substitution_cipher.sf

#!/usr/bin/ruby

# A simple substitution cipher, given a key that maps each letter to a different letter.
# The position of the letters in the key are used for mapping.
# And also the letters in the key are the letters allowed in the message.

# See also:
#   https://rosettacode.org/wiki/Substitution_cipher

class SubstitutionCipher(Array key) {

    has keylookup = Hash(key.kv.map{.flip}.flat...)
    has num2alpha = Hash(key.sort.kv.flat...)
    has alpha2num = num2alpha.flip

    method encode(String s) {
        var r = ""
        s.each {|c|
            r += (alpha2num.has(c) ? key[alpha2num{c}] : " ")
        }
        return r
    }

    method decode(String s) {
        var r = ""
        s.each {|c|
            r += (keylookup.has(c) ? num2alpha{keylookup{c}} : " ")
        }
        return r
    }
}

# Allow only lowercase a..z
var alpha = ['a'..'z'].map{.to_a}.flat

# Allow digits, lower/upper case a..z and space
# var alpha = [0..9, 'A'..'Z', 'a'..'z', ' '].map{.to_a}.flat

var key = alpha.shuffle
var obj = SubstitutionCipher(key)

say "Encoding with key: #{key.join.dump}"

with ("the quick brown fox jumps over the lazy dog who barks very loudly") { |s|

    var enc = obj.encode(s)
    var dec = obj.decode(enc)

    print <<-EOT
    Original: #{s.dump}
      -> Encoded: #{enc.dump}
      -> Decoded: #{dec.dump}
    EOT

    assert_eq(dec, s)
}