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April 28, 2000

Bit Rotation for Byte, Short and Int Variables

This is a Class to perform rotr and rotl as known in standard C implementations and a rotate with carry implementation. The technique used is to determine which bits within the variable will be affected and then calculate where those bits will wind up after the rotate. To do this we must determine the bits which will be impacted on the rotate, so we calculate a mask to use to save off the bits which will be rotated. Then we perform the rotation, then we "add" the bits back in the location where they would have been had java had a rot(l|r) with carry.

public final class Rotate {
public static final int ROTATE_LEFT = 1;
public static final int ROTATE_RIGHT = 2;

Calculate the mask to use to mask off the bits which will be rotated out of the variable. For example, if we are going to rotr 4 bits, the right most 4 bits will be rotated out of the variable, so the mask should be: 0x0000000F. Or, if we rotl 8 bits, then the mask should be 0xFF000000.

* @param bitstorotate number of bits to rotate
* @param direction which direction to rotate ROTATE_LEFT or ROTATE_RIGHT
* @return the mask for masking out the bits which will be displaced in the rotate
*/
private static int calcmask(int bitstorotate, int direction)
{
int mask = 0;
int c;
if (bitstorotate == 0)
return 0;
c = 0x80000000;
mask = (c >> bitstorotate);
if (direction == ROTATE_RIGHT)
{
mask = (c >> (32 - bitstorotate));
mask = ~mask;
}
else
mask = (c >> bitstorotate);
return mask;
}