module Statistics
  ( compute
  ) where

import qualified Data.List          as L
import qualified Data.Map           as M
import qualified Data.Time.Calendar as Calendar

import           Common.Model       (Payment (..), PaymentStats)

compute :: [Payment] -> PaymentStats
compute payments =

  M.toList $ foldl
    (\m p -> M.alter (alter p) (startOfMonth $ _payment_date p) m)
    M.empty
    payments

  where

    initMonthStats =
      M.fromList
      . map (\category -> (category, 0))
      . L.nub
      $ map _payment_category payments

    alter p Nothing           = Just (addPayment p initMonthStats)
    alter p (Just monthStats) = Just (addPayment p monthStats)

    addPayment p monthStats = M.adjust ((+) (_payment_cost p)) (_payment_category p) monthStats

    startOfMonth day =
      let (y, m, _) = Calendar.toGregorian day
      in  Calendar.fromGregorian y m 1