1 files changed, 57 insertions, 0 deletions
diff --git a/2024/05-Print_Queue/second.hs b/2024/05-Print_Queue/second.hs
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+++ b/2024/05-Print_Queue/second.hs
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+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import qualified Data.List            as L
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+exampleExpectedOutput = 123
+
+type Rule = (Int, Int)
+type Update = [Int]
+data Input = Input [Rule] [Update] deriving Show
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some digitChar
+
+parseRule :: Parser Rule
+parseRule = (,) <$> parseNumber <* char '|'
+                <*> parseNumber <* eol
+
+parseUpdate :: Parser Update
+parseUpdate = some (parseNumber <* optional (char ',')) <* eol
+
+parseInput' :: Parser Input
+parseInput' = Input <$> some parseRule <* eol
+                    <*> some parseUpdate <* 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 -> Int
+compute (Input rules updates) = sum $ map compute' invalids
+  where
+    invalids = L.filter (not . valid) updates
+    valid update = all (valid' update) rules
+    valid' update (x, y) = case (L.elemIndices x update, L.elemIndices y update) of
+      ([i], [j]) -> i < j
+      _          -> True
+    compute' update = (L.sortBy sortByRule update) !! (length update `div` 2)
+    sortByRule :: Int -> Int -> Ordering
+    sortByRule x y = if L.elem (x, y) rules then LT else GT
+
+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