1 files changed, 69 insertions, 0 deletions
diff --git a/2023/04-Scratchcards/second.hs b/2023/04-Scratchcards/second.hs
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+++ b/2023/04-Scratchcards/second.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.List qualified as L
+import Data.Map qualified as M
+import Data.Maybe
+import Data.Set qualified as S
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+
+import Debug.Trace
+
+exampleExpectedOutput = 30
+
+data Card = Card Int [Int] [Int] deriving Show
+type Input = [Card]
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some digitChar <* many (char ' ')
+
+parseCard :: Parser Card
+parseCard = do
+  id <- string "Card" *> many (char ' ') *> parseNumber <* char ':' <* many (char ' ')
+  winnings <- some parseNumber
+  void $ char '|' <* many (char ' ')
+  numbers <- some parseNumber
+  return $ Card id winnings numbers
+
+parseInput' :: Parser Input
+parseInput' = some (parseCard <* eol) <* 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'
+
+compute :: Input -> Integer
+compute = fst . L.foldl' compute' (0, L.repeat 1) . map score'
+  where
+    compute' :: (Integer, [Integer]) -> Int -> (Integer, [Integer])
+    compute' (acc, stack) score = (acc', stack')
+      where
+        copies = head stack
+        acc' = acc + copies
+        (scored, remain) = L.splitAt score $ tail stack
+        stack' = map (+ copies) scored ++ remain
+    score' :: Card -> Int
+    score' (Card _ winnings numbers) | wins == 0 = 0
+                                     | otherwise = wins
+      where
+        wins = length $ winnings `L.intersect` numbers
+
+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
+