AtbashCipher.fsx

(*
Atbash Cipher

The Atbash Cipher is simple: replace every letter with its “mirror” in the alphabet.
A is replaced by Z. B is replaced by Y. Etc. Write a function to calculate it.

Examples

(atbash "") ;=> ""
(atbash "hello") ;=> "svool"
(atbash "Clojure") ;=> "Xolqfiv"
(atbash "Yo!") ;=> "Bl!"
Please maintain capitalization and non-alphabetic characters.
*)

let pairWithMirror l = List.rev l |> List.zip l

let lowersWithMirror = pairWithMirror [ 'a' .. 'z' ]

let finder chr =
    let predicate =
        fun (o, _) -> o = (System.Char.ToLower chr)

    List.tryFind predicate lowersWithMirror
    |> Option.map snd

let folder str chr =
    let mirror =
        match (finder chr) with
        | Some m when System.Char.IsUpper chr -> System.Char.ToUpper m
        | Some m -> m
        | _ -> chr

    sprintf "%s%c" str mirror

let atbash (str: string) = Seq.fold folder System.String.Empty str

atbash "" = ""
&& atbash "hello" = "svool"
&& atbash "Clojure" = "Xolqfiv"
&& atbash "Yo!" = "Bl!"