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authorJulien Dessaux2024-12-10 09:27:43 +0100
committerJulien Dessaux2024-12-10 09:27:43 +0100
commitf8251081772e217ad537937cd48bb7e3f0541974 (patch)
tree4ab6a0c958898b2662f971f5c53a924f6024e2c0 /2024/10-Hoof_It/first.hs
parent2024-09 in haskell (diff)
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2024-10 in haskell
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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.Set as S
+import qualified Data.Vector as V
+import qualified Data.Vector.Unboxed as VU
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+
+exampleExpectedOutput = 36
+
+type Line = VU.Vector Int
+type Input = V.Vector Line
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Int
+parseNumber = read . pure <$> digitChar
+
+parseLine :: Parser Line
+parseLine = do
+ line <- some parseNumber <* eol
+ return $ VU.generate (length line) (line !!)
+
+parseInput' :: Parser Input
+parseInput' = do
+ line <- some parseLine <* eof
+ return $ V.generate (length line) (line !!)
+
+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 $ V.imap scoreLine input
+ where
+ scoreLine :: Int -> VU.Vector Int -> Int
+ scoreLine y line = VU.sum $ VU.imap (\x c -> score x y c) line
+ score :: Int -> Int -> Int -> Int
+ score x y c | c == 0 = S.size $ reachableSummits 0 (x, y)
+ | otherwise = 0
+ reachableSummits :: Int -> (Int, Int) -> S.Set (Int, Int)
+ reachableSummits h (x, y) | h == 9 = S.singleton (x, y)
+ | otherwise = S.unions $ map (reachableSummits (h+1)) succ
+ where
+ succ = filter valid [(x-1, y), (x+1, y), (x, y-1), (x, y+1)]
+ valid (x, y) = case input V.!? y of
+ Just line -> case line VU.!? x of
+ Just h' -> h+1 == h'
+ _ -> False
+ _ -> False
+
+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