2024-22 part 1 in haskell

2024/22-Monkey_Market/example

@@ -0,0 +1,4 @@
+1
+10
+100
+2024

2024/22-Monkey_Market/first.hs

@@ -0,0 +1,55 @@
+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import           Data.Bits
+import           Data.Functor
+import qualified Data.List            as L
+import qualified Data.Map             as M
+import           Data.Maybe
+import           Data.Ord             (comparing)
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+import           Debug.Trace
+
+exampleExpectedOutput = 37327623
+
+type Input = [Int]
+
+type Parser = Parsec Void String
+
+parseInput' :: Parser Input
+parseInput' = some (read <$> some digitChar <* 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 = sum $ map (nextN 2000) input
+  where
+    nextN :: Int -> Int -> Int
+    nextN 0 n = n
+    nextN i n = nextN (i-1) $ next n
+    next :: Int -> Int
+    next s = let s' = prune $ mix (shift s 6) s
+                 s'' = prune $ mix (shift s' (-5)) s'
+                 s''' = prune $ mix (shift s'' 11) s''
+             in s'''
+    mix :: Int -> Int -> Int
+    mix = xor
+    prune :: Int -> Int
+    prune n = n `mod` 16777216
+
+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