Re: Rotating through a four-bit nibble

In message:  <[email protected]>
             "Brian Davis" <[email protected]> writes:
>I'm working in NQC, and I need to rotate the bit series in a 4-bit
>(or 8-bit) nibble embedded in a 16-bit word. Annoying. Any suggestions?

What are you optimizing for?  Code clarity?  Code speed?  Code length?

There are (to my knowledge) no truly pretty solutions to this.
I would tend to go with:

  int rotbits(int input, int numbits, int offset, int rotdist) {
    int mask = (~((~0) << numbits)) << offset;
    int left = rotdist % numbits;
    int right = (numbits - left) % numbits;
    int temp = input & mask;
    return (input & ~mask) | (((temp << left) | (temp >> right)) & mask);
  }

This generic solution is (of course) slower (and a bit harder to read)
than a solution specifically tuned for a particular size, offset, and
rotation distance... but it's 7 lines of code that can then be called
wherever you need to do these sub-word manipulations.  Depending on
how good the NQC optimizer has gotten, it might even simplify down to
the faster versions when given constants for the size and offset...

- Alex