2018/Day1B.hs

module Day1B where

import Day1A (read')
import qualified Data.IntSet as S

handle :: Int -> S.IntSet -> [Int] -> Int
handle freq history (nextDelta:xs)
  | S.member freq history = freq  -- When the frequency is in the history set, return it
  | otherwise             = handle (freq + nextDelta) (S.insert freq history) xs  -- Recurse otherwise
handle _ _ _ = error "xs should not be empty :("

main :: IO ()
main = interact $ show . handle 0 S.empty . deltaFreqs
  where deltaFreqs = cycle . map read' . lines  -- Convert input to Ints and repeat infinitely