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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
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