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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.List as L
import qualified Data.Map as M
import qualified Data.Vector as V
import qualified Data.Vector.Unboxed as VU
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
example1ExpectedOutput = 140
example2ExpectedOutput = 772
example3ExpectedOutput = 1930
type Line = VU.Vector Char
type Input = V.Vector Line
type Parser = Parsec Void String
parseLine :: Parser Line
parseLine = do
line <- some letterChar <* 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'
type Coord = (Int, Int)
type Visited = M.Map Coord ()
compute :: Input -> Int
compute input = fst $ V.ifoldl' compute' (0, M.empty) input
where
compute' :: (Int, Visited) -> Int -> Line -> (Int, Visited)
compute' (acc, visited) y line = VU.ifoldl' (compute'' y) (acc, visited) line
compute'' :: Int -> (Int, Visited) -> Int -> Char -> (Int, Visited)
compute'' y acc@(cost, visited) x c = case M.lookup (x, y) visited of
Just _ -> acc -- already processed
Nothing -> let (area, perimeter, visited') = enclose c (0, 0, visited) (x, y) in (cost + area * perimeter, M.union visited visited')
enclose :: Char -> (Int, Int, Visited) -> Coord -> (Int, Int, Visited)
enclose c acc@(area, perimeter, visited) (x, y) = case M.lookup (x, y) visited of
Just _ -> acc
Nothing -> case input V.!? y of
Just line -> case line VU.!? x of
Just c' -> let n = neighbors visited c x y in L.foldl' (enclose c) (1 + area, 4 - length n + perimeter, M.insert (x, y) () visited) n
Nothing -> (0, 0, visited)
Nothing -> (0, 0, visited)
neighbors :: Visited -> Char -> Int -> Int -> [Coord]
neighbors visited c x y = filter (valid visited c) [ (x-1, y), (x+1, y), (x, y-1), (x, y+1) ]
valid :: Visited -> Char -> Coord -> Bool
valid visited c (x, y) = case input V.!? y of
Just line -> case line VU.!? x of
Just c' -> c == c'
Nothing -> False
Nothing -> False
main :: IO ()
main = do
example1 <- parseInput "example1"
let exampleOutput = compute example1
when (exampleOutput /= example1ExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show example1ExpectedOutput)
example2 <- parseInput "example2"
let exampleOutput = compute example2
when (exampleOutput /= example2ExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show example2ExpectedOutput)
example3 <- parseInput "example3"
let exampleOutput = compute example3
when (exampleOutput /= example3ExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show example3ExpectedOutput)
input <- parseInput "input"
print $ compute input
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