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
