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-rw-r--r--2020/22-Crab_Combat/second.hs86
1 files changed, 86 insertions, 0 deletions
diff --git a/2020/22-Crab_Combat/second.hs b/2020/22-Crab_Combat/second.hs
new file mode 100644
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+++ b/2020/22-Crab_Combat/second.hs
@@ -0,0 +1,86 @@
+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+import Control.Monad (void, when)
+import Data.List qualified as L
+import Data.Map qualified as M
+import Data.Maybe (catMaybes)
+import Data.Set qualified as S
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 291
+
+type Card = Int
+type Deck = [Card]
+type Input = (Deck, Deck)
+type History = [Input]
+
+type Parser = Parsec Void String
+
+parseCard :: Parser Card
+parseCard = do
+ n <- some digitChar
+ void $ optional (char '\n')
+ return $ read n
+
+parseDeck :: Parser Deck
+parseDeck = string "Player " *> digitChar *> string ":\n" *> some parseCard
+
+parseInput' :: Parser Input
+parseInput' = do
+ p1 <- parseDeck
+ void $ char '\n'
+ p2 <- parseDeck
+ void $ eof
+ return (p1, p2)
+
+parseInput :: String -> IO Input
+parseInput filename = do
+ input <- readFile filename
+ case runParser parseInput' filename input of
+ Left bundle -> die $ errorBundlePretty bundle
+ Right input' -> return input'
+
+computeScore :: Deck -> Int
+computeScore d = sum $ zipWith (*) d (reverse $ take (length d) [1..])
+
+recursing :: Input -> History -> Bool
+recursing i = or . map (== i)
+
+computeRound :: Input -> Bool
+computeRound (a:as, b:bs)
+ | (length as >= a) && (length bs >= b) = computeSubGame (take a as, take b bs) []
+ | otherwise = a > b
+
+computeSubGame :: Input -> History -> Bool
+computeSubGame ([], d) _ = False
+computeSubGame (d, []) _ = True
+computeSubGame (a:as, b:bs) h
+ | recursing (a:as, b:bs) h = True
+ | computeRound (a:as, b:bs) = computeSubGame (as ++ [a, b], bs) newH
+ | otherwise = computeSubGame (as, bs ++ [b, a]) newH
+ where
+ newH = (a:as, b:bs) : h
+
+compute' :: Input -> History -> Int
+compute' ([], d) _ = computeScore d
+compute' (d, []) _ = computeScore d
+compute' (a:as, b:bs) h
+ | recursing (a:as, b:bs) h = computeScore (a:as)
+ | computeRound (a:as, b:bs) = compute' (as ++ [a, b], bs) newH
+ | otherwise = compute' (as, bs ++ [b, a]) newH
+ where
+ newH = (a:as, b:bs) : h
+
+compute :: Input -> Int
+compute input = compute' input []
+
+main :: IO ()
+main = do
+ example <- parseInput "example"
+ let exampleOutput = compute example
+ when (exampleOutput /= exampleExpectedOutput) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+ input <- parseInput "input"
+ print $ compute input