FindBirthday.hs

module FindBirthday where

-- compile as: ghc --make FindBirthday.hs -main-is FindBirthday.main
-- or run as: runhaskell FindBirthday.hs
-- and answer "y" or "n" when the program asks you a question. :)

import Data.Time (formatTime, addDays)
import System.Locale (defaultTimeLocale)

-- use 2014 because that was a leap year, and we need all 366 days.
showNthDay :: Integer -> String
showNthDay n = formatTime defaultTimeLocale "%d %B" . addDays n . read $ "2004-01-01"

main :: IO ()
main = guess 256 0

guess :: Integer -> Integer -> IO ()
guess 0 a = putStrLn $ "Your birthday is " ++ showNthDay a
guess n a | n + a > 365 = guess (n `div` 2) a
guess n a | True = do
  let candidate = n+a
  putStrLn $ "Is your birthday on or after " ++ showNthDay candidate ++ "?"
  reply <- getLine
  let a' = if take 1 reply == "y" then candidate else a
  guess (n `div` 2) a'