module Main where

import Data.Char (digitToInt)
import Data.Either (rights)
import Debug.Trace
import System.Environment (getArgs)

-- | A candidate is either a left-most number we've just selected, or
-- a complete candidate represented as a pair.
type Candidate = Either Int (Int, Int)

main :: IO ()
main = do
  input <- readFile . head =<< getArgs
  putStrLn $ "Part 1: " ++ show (part1 input)
  putStrLn $ "Part 2: " ++ show (part2 input)

-- | Maintain a list of candidates (either partial, or complete) and
-- then select the greatest one for each line.
--
-- Insight: By keeping a list of possible candidates we can always
-- backtrack out of an accidental corner (like making a partial
-- candidate of the last digit).
part1 :: String -> Int
part1 = sum . map (maximum . map combine . rights . foldl go [] . map digitToInt) . lines
  where
    go :: [Candidate] -> Int -> [Candidate]
    go [] x = pure $ Left x
    go (Left a : candidates') x
      | x > a = Left x : Right (a, x) : candidates'
      | otherwise = Right (a, x) : candidates'
    go candidates@(Right (a, b) : candidates') x
      | x > a = Left x : Right (a, x) : candidates
      | x > b = Right (a, x) : candidates'
      | otherwise = candidates

    combine :: (Int, Int) -> Int
    combine (a, b) = a * 10 + b

part2 :: String -> Int
part2 = error "Not implemented"

testInput :: String
testInput =
  unlines
    [ "987654321111111",
      "811111111111119",
      "234234234234278",
      "818181911112111"
    ]