-- Which prime, below one-million, can be written as the sum of the most
-- consecutive primes?

import Euler

intersectOn' :: (a -> b -> Ordering) -> [a] -> [b] -> [b]
intersectOn' _ []     _      = []
intersectOn' _ _      []     = []
intersectOn' f (x:xs) (y:ys) = case f x y of
   LT ->     intersectOn' f xs (y:ys)
   EQ -> y : intersectOn' f xs     ys
   GT ->     intersectOn' f (x:xs) ys

primeSums = concat $
   map (intersectOn' (\p (_,q) -> compare p q) primes) $
   map (zip [1..] . takeWhile (< 1000000) . tail . scanl' (+) 0) $
   tails $ takeWhile (< 50000) $ primes

main = print $ snd $ maximumBy (compare `on` fst) $ primeSums