Question

I am trying to mimick a old legacy C program behavior in Java. The C code is :

void calc_crc(unsigned char *datbuff,unsigned int length)
{
static unsigned char tmp;
static unsigned int crc,zaehler;
crc = 0;
for (zaehler = 0;zaehler < length ;zaehler ++)
{
tmp=(unsigned char) (crc>>8) ;
crc=(crc<<8) ^ crctab[tmp] ^ *datbuff;
datbuff++;
}
}

static unsigned short crctab[256] =
{ // Some values
};

The first issue is this code uses unsigned where as java is signed. Trying hard to retain those values.
Can we get the same results in java too.

Answer 1

The easiest way is to always use at least one size up. i.e. where C uses a char, you use a short or an int. In this case it will require more fiddling with the values to keep them in check, but that can be handled with masking for the most part.

The longer answer is that you can do the same operations on signed variables, but translate all the values. Also, be very careful with your shifts in java, make sure to use >>> or you’ll just confuse yourself. >> is a signed right shift, >>> is unsigned. For the most part, your code may just work if you make sure to use the correct size variables and the correct shift operators.