Bit Manipulation Challenge

>
> I want to take an IP address routing prefix in CIDR [1] notation (i.e. the 24
> in "192.168.100.14/24") and convert it into subnet form (i.e.
> 255.255.255.0). I came up with 4 ways to do that (see below), but none stand
> out as best (although #3 and #4 seem a bit more straightforward as they
> avoid the `max` temp).
>
> How would you go about it?
>
> Here are the ways I came up with:
> cidr := 24.
> shift := 32 - cidr.
> max := #[ 255 255 255 255 ] asInteger.
> "1." ((max bitShift: shift negated) bitShift: shift) asByteArray.
> "2." (max bitClear: (2 raisedTo: shift) - 1) asByteArray.
> "3." ((2 raisedTo: shift) - 1) bitInvert32 asByteArray.
> "4." (((2 raisedTo: cidr) - 1) bitShift: shift) asByteArray

> On Mon, 20 Jul 2020 at 23:19, Sean P. DeNigris <[hidden email]> wrote:
> >
> > I want to take an IP address routing prefix in CIDR [1] notation (i.e. the 24
> > in "192.168.100.14/24") and convert it into subnet form (i.e.
> > 255.255.255.0). I came up with 4 ways to do that (see below), but none stand
> > out as best (although #3 and #4 seem a bit more straightforward as they
> > avoid the `max` temp).
> >
> > How would you go about it?
> >
> > Here are the ways I came up with:
> > cidr := 24.
> > shift := 32 - cidr.
> > max := #[ 255 255 255 255 ] asInteger.
> > "1." ((max bitShift: shift negated) bitShift: shift) asByteArray.
> > "2." (max bitClear: (2 raisedTo: shift) - 1) asByteArray.
> > "3." ((2 raisedTo: shift) - 1) bitInvert32 asByteArray.
> > "4." (((2 raisedTo: cidr) - 1) bitShift: shift) asByteArray
>
> To me the most complicated ones have the "subtract 1" - it makes it
> harder to reason about.  I'd also guess "3" and "4" are slowest.
>
> "1." can be simplified by hardcoding max....
> 5. (4294967295 bitShift: shift negated) bitShift: shift) asByteArray
>
> Additional alternative...
> 6. ((4294967295 bitShift: shift) rem: 4294967296) asByteArray
>
> And why not low-brow...
> 7. CIDRLookup at: cidr.
>
> cheers -ben