Experimental Physics and
Industrial Control System

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.

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…

References:

Select MS Bit from an integer Dudley, David

Re: Select MS Bit from an integer Till Straumann

Re: Select MS Bit from an integer Andrew Johnson

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