54 lines
1.5 KiB
Haskell
54 lines
1.5 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.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 = 2
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type History = [Int]
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type Input = [History]
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type Parser = Parsec Void String
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parseNumber :: Parser Int
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parseNumber = read <$> some (char '-' <|> digitChar) <* hspace
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parseInput' :: Parser Input
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parseInput' = some (some parseNumber <* eol) <* 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 (next . reverse)
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where
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next :: History -> Int
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next = sum . map last . takeWhile (any (/= 0)) . iterate differences
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differences :: History -> History
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differences xs = zipWith (flip (-)) xs (tail xs)
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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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