1 files changed, 47 insertions, 0 deletions
diff --git a/2024/01-Historian_Hysteria/second.hs b/2024/01-Historian_Hysteria/second.hs
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+++ b/2024/01-Historian_Hysteria/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 = 31
+
+type Pair = (Int, Int)
+type Input = [Pair]
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some digitChar <* optional hspace
+
+parsePair :: Parser Pair
+parsePair = (,) <$> parseNumber
+                <*> parseNumber
+
+parseInput' :: Parser Input
+parseInput' = some (parsePair <* 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 -> Int
+compute input = L.foldl' similarity 0 l
+  where
+    (l, r) = unzip input
+    similarity :: Int -> Int -> Int
+    similarity acc i = acc + i * (length $ L.elemIndices i r)
+
+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