-- This Haskell program converts positive integers to the English phrase equivalent.  It only deals with integers of up to 12 digits.

-- The main function is cvt.  Type cvt 13472 for example.
-- There is also a 'display' version that prints the string to standard output rather than returning the string as result.

import Char(toUpper)

convert2 :: Int -> String
convert2 n = combine2 (tens n)

tens :: Int -> (Int,Int)
tens n = (n `div` 10, n `mod` 10)

unitwords, teenwords, tenwords :: [String]
unitwords = ["one","two","three","four","five","six","seven","eight","nine"]
teenwords = ["ten","eleven","twelve","thirteen","fourteen","fifteen",
             "sixteen","seventeen","eighteen","nineteen"]
tenwords  = ["twenty","thirty","forty","fifty","sixty","seventy",
             "eighty","ninety"]

combine2 :: (Int,Int) -> String
combine2 (0, u+1)   = unitwords!!u		-- remember indexing is from 0
combine2 (1, u)     = teenwords!!u
combine2 (t+2, 0)   = tenwords!!t
combine2 (t+2, u+1) = tenwords!!t ++ "-" ++ unitwords!!u


convert3 :: Int -> String
convert3 n = combine3 (hundreds n)

hundreds :: Int -> (Int,Int)
hundreds n = (n `div` 100, n `mod` 100)

combine3 :: (Int,Int) -> String
combine3 (0, u)   = convert2 u
combine3 (h+1, 0) = unitwords!!h ++ " hundred"
combine3 (h+1, u) = unitwords!!h ++ " hundred and " ++ convert2 u


convert6 :: Int -> String
convert6 n = combine6 (thousands n)

thousands :: Int -> (Int,Int)
thousands n = ( n `div` 1000, n `mod` 1000)

combine6 :: (Int,Int) -> String
combine6 (0, u)  = convert3 u
combine6 (th, 0) = convert3 th ++ " thousand"
combine6 (th, u) = convert3 th ++ " thousand" ++ link u ++ convert3 u

link h 
     | h < 100    = " and "
     | otherwise  = " "			-- alternatively ", "

-- Extension to billions. (i.e. up to 12 digits.)

convert12 :: Int -> String
convert12 n = combine12 (millions n)

millions :: Int -> (Int,Int)
millions n = (n `div` 1000000, n `mod` 1000000)

combine12 :: (Int,Int) -> String
combine12 (0, u) = convert6 u
combine12 (m, 0) = convert6 m ++ " million"
combine12 (m, u) = convert6 m ++ " million" ++ link u ++ convert6 u

-- Extension to full stop and capitalise first word.
	
cvt :: Int -> String
cvt 0 = "Zero."
cvt n = (toUpper ch):chs
  where (ch:chs) = convert12 n ++ "."

-- Print the converted result to standard output.

wordify :: Int -> IO()
wordify n = putStr (cvt n)