{-# LANGUAGE FlexibleInstances #-}

{- Copyright 2016 Joey Hess <id@joeyh.name>
 -
 - Licensed under the GNU AGPL version 3 or higher.
 -}

module Entropy where

import Data.List
import qualified Data.ByteString.UTF8 as B

class ToChars t where
	toChars :: t -> [Char]

instance ToChars String where
	toChars = id

instance ToChars B.ByteString where
	toChars = B.toString

shannonEntropy :: ToChars s => s -> Double
shannonEntropy = sum . map lg' . fq' . map (fromIntegral.length) . group . sort . toChars
  where
	lg' c = (c * ) . logBase 2 $ 1.0 / c
	fq' c = let sc = sum c in map (/ sc) c

totalEntropy :: ToChars s => s -> Double
totalEntropy s = shannonEntropy s * fromIntegral (length (toChars s))