ProjectEuler/haskell/src/0017.hs

{-
If the numbers 1 to 5 are written out in words: one, two, three, four, five, then there are 3 + 3 + 5 + 4 + 4 = 19 letters used in total.
If all the numbers from 1 to 1000 (one thousand) inclusive were written out in words, how many letters would be used?
NOTE: Do not count spaces or hyphens. For example, 342 (three hundred and forty-two) contains 23 letters and 115 (one hundred and fifteen) contains 20 letters. The use of "and" when writing out numbers is in compliance with British usage.
-}

import Data.Char
import Data.Function

(|>) = (&)

-- Round Down to Nearest Ten
rd2nt :: Int -> Int
rd2nt x
  | x < 10  = 0
  | x < 20  = 10
  | x < 30  = 20
  | x < 40  = 30
  | x < 50  = 40
  | x < 60  = 50
  | x < 70  = 60
  | x < 80  = 70
  | x < 90  = 80
  | x < 100 = 90
  | x >= 100 = rd2nt (mod x 100)

digit_to_name :: Int -> String
digit_to_name 0  = ""
digit_to_name 1  = "one"
digit_to_name 2  = "two"
digit_to_name 3  = "three"
digit_to_name 4  = "four"
digit_to_name 5  = "five"
digit_to_name 6  = "six"
digit_to_name 7  = "seven"
digit_to_name 8  = "eight"
digit_to_name 9  = "nine"
digit_to_name 10 = "ten"
digit_to_name 11 = "eleven"
digit_to_name 12 = "twelve"
digit_to_name 13 = "thirteen"
digit_to_name 14 = "fourteen"
digit_to_name 15 = "fifteen"
digit_to_name 16 = "sixteen"
digit_to_name 17 = "seventeen"
digit_to_name 18 = "eighteen"
digit_to_name 19 = "nineteen"
digit_to_name 20 = "twenty"
digit_to_name 30 = "thirty"
digit_to_name 40 = "forty"
digit_to_name 50 = "fifty"
digit_to_name 60 = "sixty"
digit_to_name 70 = "seventy"
digit_to_name 80 = "eighty"
digit_to_name 90 = "ninety"
digit_to_name 1000 = "one thousand"
digit_to_name x
  | (mod x 100) == 0 = digit_to_name (div x 100) ++ " hundred"
  | x < 100          = digit_to_name (rd2nt x) ++ " " ++ digit_to_name (mod x 10)
  | x < 1000         = digit_to_name (div x 100) ++ " hundred and " ++ digit_to_name (mod x 100)


isNotSpace :: Char -> Bool
isNotSpace x = isSpace x |> not

solution :: Int
solution = map digit_to_name [1..1000] |> map (filter isNotSpace) |> map length |> sum

main :: IO ()
main = putStrLn ("Solution: " ++ show solution)