# Bit Manipulation Challenge

4 messages
Open this post in threaded view
|

## Bit Manipulation Challenge

 Administrator 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 1. https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing----- Cheers, Sean -- Sent from: http://forum.world.st/Pharo-Smalltalk-Users-f1310670.html Cheers, Sean
Open this post in threaded view
|

## Re: Bit Manipulation Challenge

 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