Practicing haskell with a short encoding problem to help me create alphabet shifts for my children. Loaded into ghci you would do `encode 1 "Welcome to spy class!"` and get `23-5-12-3-15-13-5 20-15 19-16-25 3-12-1-19-19!`

encode.hs

module Encode (encode) where

import Data.Char (isAlpha, isDigit, isUpper, toLower)

switchChar :: Int -> Char -> String
-- needs to wrap around if shift is greater than alphabet ??
switchChar shift x =
  show
  $ head
  $ map snd
  $ filter ((== toLower x).fst)
  $ zip ['a'..'z'] [shift..]

encodeSingle :: Int -> Char -> String
encodeSingle shift x
  | isAlpha x = switchChar shift x ++ "-"
  | otherwise = [x]

trimEnds :: String -> String
trimEnds = reverse . trimDashes . reverse

trimDashes :: String -> String
trimDashes xs
  | null xs = ""
  | length xs == 1 = xs
  | not (isDigit (head xs)) && head (tail xs) == '-' = head xs : trimDashes (tail (tail xs))
  | otherwise = head xs : trimDashes (tail xs)

encode :: Int -> String -> String
encode shift plain = trimEnds (encodeSingle shift =<< plain)