2023-12 part 1 in haskell

2023/12-Hot_Springs/example

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+???.### 1,1,3
+.??..??...?##. 1,1,3
+?#?#?#?#?#?#?#? 1,3,1,6
+????.#...#... 4,1,1
+????.######..#####. 1,6,5
+?###???????? 3,2,1

2023/12-Hot_Springs/first.hs

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+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+
+import Control.Applicative.Permutations
+import Control.Monad (void, when)
+import Data.Char qualified as C
+import Data.Either
+import Data.Functor
+import Data.List qualified as L
+import Data.Map qualified as M
+import Data.Maybe
+import Data.Set qualified as S
+import Data.Vector qualified as V
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+
+import Debug.Trace
+
+exampleExpectedOutput = 21
+
+data Tile = Broken | Operational | Unknown deriving Eq
+instance Show Tile where
+  show Broken = "#"
+  show Operational = "."
+  show Unknown = "?"
+data Row = Row [Tile] [Int] deriving Show
+type Input = [Row]
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some digitChar <* optional (char ',')
+
+parseTile :: Parser Tile
+parseTile = char '#' $> Broken
+        <|> char '.' $> Operational
+        <|> char '?' $> Unknown
+
+parseRow :: Parser Row
+parseRow = Row <$> some parseTile <* space
+               <*> some parseNumber <* eol
+
+parseInput' :: Parser Input
+parseInput' = some parseRow <* eof
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle -> error $ errorBundlePretty bundle
+    Right input' -> return input'
+
+valid :: [Int] -> [Tile] -> Bool
+valid record tiles = map length (L.filter (L.any (/= Operational)) $ L.group tiles) == record
+
+permutations :: [Tile] -> [[Tile]]
+permutations [] = [[]]
+permutations (Unknown:xs) = permutations (Operational:xs) ++ permutations (Broken:xs)
+permutations (x:xs) = map (x:) $ permutations xs
+
+compute :: Input -> Int
+compute = sum . map compute'
+  where
+    compute' :: Row -> Int
+    compute' (Row tiles record) = length . filter (valid record) $ permutations tiles
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input