Milray

Milray.hs

import Control.Applicative
import Data.Int
import Data.List

newtype Milray = Milray Int64 deriving Show

projectShareValue :: [Int64] -> Milray
projectShareValue shares =
    Milray $ floor $ fromIntegral (10::Int64) * lg (g * 2) * (patrons - 1)
  where
    lg      = logBase 2
    patrons = toEnum $ length shares :: Double
    -- (Π_{i=1,n} a_i)^{1/n} = exp [1/n Σ_{i=1,n} ln a_i]
    -- https://en.wikipedia.org/wiki/Geometric_mean
    --
    -- This (straightforward) way of calculating the geometric mean:
    --   g = (fromIntegral $ foldl1' (*) shares) ** (1 / patrons)
    -- may produce a very large number when multiplying.  E.g.,
    --   projectShareValue $ replicate 101 4
    -- overflows Int64 and returns 0 here.
    -- So use the following method instead:
    g       = exp $ 1/patrons * (sum $ (log . fromIntegral) <$> shares)

main = print $ projectShareValue $ replicate 101 4