第一回 オフラインリアルタイムどう書く

第一回 オフラインリアルタイムどう書くという勉強会があったそうです。
残念ながら僕は参加できなかったんですが、当日のお題をHaskellで解いてみました。
なるべくシンプルに実装したつもりです。

toukaku.hs

module Main where

import Data.List (transpose, insert)

data Cell   = O | X | None deriving (Eq, Show, Ord, Enum)
type Board  = [Cell]
data Result = OWon | XWon | FoulOWon | FoulXWon | Draw deriving Enum

instance Show Result where
  show OWon     = "o won."
  show XWon     = "x won."
  show FoulOWon = "Foul: o won."
  show FoulXWon = "Foul: x won."
  show Draw     = "Draw game."

main :: IO ()
main = mapM_ (print . judge) . lines =<< getContents

judge :: String -> Result
judge = judge' O (replicate 9 None) . take 9 . map (subtract 49 . fromEnum)

judge' :: Cell -> Board -> [Int] -> Result
judge' _ _ []     = Draw
judge' c b (i:is) = case put c i b of
  Nothing -> toEnum (3 - fromEnum c)
  Just b' -> if win c b'
    then toEnum $ fromEnum c
    else judge' (toEnum (1 - fromEnum c)) b' is

win :: Cell -> Board -> Bool
win c b = any (all (== c)) $ mat ++ transpose mat ++ map (zipWith (!!) mat) [[0,1,2], [2,1,0]]
  where
    mat = take 3 . map (take 3) $ iterate (drop 3) b

put :: Cell -> Int -> Board -> Maybe Board
put c n b = case b !! n of
  None -> Just . map snd . insert (n, c) . filter ((/= n) . fst) $ zip [0..] b
  _    -> Nothing