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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 = 80
example2ExpectedOutput = 436
example3ExpectedOutput = 1206
example4ExpectedOutput = 236
example5ExpectedOutput = 368
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 ()
type Region = [Coord]
compute :: Input -> Int
compute input = sum $ map (\r -> corners r * length r) regions
where
(regions, _) = V.ifoldl' regions' ([], M.empty) input
regions' :: ([Region], Visited) -> Int -> Line -> ([Region], Visited)
regions' acc y line = VU.ifoldl' (regions'' y) acc line
regions'' :: Int -> ([Region], Visited) -> Int -> Char -> ([Region], Visited)
regions'' y acc@(rs, visited) x c = case M.lookup (x, y) visited of
Just _ -> acc -- already processed
Nothing -> let (r, visited') = flood c ([], visited) (x, y) in (r:rs, visited')
flood :: Char -> (Region, Visited) -> (Int, Int) -> (Region, Visited)
flood c acc@(rs, visited) (x, y) = case M.lookup (x, y) visited of
Just _ -> acc
Nothing -> L.foldl' (flood c) ((x, y):rs, M.insert (x, y) () visited) $ neighbors visited c x y
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 -> (Int, Int) -> 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
corners :: Region -> Int
corners r = sum $ map corners' r
where
inside c = L.elem c r
outside = not . inside
corners' (x, y) = length $ filter id [ outside (x-1, y) && outside (x, y-1) -- top left in
, outside (x+1, y) && outside (x, y-1) -- top right in
, outside (x-1, y) && outside (x, y+1) -- bottom left in
, outside (x+1, y) && outside (x, y+1) -- bottom right in
, inside (x-1, y) && outside (x-1, y-1) && inside (x, y-1) -- top left out
, inside (x+1, y) && outside (x+1, y-1) && inside (x, y-1) -- top right out
, inside (x-1, y) && outside (x-1, y+1) && inside (x, y+1) -- bottom left out
, inside (x+1, y) && outside (x+1, y+1) && inside (x, y+1) ] -- bottom right out
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)
example4 <- parseInput "example4"
let exampleOutput = compute example4
when (exampleOutput /= example4ExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show example4ExpectedOutput)
example5 <- parseInput "example5"
let exampleOutput = compute example5
when (exampleOutput /= example5ExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show example5ExpectedOutput)
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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