63 lines
2 KiB
Haskell
63 lines
2 KiB
Haskell
-- requires cabal install --lib megaparsec parser-combinators heap vector
|
|
module Main (main) where
|
|
|
|
import Control.Monad (void, when)
|
|
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 = 81
|
|
|
|
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 = length $ reachableSummits 0 (x, y)
|
|
| otherwise = 0
|
|
reachableSummits :: Int -> (Int, Int) -> [(Int, Int)]
|
|
reachableSummits h (x, y) | h == 9 = [(x, y)]
|
|
| otherwise = concatMap (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
|