2024-14 in haskell

2024/14-Restroom_Redoubt/second.hs

@@ -0,0 +1,54 @@
+-- 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
+
+type Pair = (Int, Int)
+type Robot = (Pair, Pair)
+type Input = [Robot]
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read <$> some (digitChar <|> char '-')
+
+parsePair :: Parser Pair
+parsePair = (,) <$> parseNumber <* char ','
+                <*> parseNumber
+
+parseRobot :: Parser Robot
+parseRobot = (,) <$> (string "p=" *> parsePair)
+                 <*> (string " v=" *> parsePair)
+
+parseInput' :: Parser Input
+parseInput' = some (parseRobot <* 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 :: Int -> Int -> Input -> Int -- the minimum safety score is the answer
+compute width height input = fst $ L.foldl' (\acc@(_, a) n@(_, b) -> if a > b then n else acc) (0, 1000000000) safeties
+  where
+    safeties = [(i, safety i)|i<-[0..width*height]]
+    mx = width `div` 2
+    my = height `div` 2
+    safety i = product $ L.foldl' score [0, 0, 0, 0] $ map (step i) input
+    step i ((x, y), (v, w)) = (((x + v * i) `mod` width), ((y + w * i) `mod` height))
+    score acc@[a, b, c, d] (x, y) | x < mx && y < my = [a+1, b, c, d]
+                                  | x > mx && y < my = [a, b+1, c, d]
+                                  | x < mx && y > my = [a, b, c+1, d]
+                                  | x > mx && y > my = [a, b, c, d+1]
+                                  | otherwise = acc
+
+main :: IO ()
+main = do
+  input <- parseInput "input"
+  print $ compute 101 103 input