CODEWITHSUNDEEP
matlab
gavishna
gavishna

Reputation: 63

Bit Rotation operation

Is there a way to make the Bit rotation operation reversible? I mean, if there is an image X (size 256 * 256 *3) then on doing bit rotation, image Y is obtained. Then on subjecting Y to a bit rotation, we get back image X. Also, How to tackle the bit overflow so that there is no information loss.

Upvotes: 2

Views: 1776

Answers (2)

gnovice
gnovice

Reputation: 125874

UPDATE: I've taken the code I posted below and refined it into a complete function with error-checking, help documentation, and the ability to operate on arrays of unsigned integers and double-precision variables just like the related built-in function BITSHIFT. I suggest using the newer version I link to above instead of the older version posted below.

MATLAB does not have a built-in bit rotation function, and the BITSHIFT function will drop bits that overflow. However, you could implement your own bit rotation function based on the existing bit operations. Here's a simple first-pass version I threw together (sans error-checking):

function data = bit_rotate(data,nBits)
  dataBits = log2(double(intmax(class(data)))+1);  %# Number of bits in data
  nBits = rem(nBits,dataBits);  %# No need to rotate by dataBits bits or more
  if nBits == 0  %# No bit rotation needed, just return
    return
  end
  shiftedData = bitshift(data,nBits);              %# Bit shift the data
  lostData = bitxor(data,bitshift(shiftedData,-nBits));  %# Find the lost bits
  rotatedData = bitshift(lostData,nBits-sign(nBits)*dataBits);  %# Rotate them
  data = shiftedData+rotatedData;  %# Add the rotated bits to the shifted bits
end

And here's some test data:

>> B = uint8(208);     %# An unsigned 8-bit integer value
>> disp(dec2bin(B,8))  %# Display the bit pattern of B
11010000
>> disp(dec2bin(bit_rotate(B,2),8))   %# Rotate left by 2 bits
01000011
>> disp(dec2bin(bit_rotate(B,-2),8))  %# Rotate right by 2 bits
00110100

Note that bit_rotate will also operate on any size matrix input for data as long as it's an unsigned integer type.

Upvotes: 1

Olli Etuaho
Olli Etuaho

Reputation: 303

Sure, bit rotation is reversible, just rotate the other way around by the same amount.

Wikipedia has nice info on how to implement it in C with basic bit shifts so that you don't get overflow: http://en.wikipedia.org/wiki/Circular_shift

And I suppose that if the operation is called "bit rotation" in Matlab, it surely doesn't overflow anyway.

Upvotes: 0

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