Circular Shifting with only three bits

[Arokh]

Hi,

I'm in need of circular shifting, which isn't supported in VB.NET.

To make things a bit more complicated (no pun intended :p ),

I only want to use the first 3 bits to do the rotation. (Just imagine a 3 bit integer)

 

I've come up with some solutions,

but I hope there is a more elegant, maybe faster solution.

 

Shift to the right:

X = ((X >> 1) Or ((X Mod 2) << 2))

 

Shift to the left:

X = (X << 1 Or ( (8 And (X << 1) ) >> 3)) And 7

 

Note:

I only need to shift by one, never more.

[snarfblam]

I think that that is as simple as it is going to get. If you are going to do a bit rotation with a non-supported bit-size, you will always have to perform the carry yourself (i.e. the extra shift and 'or'). If you can safely operate under the assumption that only the least significant 3 bits are ever set, you can do away with some bit masking there.

[MrPaul]

Small improvement

 

For the shl, you can do away with one of those shifts, and for the shr, you can use And instead of Mod.

 

'Assuming only the 3 least significant bits are set
'Shift right
X = (X >> 1) Or ((X And 1) << 2)
'Shift left
X = ((X And 3) << 1) Or ((X And 4) >> 2)

 

I doubt it gets much simpler than that.

 

Good luck :cool: