data = value.getBytes("US-ASCII");        
    long checksum = 0L;

    for( byte b : data )  {
        checksum += b; 
    }

    checksum = checksum % 256;

    return new Long( checksum ).intValue();

}

Answer 4

Don t you think in your Java code

checksum += (checksum + (long) chars[ i ]) & 0xFF;

should be

checksum += ((long) chars[ i ]) & 0xFF;

?

Also in your C function, you are using incorrect format specifier for unsigned int. It should be %u instead of %d.

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