76 lines
2.1 KiB
Haskell
76 lines
2.1 KiB
Haskell
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-- requires cabal install --lib megaparsec parser-combinators
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module Main (main) where
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import Control.Applicative.Permutations
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import Control.Monad (void, when)
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import Data.Char qualified as C
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import Data.Either
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import Data.Functor
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import Data.List qualified as L
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import Data.Map qualified as M
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import Data.Maybe
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import Data.Set qualified as S
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import Data.Vector qualified as V
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import Data.Void (Void)
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import Text.Megaparsec
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import Text.Megaparsec.Char
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import Debug.Trace
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exampleExpectedOutput = 400
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data Tile = Ash | Rock deriving (Eq)
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instance Show Tile where
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show Ash = "."
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show Rock = "#"
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type Row = [Tile]
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type Pattern = [Row]
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type Input = [Pattern]
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type Parser = Parsec Void String
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parseTile :: Parser Tile
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parseTile = char '.' $> Ash
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<|> char '#' $> Rock
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parseRow :: Parser Row
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parseRow = some parseTile <* eol
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parsePattern :: Parser Pattern
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parsePattern = some parseRow <* eol
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parseInput' :: Parser Input
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parseInput' = some parsePattern <* eof
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parseInput :: String -> IO Input
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parseInput filename = do
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input <- readFile filename
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case runParser parseInput' filename input of
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Left bundle -> error $ errorBundlePretty bundle
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Right input' -> return input'
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compute :: Input -> Int
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compute = sum . map findMirror
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where
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findMirror :: Pattern -> Int
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findMirror xs = 100 * findMirrors xs 1 + findMirrors (L.transpose xs) 1
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findMirrors :: Pattern -> Int -> Int
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findMirrors xs i | i == length xs = 0
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| diffs == 1 = i
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| otherwise = next
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where
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(is, ts) = L.splitAt i xs
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next = findMirrors xs (i+1)
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is' = reverse is -- we need to flip the left side
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diffs = sum $ zipWith diff is' ts
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diff :: Row -> Row -> Int
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diff as bs = length . L.filter id $ zipWith (/=) as bs
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main :: IO ()
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main = do
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example <- parseInput "example"
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let exampleOutput = compute example
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when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
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input <- parseInput "input"
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print $ compute input
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