spacetalky問題回答

spacetalky.hs

import System.Environment
import System.Exit
import Control.Monad
import Data.Char
import qualified Data.List as List

main = do
  argv <- getArgs
  when (length argv /= 1) $ do
    putStrLn "usage:command filename"
    exitFailure

  contents <- readFile(argv !! 0)
  mapM_ putStrLn . map (anserString .List.dropWhileEnd isSpace) . lines $  contents

anserString :: String -> String
anserString line =
                  if canEncode line
                  then stretchString line ++ ":" ++ line
                  else "X:" ++ line;

canEncode :: String -> Bool
canEncode xs =  let len = length xs
                in
                  if len <= 2
                  then True
                  else 
                    if (len `mod` 2)  == 1
                    then False
                    else
                      let (x:(y:(z:zs))) = xs
                      in
                        if x == z && y /= 'z'
                        then False
                        else canEncode (z:zs)

stretchTwoChar :: String -> String
stretchTwoChar [] = []
stretchTwoChar (x:[]) = [x]
stretchTwoChar (x:xs) = let num = toDecimal.head $ xs
                        in  take num $ repeat x

stretchString :: String -> String
stretchString = concat . map stretchTwoChar . splitN 2

splitN :: Int -> [a] -> [[a]]
splitN n array = let (before, remain) = splitAt n array
                 in case remain of
                    [] -> [before]
                    _  -> before : splitN n remain

toDecimal :: Char -> Int
toDecimal c = let a = "abcdefghijklmnopqrstuvwxyz"
              in  case (c `List.elemIndex` a) of
                  Nothing -> 0
                  Just n  -> n + 1