C# Rotate bits to left overflow issue

Question

I've been trying to get this to work for several days now, i've read a thousand guides and people's questions, but still, i cant find a way to do it properly.

What i want to do is to rotate the bits to the left, here's an example.

Original number = 10000001 = 129 What i need = 00000011 = 3

I have to rotate the bits to left a certain ammount of times (it depends on what the user types), here's what i did:

byte b = (byte)129;
byte result = (byte)((byte)b << 1);
Console.WriteLine(result);

Console.Write("Press any key to continue . . . ");
Console.ReadKey(true);

The issue with this it that it causes an error (OverflowException) when i try to use the (<<) operator with that number (note that if i put a number wich first bit is a 0; example: 3 = 00000011; it works as intended and it returns a 6 as a result.

The problem is, if the first bit is a 1, it gives me the (OverflorException) error. I know this isnt rotating, its just a shifting, the first bit goes away and on the end of the byte a 0 pops up, and i can then change it with an OR 000000001 operation to make it a 1 (if the first bit was a 1, if it was a 0 i just leave it there).

Any ideas? Thanks in advance!

Answer 1

You're getting an overflow exception because you're operating in a checked context, apparently.

You can get around that by putting the code in an unchecked context - or just by making sure you don't perform the cast back to byte on a value that can be more than 255. For example:

int shifted = b << rotateLeftBits;
int highBits = shifted & 0xff;
int lowBits = shifted >> 8; // Previously high bits, rotated
byte result = (byte) (highBits | lowBits);

This will work for rotate sizes of up to 8. For greater sizes, just use rotateLeftBits % 8 (and normalize to a non-negative number if you might sometimes want to rotate right).

Answer 2

<< is a shift operator, not a rotate one.

If you want to rotate, you can use (with suitable casting):

b = (b >> 7) | ((b & 0x7f) << 1);

The first part of that gets the leftmost bit down to the rightmost, the second part shifts all the other left.

The or-ing them with | combines the two.

Answer 3

Thanks for your answers!

Shortly after i made this post i came up with an idea to solve this problem, let me show you (Before you ask, it works!):

byte b = (byte)129; 
b = (byte)((byte)b & 127); 
byte result = (byte)((byte)b << 1); 
result = (byte)((byte)result | 1); 
Console.WriteLine(result); 

What this does is, removes the first bit (in case if it is a 1) it shifts to the left that zero (doesnt generate overflow) and once the shift is over, it changes that 0 back to 1. If that first bit was a 0, it will just move that zero (note that this is just a piece of the whole code, and as it is partially written in spanish (the comments and variables) i doubt you will understand most of it, so i decided to take out the problematic part to show it to you guys!

I will still try the things you told me and see how it goes, again, thanks a lot for your answers!

Answer 4

please try this function - rotates in both directions (left and right rotation of an 8 bit value) [I didn't tested that function!]

// just for 8Bit values (byte)
byte rot(byte value, int rotation)
    {
        rotation %= 8;

        int result;
        if(rotation < 0)
        {
            result = value << (8 + rotation);
        }
        else
        {
            result = value << rotation;
        }

        byte[] resultBytes = BitConverter.GetBytes(result);
        result = resultBytes[0] | resultBytes[1];

        return (byte)result;
    }

short rot(short value, int rotation) { ... }
int rot(int value, int rotation) { ... }