2020-22 part 2 in haskell
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2020/22-Crab_Combat/second.hs
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86
2020/22-Crab_Combat/second.hs
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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 (catMaybes)
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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 = 291
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type Card = Int
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type Deck = [Card]
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type Input = (Deck, Deck)
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type History = [Input]
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type Parser = Parsec Void String
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parseCard :: Parser Card
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parseCard = do
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n <- some digitChar
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void $ optional (char '\n')
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return $ read n
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parseDeck :: Parser Deck
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parseDeck = string "Player " *> digitChar *> string ":\n" *> some parseCard
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parseInput' :: Parser Input
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parseInput' = do
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p1 <- parseDeck
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void $ char '\n'
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p2 <- parseDeck
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void $ eof
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return (p1, p2)
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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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computeScore :: Deck -> Int
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computeScore d = sum $ zipWith (*) d (reverse $ take (length d) [1..])
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recursing :: Input -> History -> Bool
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recursing i = or . map (== i)
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computeRound :: Input -> Bool
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computeRound (a:as, b:bs)
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| (length as >= a) && (length bs >= b) = computeSubGame (take a as, take b bs) []
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| otherwise = a > b
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computeSubGame :: Input -> History -> Bool
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computeSubGame ([], d) _ = False
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computeSubGame (d, []) _ = True
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computeSubGame (a:as, b:bs) h
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| recursing (a:as, b:bs) h = True
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| computeRound (a:as, b:bs) = computeSubGame (as ++ [a, b], bs) newH
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| otherwise = computeSubGame (as, bs ++ [b, a]) newH
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where
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newH = (a:as, b:bs) : h
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compute' :: Input -> History -> Int
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compute' ([], d) _ = computeScore d
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compute' (d, []) _ = computeScore d
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compute' (a:as, b:bs) h
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| recursing (a:as, b:bs) h = computeScore (a:as)
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| computeRound (a:as, b:bs) = compute' (as ++ [a, b], bs) newH
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| otherwise = compute' (as, bs ++ [b, a]) newH
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where
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newH = (a:as, b:bs) : h
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compute :: Input -> Int
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compute input = compute' 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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