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-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where
import Control.Monad (void, when)
import Data.List (foldl')
import Data.Map qualified as M
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import System.Exit (die)
exampleExpectedOutput = 436
type Input = [Int]
type Parser = Parsec Void String
parseInt :: Parser Int
parseInt = do
n <- some digitChar
void $ optional (char ',')
return $ read n
parseOps :: Parser Input
parseOps = some parseInt <* char '\n' <* eof
parseInput :: String -> IO Input
parseInput filename = do
input <- readFile filename
case runParser parseOps filename input of
Left bundle -> die $ errorBundlePretty bundle
Right ops -> return ops
compute :: Input -> Int
compute input = compute' (length input) (foldl' prepare M.empty $ zip (init input) [1..]) (last input)
where
compute' :: Int -> M.Map Int (Int, Int) -> Int -> Int
compute' 2020 _ n = n
compute' t m n = case M.lookup n m of
Just (t', i) -> compute' (t+1) (M.insert n (t, i+1) m) (t - t')
Nothing -> compute' (t+1) (M.insert n (t, 0) m) 0
prepare :: M.Map Int (Int, Int) -> (Int, Int) -> M.Map Int (Int, Int)
prepare acc (n, t) = case M.lookup n acc of
Just (_, i) -> M.insert n (t, i+1) acc
Nothing -> M.insert n (t, 0) acc
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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