-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where

import Control.Applicative.Permutations
import Control.Monad (void, when)
import Data.Char qualified as C
import Data.Either
import Data.Functor
import Data.List qualified as L
import Data.Map qualified as M
import Data.Maybe
import Data.Set qualified as S
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char

import Debug.Trace

exampleExpectedOutput = 2

type History = [Int]
type Input = [History]

type Parser = Parsec Void String

parseNumber :: Parser Int
parseNumber = read <$> some (char '-' <|> digitChar) <* hspace

parseInput' :: Parser Input
parseInput' = some (some parseNumber <* eol) <* eof

parseInput :: String -> IO Input
parseInput filename = do
  input <- readFile filename
  case runParser parseInput' filename input of
    Left bundle -> error $ errorBundlePretty bundle
    Right input' -> return input'

compute :: Input -> Int
compute = sum . map (next . reverse)
  where
    next :: History -> Int
    next = sum . map last . takeWhile (any (/= 0)) . iterate differences
    differences :: History -> History
    differences xs = zipWith (flip (-)) xs (tail xs)

main :: IO ()
main = do
  example <- parseInput "example"
  let exampleOutput = compute example
  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
  input <- parseInput "input"
  print $ compute input