2020-14 part 2 in haskell

2020/14-Docking_Data/example2

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+mask = 000000000000000000000000000000X1001X
+mem[42] = 100
+mask = 00000000000000000000000000000000X0XX
+mem[26] = 1

2020/14-Docking_Data/second.hs

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+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+import Control.Monad (void, when)
+import Data.Bits (clearBit, setBit)
+import Data.List (foldl')
+import Data.Map qualified as M
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 208
+
+type Bits = [Maybe Bool]
+data Op = Mask Bits| Mem Int Int
+type Input = [Op]
+
+type Parser = Parsec Void String
+
+parseMask :: Parser Op
+parseMask = do
+  void $ string "mask = "
+  bits <- some $ (char '1' *> (return $ Just True)) <|> (char '0' *> (return $ Just False)) <|> (char 'X' *> (return Nothing))
+  return $ Mask bits
+
+parseMem :: Parser Op
+parseMem = do
+  void $ string "mem["
+  index <- some digitChar
+  void $ string "] = "
+  value <- some digitChar
+  return $ Mem (read index) (read value)
+
+parseOps :: Parser Input
+parseOps = do
+  ops <- some $ (parseMask <|> parseMem) <* (optional $ char '\n')
+  void $ eof
+  return $ ops
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseOps filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right ops -> return ops
+
+computeOp :: Int -> Bits -> [Int]
+computeOp index bits = fst $ foldl' evalBit ([index], 35) bits
+  where
+    evalBit :: ([Int], Int) -> Maybe Bool -> ([Int], Int)
+    evalBit (acc, bit) (Just False) = (acc, bit-1)
+    evalBit (acc, bit) (Just True) = (map (\x -> setBit x bit) acc, bit-1)
+    evalBit (acc, bit) Nothing = ((map (\x -> setBit x bit) acc) ++ (map (\x -> clearBit x bit) acc), bit-1)
+
+compute :: Input -> Int
+compute = extract . foldl' compute init
+  where
+    extract :: (M.Map Int Int, Bits) -> Int
+    extract = sum . M.elems . fst
+    compute :: (M.Map Int Int, Bits) -> Op -> (M.Map Int Int, Bits)
+    compute (acc, _) (Mask bits) = (acc, bits)
+    compute (acc, bits) (Mem index value) = (newAcc, bits)
+      where
+        newAcc :: M.Map Int Int
+        newAcc = foldl' eval acc (computeOp index bits)
+        eval :: M.Map Int Int -> Int -> M.Map Int Int
+        eval acc' index' = M.insert index' value acc'
+    init :: (M.Map Int Int, Bits)
+    init = (M.empty, [])
+
+main :: IO ()
+main = do
+  example <- parseInput "example2"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input