Project 6 — Caesar Cipher

e
3
Hello, World!

ek9 caesarcipher.ek9 < input.txt

Caesar Cipher, by Al Sweigart al@inventwithpython.com
The Caesar cipher encrypts letters by shifting them over by a
key number. For example, a key of 2 means the letter A is
encrypted into C, the letter B encrypted into D, and so on.

Do you want to (e)ncrypt or (d)ecrypt?
Please enter the key (0 to 25) to use.
Enter the message to encrypt.
KHOOR, ZRUOG!

#!ek9
defines module bigbook.caesarcipher

  defines constant

    Symbols <- "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

  defines type

    CipherMode
      Encrypt
      Decrypt

  defines class

    //THE input idiom of this book, copied unchanged from lib/LineInput.ek9 (the reasons are
    //there): read with next(), and every input loop also stops when ended() is true.
    LineInput
      stdin as Stdin: Stdin()
      inputEnded as Boolean: false

      next()
        <- rtn as String: stdin.next()
        if not rtn?
          inputEnded: true

      ended() as pure
        <- rtn as Boolean: Boolean(inputEnded)

      override operator ? as pure
        <- rtn as Boolean: true

    //THE INPUT AND OUTPUT: everything the player sees or types goes through here, worded by Say.
    Console
      stdout as Stdout: Stdout()
      input as LineInput: LineInput()
      say as Say: Say("en")

      Console()
        -> words as Say
        say: words

      showIntroduction()
        stdout.println(say.introduction())
        stdout.println("")

      //Asks until the answer is a mode; unset only if the input ends first.
      nextMode()
        <- rtn as CipherMode: CipherMode()
        while not rtn? and not input.ended()
          stdout.println(say.askMode())
          rtn: modeOf(input.next())
          if not rtn? and not input.ended()
            stdout.println(say.notAMode())

      //Asks until the answer is a key; unset only if the input ends first.
      nextKey()
        <- rtn as Integer: Integer()
        while not rtn? and not input.ended()
          stdout.println(say.askKey(maxKey()))
          rtn: keyFrom(input.next())

      //The message to translate; unset only if the input ends first.
      nextMessage()
        -> mode as CipherMode
        <- rtn as String: String()
        stdout.println(mode == CipherMode.Encrypt <- say.askMessageToEncrypt() : say.askMessageToDecrypt())
        rtn: input.next()

      //The translation is computed, not worded: it is shown as the cipher made it.
      showTranslated()
        -> translated as String
        stdout.println(translated)

      override operator ? as pure
        <- rtn as Boolean: true

  defines function

    //THE GAME: the cipher, with no input or output.

    //The first letter of the reply picks the mode; any other reply leaves it unset.
    modeOf() as pure
      -> reply as String
      <- rtn as CipherMode: CipherMode()
      initial <- reply.trim().lowerCase().first()
      if initial == 'e'
        rtn: CipherMode.Encrypt
      else if initial == 'd'
        rtn: CipherMode.Decrypt

    maxKey() as pure
      <- rtn as Integer: length Symbols - 1

    //Integer(text) is unset when the text is not a number; a number outside 0 to maxKey()
    //is not a key either.
    keyFrom() as pure
      -> reply as String
      <- rtn as Integer: Integer()
      if candidate <- Integer(reply.trim()) then candidate >= 0 and candidate <= maxKey()
        rtn: candidate

    //A symbol's position comes from indexOf, which is unset for anything outside Symbols,
    //so the guard is also the "is this a letter?" test. mod is never negative, so one
    //expression wraps both directions.
    shiftedSymbol() as pure
      ->
        character as Character
        shift as Integer
      <- rtn as String: $character

      if position <- Symbols.indexOf(character)
        wrapped <- (position + shift) mod length Symbols
        rtn: Symbols.subString(wrapped, wrapped + 1)

    //The cipher works on capital letters only, so the message is uppercased first.
    translate()
      ->
        message as String
        mode as CipherMode
        key as Integer
      <- rtn as String: ""

      shift <- mode == CipherMode.Encrypt <- key : -key
      for character in message.upperCase()
        rtn += shiftedSymbol(character, shift)

  defines function

    //THE language of the words, copied unchanged from lib/Language.ek9: BIGBOOK_LANG when it
    //is set (fr, zh, ...), otherwise en.
    language()
      <- rtn as String: EnvVars().get("BIGBOOK_LANG") ?: "en"

  //THE WORDS: every word the player sees.
  defines text for "en"

    Say

      introduction()
        `Caesar Cipher, by Al Sweigart al@inventwithpython.com
The Caesar cipher encrypts letters by shifting them over by a
key number. For example, a key of 2 means the letter A is
encrypted into C, the letter B encrypted into D, and so on.`

      askMode()
        "Do you want to (e)ncrypt or (d)ecrypt?"

      notAMode()
        "Please enter the letter e or d."

      askKey()
        -> highestKey as Integer
        `Please enter the key (0 to ${highestKey}) to use.`

      askMessageToEncrypt()
        "Enter the message to encrypt."

      askMessageToDecrypt()
        "Enter the message to decrypt."

  defines program

    //Caesar Cipher (Big Book project 6). Encrypts or decrypts a message with a chosen key. The
    //game decides; the Console asks and tells, in the player's words.
    CaesarCipher()
      //A language with no words of its own is unset: ?: falls back to English.
      console <- Console(Say(language()) ?: Say("en"))

      console.showIntroduction()
      mode <- console.nextMode()
      key <- console.nextKey()
      if mode? and key?
        if message <- console.nextMessage(mode)
          console.showTranslated(translate(message, mode, key))