numwords.hs

module Main where

main :: IO ()
main = interact (unlines . map (go . read) . lines)

go :: Int -> String
go n = case term n
  of [] -> "nothing"
     [(x,"")] -> numb x
     xs -> unwords $ map item xs

item (0,"") = ""
item (0,y)  = y
item (x,"") = "and " ++ numb x
item (x,y)  = numb x ++ " " ++ y

term :: Int -> [(Int, String)]
term n
  | n > quadrilion  = let (d,m) = divMod n quadrilion in (d,"quadrilion") : term m
  | n > trilion     = let (d,m) = divMod n trilion    in (d,"trilion")    : term m
  | n > billion     = let (d,m) = divMod n billion    in (d,"billion")    : term m
  | n > million     = let (d,m) = divMod n million    in (d,"million")    : term m
  | n > thousand    = let (d,m) = divMod n thousand   in (d,"thousand")   : term m
  | n > hundred     = let (d,m) = divMod n hundred    in (d,"hundred")    : term m
  | otherwise       = [(n,"")]

hundred    = 100
thousand   = 1000
million    = thousand * thousand
billion    = thousand * million
trilion    = thousand * billion
quadrilion = thousand * trilion

prop_test1 :: Bool
prop_test1 = go 1000000 == "one million"

numb :: Int -> String
numb 0 = ""
numb 1 = "one"
numb 2 = "two"
numb 3 = "three"
numb 4 = "four"
numb 5 = "five"
numb 6 = "six"
numb 7 = "seven"
numb 8 = "eight"
numb 9 = "nine"
numb 10 = "ten"
numb 11 = "eleven"
numb 12 = "twelve"
numb 13 = "thirteen"
numb 14 = "fourteen"
numb 15 = "fifteen"
numb 16 = "sixteen"
numb 17 = "seventeen"
numb 18 = "eighteen"
numb 19 = "nineteen"
numb 20 = "twenty"
numb x | x < 100  = let (d,m) = divMod x 10 in tees d ++ " " ++ numb m
       | x < 1000 = let (d,m) = divMod x 100 in hundreds d ++ " " ++ numb m
numb x = error $ "numb: " ++ show x

tees :: Int -> String
tees 1 = "ten"
tees 2 = "twenty"
tees 3 = "thirty"
tees 4 = "fourty"
tees 5 = "fifty"
tees 6 = "sixty"
tees 7 = "seventy"
tees 8 = "eighty"
tees 9 = "ninety"
tees x = error $ "tees: " ++ (show x)

hundreds n = numb n ++ " hundred"