rotateLeft() - Left Rotating All Bits in a Byte Array

Java Tutorials - Herong's Tutorial Examples

∟rotateLeft() - Left Rotating All Bits in a Byte Array

This section provides a tutorial example on how to perform a left rotation of all bits in a byte array - rotateLeft().

If you are trying to implement DES (Data Encryption Standard) encryption algorithm, you will need to rotate bit values in bit strings. With the getBit() and setBit() methods defined above, rotating a bit string stored in a byte array becomes easy to do. Here my rotateLeft() method.

   private static byte[] rotateLeft(byte[] in, int len, int step) {
      int numOfBytes = (len-1)/8 + 1;
      byte[] out = new byte[numOfBytes];
      for (int i=0; i<len; i++) {
         int val = getBit(in,(i+step)%len);
         setBit(out,i,val);
      }
      return out;
   }

Explanations:

The method interface expects the caller to provide the byte array that stores the input bit string, the length of the input bit string, and the number of steps to rotate.
The first statement calculates the number of bytes needed to store the output bit string.
The second statement defines a new byte array to store the output bit string.
The third statement loops through every bit in the output bit string.
The fourth statement in the loop gets the bit value by calling getBit() on the input bit string from a bit position rotated to the right of the current bit position. The number of steps to be rotated is given by the caller.
The fifth statement in the loop sets the bit value by calling setBit() on the output bit string at the current bit position.