Re: Select MS Bit from an integer

[Till Straumann <strauman@slac.stanford.edu>]

On 09/04/2015 10:10 AM, Andrew Johnson wrote:
> On 09/04/2015 11:12 AM, Till Straumann wrote:
> > You could do it with a bunch of calc (C1..C5, V) records
> >
> >   C1 computes  (IN & 0xffff0000)
> >   C2 computes  (c1 ? c1 : IN ) & 0xff00ff00
> >   C3 computes  (c2 ? c2 : c1 ? c1 : IN) & 0xf0f0f0f0
> >   C4 computes  (c3 ? c3 : c2 ? c2 : c1 ? c1 : IN) & 0xcccccccc
> >   C5 computes  (c4 ? c4 : c3 ? c3 : c2 ? c2 : c1 ? c1 : IN) & 0xaaaaaaaa
> >
> > V computes  (C1 ? 16 : 0) + (C2 ? 8 : 0) + (C3 ? 4 : 0) + (C4 ? 2 : 0) +
> > (C5 ? 1 : 0)
> If I've understood this code properly V will be the bit number of the
> highest set bit, from 0 through 31. It doesn't distinguish between no
> bits set and only the LSB set though, you'd have to detect that separately.
Yes - depends on what the result should be. I just intended to present 
the idea.
The last line could e.g., be

IN ? (C1 ? 16 : 0) + (C2 ? 8 : 0) + (C3 ? 4 : 0) + (C4 ? 2 : 0) + (C5 ? 1 : 0) : -1

which would yield -1 if no bits are set.

The expressions could be simplified at the expense of more records

C1  = (IN & 0xffff0000)
C1C = C1 ? C1 : IN;
C2  = C1C & 0xff00ff00
C2C = C2 ? C2 : C1C
C3  = C2C & 0xf0f0f0f0
C3C = C3 ? C3 : C2C
C4  = C3C & 0xcccccccc
C4C = C4 ? C4 : C3C
C5  = C4C & 0xaaaaaaaa



> If you have the synapps calc module installed a single transform record
> could do all these calculations, transform handles up to 16 expressions.
>
> You can find lots of bit-manipulation hacks at
>      https://graphics.stanford.edu/~seander/bithacks.html
>
> - Andrew
>
> > On 09/04/2015 07:59 AM, Dudley, David wrote:
> > > Quick question that I didn’t see an immediate way to perform
> > >
> > > How do I select the most-significant set bit in an integer?  I was
> > > thinking about using a SEL, but that seems like a lot of work just to
> > > do a simple thing.  Other possibility would be a SUB, but I’d like to
> > > do it with base record types if I can.
> > >
> > > Something simple please…