d5.hs

import Data.Vector as V

-- parsing
parse :: String -> V.Vector Int
parse = fromList . fmap read . lines

run :: V.Vector Int -> Int
run v = go v 0
    where go v n 
      | n < 0 || n >= V.length v = 0
      | otherwise = let x  = v ! n
                        nv = v V.\\ [(n, succ x)]
                     in 1 + go nv (n + x)