alphabet_cipher

open System

let resolveEncryptedLetter (letterToEncrypt:char, keywordLetter) =
    let alphabet = "abcdefghijklmnopqrstuvwxyz"
    alphabet.Split  [| keywordLetter |]
    |> Array.rev
    |> Array.append [| keywordLetter |> string |]
    |> Array.reduce (+)
    |> Seq.toArray
    |> fun rowOrderedByKeywordLetter 
        -> rowOrderedByKeywordLetter, alphabet
            |> Seq.toArray
            |> Array.findIndex (fun letter -> letter.Equals(letterToEncrypt))
    |> fun (rowOrderedByKeywordLetter, letterToEncryptIndex) ->  Array.get rowOrderedByKeywordLetter letterToEncryptIndex
    |> fun keywordAlphabet -> keywordAlphabet.ToString()

let buildKeywordText messageToEncrypt (keyWord:char[]) = 
    Seq.fold (fun (messageCharIndex:int, keywordText:string) element -> 
        messageCharIndex % keyWord.Length
        |> fun keywordCharIndex -> (messageCharIndex + 1, keywordText + keyWord.[keywordCharIndex].ToString())) (0,"") messageToEncrypt
    |>
    snd

let encryptMessage(keyWord:string, messageToEncrypt:string) = 
    let keyWordText = keyWord |> Seq.toArray |> buildKeywordText messageToEncrypt
    Seq.fold (fun (counter:int, result:string) letterToEncrypt ->
        counter + 1, result + resolveEncryptedLetter(letterToEncrypt,  keyWordText.[counter])) (0, "") messageToEncrypt
    |> snd

printfn "Ans %A" (encryptMessage ("scones", "meetmebythetree"))