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-- 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 (fromJust)
import Data.Set qualified as S
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import System.Exit (die)
exampleExpectedOutput = 67384529
type Cup = Int
type Input = [Cup]
type History = [Input]
type Parser = Parsec Void String
parseCup :: Parser Cup
parseCup = do
n <- digitChar
return $ read [n] - 1 -- so that there is a 0 in there and then we can use modulo
parseInput' :: Parser Input
parseInput' = some parseCup <* optional (char '\n') <* eof
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'
score :: Input -> Int
score (0:xs) = read . concat $ map (\i -> show (i+1)) xs
score (x:xs) = score $ xs ++ [x]
compute' :: Input -> Int -> Int
compute' input 0 = score input
compute' (current:one:two:three:xs) remainingMoves = compute' step (remainingMoves - 1)
where
l = length xs + 4
destinationIndex :: Int
destinationIndex = case L.elemIndex ((current - 1) `mod` l) xs of
Just i -> i
Nothing -> case L.elemIndex ((current - 2) `mod` l) xs of
Just i -> i
Nothing -> case L.elemIndex ((current - 3) `mod` l) xs of
Just i -> i
Nothing -> fromJust $ L.elemIndex ((current - 4) `mod` l) xs
destinationIndex' = destinationIndex + 1
step :: Input
step = concat [take destinationIndex' xs, [one, two, three], drop destinationIndex' xs, [current]]
compute :: Input -> Int
compute input = compute' input 100
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
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