On Tue, Jul 21, 2020 at 02:17:59AM +0800, Ben Coman wrote: > On Mon, 20 Jul 2020 at 23:19, Sean P. DeNigris <s...@clipperadams.com> 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
This also seems to work for IPV4: (max << shift) asByteArray last: 4 Dave
