Main.hs

module Main (main) where

import Control.Monad.State

main :: IO ()
main = print . solve =<< readFile "input.txt"

data Dir = L Int | R Int

parseDir :: String -> Dir
parseDir = \case
  'L' : xs -> L (read xs)
  'R' : xs -> R (read xs)
  _ -> undefined

applyDir :: Dir -> State Int Int
applyDir dir = do
  curr <- get
  put case dir of
    L n -> (curr - n) `mod` 100
    R n -> (curr + n) `mod` 100
  get

solve :: String -> Int
solve = length . filter (0 ==) . flip evalState 50 . traverse (applyDir . parseDir) . lines

Main2.hs

module Main (main) where

import Control.Monad.State

main :: IO ()
main = print . solve =<< readFile "input.txt"

data Dir = L Int | R Int

parseDir :: String -> Dir
parseDir = \case
  'L' : xs -> L (read xs)
  'R' : xs -> R (read xs)
  _ -> undefined

clicks :: Int -> Dir -> Int
clicks curr = \case
  L n -> (n `div` 100) + if curr > 0 && curr <= (n `mod` 100) then 1 else 0
  R n -> (n `div` 100) + if curr > 0 && (100 - curr) <= (n `mod` 100) then 1 else 0

applyDir :: Dir -> State Int Int
applyDir dir = do
  curr <- get
  put case dir of
    L n -> (curr - n) `mod` 100
    R n -> (curr + n) `mod` 100
  pure $ clicks curr dir

solve :: String -> Int
solve = sum . flip evalState 50 . traverse (applyDir . parseDir) . lines