73 lines
2.3 KiB
Haskell
73 lines
2.3 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.Monad (void, when)
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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 (fromJust)
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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 System.Exit (die)
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exampleExpectedOutput = 149245887792
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type Cup = Int
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type Input = [Cup]
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type Input' = M.Map Cup Cup
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type Parser = Parsec Void String
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parseCup :: Parser Cup
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parseCup = do
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n <- digitChar
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return $ read [n] - 1 -- so that there is a 0 in there and then we can use modulo
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parseInput' :: Parser Input
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parseInput' = some parseCup <* optional (char '\n') <* 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 -> die $ errorBundlePretty bundle
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Right input' -> return input'
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score :: Input' -> Int
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score input = (one + 1) * (two + 1)
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where
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one = input M.! 0
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two = input M.! one
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compute' :: Cup -> Input' -> Int -> Int
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compute' _ input 0 = score input
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compute' current input remainingMoves = compute' next step (remainingMoves - 1)
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where
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one = input M.! current
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two = input M.! one
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three = input M.! two
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next = input M.! three
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cups = [one, two, three]
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mone = (current - 1) `mod` 1_000_000
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mtwo = (current - 2) `mod` 1_000_000
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mthree = (current - 3) `mod` 1_000_000
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mfour = (current - 4) `mod` 1_000_000
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destination :: Int
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destination = if L.elem mone cups then (if L.elem mtwo cups then (if L.elem mthree cups then mfour else mthree) else mtwo) else mone
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afterDestination = input M.! destination
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step :: Input'
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step = M.insert current next $ M.insert destination one $ M.insert three afterDestination input
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compute :: Input -> Int
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compute input = compute' (head input) input' 10_000_000
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where
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input' = M.fromList (zip input (tail input) ++ [(last input, 9)] ++ (zip [9..999_998] [10..999_999]) ++ [(999_999, head input)])
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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) (die $ "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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