Is there a way to perform a circular bit shift in C#?

Question

I know that the following is true

int i = 17; //binary 10001
int j = i << 1; //decimal 34, binary 100010

But, if you shift too far, the bits fall off the end. Where this happens is a matter of the size of integer you are working with.

Is there a way to perform a shift so that the bits rotate around to the other side? I'm looking for a single operation, not a for loop.

Answer 1

If you know the size of type, you could do something like:

uint i = 17;
uint j = i << 1 | i >> 15;

... which would perform a circular shift of a 16 bit value.

As a generalization to circular shift left n bits, on a b bit variable:

var input = 17;
var result = i << n | i >> (b - n);

---

@The comment, it appears that C# does treat the high bit of signed values differently. I found some info on this here. I also changed the example to use a uint.

Answer 2

I have to admit, I just did a search for "C# bit rotate" and found a link to a page with a Java class that would be easily adapted to C#

I also found this in Google Book which is a C++ function with similar behavior

Answer 3

Chris's answer is good, and I won't add to it.

To answer Keith's question, word rotations have a lot of uses, especially in cryptography. Have a look at various message digest algorithms, such as MD5, or the SHA family, for starters.

Answer 4

@Keith Sirmons,

I believe (don't quote me on it) that this might have some application in cryptography. Basically any situation where you want a reversible operation performed on data. Doing a regular bit shift causes a loss of data when bits fall off the end.

Personally, I was experimenting with encoding a list of 8-8bit integers in a single Int64 variable and rotating the list. Rather than the traditional 9 operations (store the first element, move 7 items, restore temp to the end of the list), it is now one operation. Not entirely useful for day-to-day stuff, but I wanted to see if it could be done quicker.

Answer 5

One year ago I've to implement MD4 for my undergraduate thesis. Here it is my implementation of circular bit shift using a UInt32.

private UInt32 RotateLeft(UInt32 x, Byte n)
{
      return (UInt32)(((x) << (n)) | ((x) >> (32 - (n))));
}

Answer 6

Just as reference on how to do it, this two functions work perfectly for rotating the bits of 1/2word:

static public uint ShiftRight(uint z_value, int z_shift)
{
    return ((z_value >> z_shift) | (z_value << (16 - z_shift))) & 0x0000FFFF;
}

static public uint ShiftLeft(uint z_value, int z_shift)
{
    return ((z_value << z_shift) | (z_value >> (16 - z_shift))) & 0x0000FFFF;
}

It would be easy to extend it for any given size.