module Main where

import qualified Data.Char
import qualified Data.List.Split
import qualified Data.List

data Cell = Fixed Int | Possible [Int] deriving (Show, Eq)
type Row  = [Cell]
type Grid = [Row]

possibleVals :: Cell -> [Int]
possibleVals (Fixed x)     = [x]
possibleVals (Possible xs) = xs

readGrid :: String -> Maybe Grid
readGrid s
  | length s == 81 =
      sequence . map (sequence . map readCell) . Data.List.Split.chunksOf 9 $ s
  | otherwise      = Nothing
  where
    readCell '.' = Just $ Possible [1..9]
    readCell c
      | Data.Char.isDigit c && c > '0' = Just . Fixed . Data.Char.digitToInt $ c
      | otherwise = Nothing

showGrid :: Grid -> String
showGrid = unlines . map (unwords . map showCell)
  where
    showCell (Fixed x) = show x
    showCell _ = "."

showGridWithPossibilities :: Grid -> String
showGridWithPossibilities = unlines . map (unwords . map showCell)
  where
    showCell (Fixed x)     = show x ++ "          "
    showCell (Possible xs) =
      (++ "]")
      . Data.List.foldl' (\acc x -> acc ++ if x `elem` xs then show x else " ") "["
      $ [1..9]


main = undefined