2024-03 in haskell

2024/03-Mull_It_Over/second.hs

@@ -0,0 +1,57 @@
+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import           Data.Functor
+import qualified Data.List            as L
+import           Data.Maybe
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+exampleExpectedOutput = 48
+
+data Op = Mul Int Int | Do | Dont | Nop deriving (Eq, Show)
+type Input = [Op]
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some digitChar
+
+parseMul :: Parser Op
+parseMul = Mul <$> (string "mul(" *> parseNumber)
+               <*> (char ',' *> parseNumber <* char ')')
+
+parseOp :: Parser Op
+parseOp = try parseMul
+      <|> string "do()" $> Do
+      <|> string "don't()" $> Dont
+      <|> anySingle $> Nop
+
+parseInput' :: Parser Input
+parseInput' = filter (/= Nop) <$> some parseOp <* 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 = fst . L.foldl' compute' (0, True)
+  where
+    compute' :: (Int, Bool) -> Op -> (Int, Bool)
+    compute' (acc, False) Do        = (acc, True)
+    compute' (acc, True)  Dont      = (acc, False)
+    compute' (acc, True)  (Mul x y) = (acc + (x * y), True)
+    compute' acc _                  = acc
+
+main :: IO ()
+main = do
+  example <- parseInput "example2"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input