Convert a CIDR to Subnet Netmask address in PicoLisp

cidr-to-netmask.l

# MIT Licensed
#
#(c) 2017 Alexander Williams, Unscramble <[email protected]>
#
# IPv4 "24" -> "255.255.255.0"
[de cidr-to-netmask (Mask)
  (let (Pos (- 5 (/ Mask 8))
        Val (& (>> (- (- 8 (% Mask 8))) 255) 255) )

    (glue "."
      (let N 5
        (make
          (loop
            (cond
              ((= N Pos) (link Val))
              ((< N Pos) (link 0))
              ((> N Pos) (link 255)) )
            (T (=1 (dec 'N))) ]

Usage examples

: (cidr-to-netmask "24")
!? (/ Mask 8)
"24" -- Number expected
? 
: (cidr-to-netmask 1)   
-> "128.0.0.0"
: (cidr-to-netmask 8)
-> "255.0.0.0"
: (cidr-to-netmask 18)
-> "255.255.192.0"
: (cidr-to-netmask 24)
-> "255.255.255.0"
: (cidr-to-netmask 32)
-> "255.255.255.255"