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第9回オフラインリアルタイムどう書くの参考問題をHaskellで書いた

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第9回オフラインリアルタイムどう書くの参考問題をHaskellで書きました。

オフラインリアルタイムどう書くの過去問でHaskellスキルアップを目指します。

makeNumber.hs
import Data.List
import Control.Applicative
import Text.Regex

--先頭が0でない長さ4の順列
permutations' :: [Int] -> [[Int]]
permutations' ns = nub $ filter (\x -> head x /= 0) $ map (take 4) $ permutations ns

--6を9に変換する
convert6To9 :: [Int] -> [[Int]]
convert6To9 [] = [[]]
convert6To9 (n:ns) | n /= 6 = map (n:) $ convert6To9 ns
                   | otherwise = [(n:), (9:)] <*> convert6To9 ns

make4DigitsNumber :: [Int] -> [[Int]]
make4DigitsNumber ns = sort $ concatMap convert6To9 $ permutations' ns

listToString :: [Int] -> String
listToString [] = ""
listToString (n:ns) = show n ++ listToString ns

solve :: Int -> [Int] -> String
solve n ns = let list = make4DigitsNumber ns in
                if length list >= n
                then listToString $ list !! (n - 1)
                else "-"

parseTestData :: String -> [(Int, [Int], String)]
parseTestData str = map parseLine $ lines str
    where
        parseLine :: String -> (Int, [Int], String)
        parseLine line = let [input, output] = words line
                             (index, numList) = parseInput input
                         in (index, numList, output)
        parseInput :: String -> (Int, [Int])
        parseInput input = let [index, numList] = splitRegex (mkRegex ":") input
                           in (read index :: Int, parseNumList numList)
        parseNumList :: String -> [Int]
        parseNumList [] = []
        parseNumList (n:ns) = (read (n:[]) :: Int):(parseNumList ns)

main = do
    testData <- parseTestData <$> getContents
    mapM (\(index, numList, output) -> print $ (solve index numList) == output) testData

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