aoc-day1.hs

#!/usr/bin/env cabal
{-# LANGUAGE OverloadedStrings #-}
{- cabal:
build-depends:
  , base ^>= 4.13.0.0
  , unliftio ^>= 0.2.13.1
  , text ^>= 1.2.4.0
  , transformers ^>= 0.5.6.2
-}

import Control.Monad.IO.Class (MonadIO, liftIO)
import System.IO (IOMode(ReadMode), openFile)
import Data.Text.IO (hGetContents)
import Data.Text as T ( Text, words, unpack)
import Data.Text.Read (decimal)
import Data.Either (rights)
import Control.Monad.Trans.Maybe (runMaybeT, MaybeT(MaybeT))
import Data.Foldable as F ( Foldable(foldl) )
import Debug.Trace ( trace )


main :: (MonadIO m)  => m ()
main = do
  expenses <- readExpenses "../data/day1.txt" 
  results <- runMaybeT $ do
    step1 <- fmap calculate (findTwoErrors 2020 expenses)
    step2 <- fmap calculate (findThreeErrors 2020 expenses)
    return (step1, step2)
  liftIO $
    case results of
      Nothing -> print "Error"
      Just r -> print r


readExpenses :: MonadIO m => Text -> m [Int]
readExpenses path = do
  fileHandle <- liftIO . openFile (T.unpack path) $ ReadMode
  content <- liftIO . hGetContents $ fileHandle
  pure $ ints content
  where
    ints = fmap fst . rights . fmap decimal . T.words


calculate :: [Int] -> Int
calculate = F.foldl (*) 1

findThreeErrors ::  (Monad m) => Int -> [Int] -> MaybeT m [Int]
findThreeErrors _ [] = MaybeT $ do return Nothing
findThreeErrors searchTarget (expense:xs) = MaybeT $ do
  r <- runMaybeT $ findTwoErrors (searchTarget - expense) xs
  case r of
    Nothing -> runMaybeT $ findThreeErrors searchTarget xs
    Just [a,b] -> return $ Just [expense,a,b]
    Just wtf -> trace (show wtf) (return Nothing)


findTwoErrors :: Monad m => Int -> [Int] -> MaybeT m [Int]
findTwoErrors _ [] = MaybeT $ do return Nothing
findTwoErrors searchTarget (expense:xs) = MaybeT $ do
  if localTarget `elem` xs
  then return $ Just [localTarget,expense]
  else runMaybeT $ findTwoErrors searchTarget xs
  where
    localTarget = searchTarget - expense