Prob15.hs

import Data.List
import qualified Data.IntMap.Strict as M
data GameState = GS Int Int (M.IntMap Int) deriving Show
parse:: String -> [Int]
parse [] = []
parse l =
  case reads l of
    (z,',':rest):_ -> z:parse rest
    (z,_):_ -> [z]
    x -> error ("not handled: " ++ show (l, x))
next (GS !n !prev !posns) =
  case M.lookup prev posns of
    Nothing -> 0
    Just x -> n-x
    
play n (gs@(GS !n' !prev !posns))
  | n == n' = gs
  | otherwise =
      let new = next gs
          posns' = M.insert prev n' posns
      in play n (GS (n'+1) new posns')

main = do
  raw <-  getContents
  let nums = (parse raw)
      gs = GS (length nums) (last nums) (M.fromList (zip (init nums) [1..]))
      GS n res m = play (30000000) gs
  print res
  print (M.size m)