My solution to the Twitter waterflow problem. Other solutions here: http://chrisdone.com/posts/twitter-problem-loeb

gistfile1.hs

puddles :: [Int] -> Int
puddles l = go 0 [] (zip l (repeat 1)) where
    go acc [] (a:b:xs) 
       | fst a <= fst b  = go acc [] (b:xs)
       | otherwise       = go acc [a] (b:xs)
    go acc (p@(pp,cp):ps) (a@(aa,ca):b@(bb,cb):xs) 
       | aa == bb = go acc (p:ps) ((aa,ca+cb):xs)
       | aa <  bb = let acc' = acc + ca * min (pp-aa) (bb-aa) in
                    if pp > bb 
                       then go acc' (p:ps) ((bb,cb+ca):xs)
                       else go acc' ps     ((pp,cp+ca):b:xs)
       | otherwise = go acc (a:p:ps) (b:xs)
    go acc _ _ = acc

main :: IO ()
main = do
   putStrLn . show $ puddles $ [0,4,6,3]
   putStrLn . show $ puddles $ [0,1,0,1]
   putStrLn . show $ puddles $ [0,2,0,2]
   putStrLn . show $ puddles $ [0,2,0,2,0,1]
   putStrLn . show $ puddles $ [0,2,0,0,2,0,1]
   putStrLn . show $ puddles $ [0,2,0,1,4,0,1]
   putStrLn . show $ puddles $ [2,5,1,2,3,4,7,7,6]