import Data.Char (chr)

generate' _ 0 _ _ _ _ = Nothing
generate' goal generations_left generation 0 _ _ =
    generate' goal (generations_left - 1) (generation + 1) (generation + 1) 0 [0]
generate' goal generations_left generation left aggregate (x:xs)
    | value == goal = Just (reverse (map (chr . fromInteger . (+ 97)) (x:xs)))
    | value > goal  = next_generate (left - 1) next_aggregate (0:(x - 1):xs)
    | x == 25       = next_generate (left - 1) (next_aggregate + 1) (0:x:xs)
    | otherwise     = next_generate left aggregate ((x + 1):xs)
    where next_generate = generate' goal generations_left generation
          next_aggregate = aggregate * 17 + x
          padded_value 0       v = v
          padded_value padding v = padded_value (padding - 1) (v * 17 + 1)
          value = padded_value (left - 1) (next_aggregate + 1)

generate max_length = generate' 248410397744610 max_length 1 1 0 [0]