import Data.List
import Data.Maybe
import Data.Tuple
import Data.Vector (fromList, (!?))

item k xs = maybeToList . (fromList xs !?) . subtract k

units = item 1
  [ "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"
  , "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen"
  , "Seventeen", "Eighteen", "Nineteen" ]

tens = item 2
  [ "Twenty", "Thirty", "Forty", "Fifty"
  , "Sixty", "Seventy", "Eighty", "Ninety" ]

infixr 5 <:

_  <: [] = []
"" <: xs = xs
x  <: xs = x : xs

part s n = s <: units u ++ tens t ++ "Hundred" <: units h
  where
    (h, m) = n `divMod` 100
    (t, u) | m < 20    = (0, m)
           | otherwise = m `divMod` 10

whole 0 = "Zero"
whole n = unwords . reverse . concat . zipWith part
  [ "", "Thousand", "Million", "Billion", "Trillion", "Quadrillion"
  , "Quintillion", "Sextillion", "Septillion", "Octillion", "Nonillion" ]
  $ unfoldr go n

go 0 = Nothing
go n = Just . swap $ n `divMod` 1000

main = putStrLn $ whole 79685746352413