On Thu, Jul 14, 2022 at 11:47 AM Daniel Thompson
[off-list ref] wrote:

On Thu, Jul 14, 2022 at 11:27:07AM +0200, Andy Shevchenko wrote:

quoted

On Thu, Jul 14, 2022 at 9:13 AM ChiaEn Wu [off-list ref] wrote:

quoted

I have tried two methods so far, as follows
-------------------------------------------------------------
/*
 * prop_val =  1      -->  1 steps --> b'00
 * prop_val =  2 ~  4 -->  4 steps --> b'01
 * prop_val =  5 ~ 16 --> 16 steps --> b'10
 * prop_val = 17 ~ 64 --> 64 steps --> b'11
*/

So, for 1 --> 0, for 2 --> 1, for 5 --> 2, and for 17 --> 3.
Now, consider x - 1:
0  ( 0 ) --> 0
1  (2^0) --> 1
4  (2^2) --> 2
16 (2^4) --> 3
64 (2^6) --> ? (but let's consider that the range has been checked already)

Since we take the lower limit, it means ffs():

  y = (ffs(x - 1) + 1) / 2;

Does it work for you?

To be honest, for this tiny table, writing code that *doesn't* require intricate
deciphering together with a huge comment saying what is does would probably be
better:

                prop_val = (prop_val <=  1 ? 0 :
                            prop_val <=  4 ? 1 :
                            prop_val <= 16 ? 2 :
                                             3);

This would be "obviously correct" and require no comment.

Agree. It will also limit checking (and whatever needed for that).

-- 
With Best Regards,
Andy Shevchenko