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-rw-r--r--2024/05-Print_Queue/first.hs55
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diff --git a/2024/05-Print_Queue/first.hs b/2024/05-Print_Queue/first.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 = 143
+
+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) = L.foldl' compute' 0 updates
+ where
+ compute' acc update | valid update = acc + update !! (length update `div` 2)
+ | otherwise = acc
+ 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
+
+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