module Main where

import Data.Time.Clock.POSIX
import System.IO

main :: IO ()
main = do
  putStrLn "Tap to the beat of a song to determine its BPM."
  hSetEcho stdin False
  hSetBuffering stdin NoBuffering
  loop [1]

average :: (Foldable t, Integral a) => t a -> a
average n = sum n `div` fromIntegral (length n)

takeLast :: Int -> [t] -> [t]
takeLast n [] = []
takeLast n l = drop (length l - n) l

diff :: Num t => [t] -> [t]
diff [] = []
diff ls = zipWith (-) (tail ls) ls

loop :: [Integer] -> IO a
loop lst = do
  getChar
  t <- getPOSIXTime
  let lastFew = takeLast 4 lst ++ [round (t * 1000)]
  print (60000 `div` average (diff lastFew))
  loop lastFew