module Statistics
  ( paymentsAndIncomes
  ) where

import           Control.Arrow      ((&&&))
import qualified Data.List          as L
import           Data.Map           (Map)
import qualified Data.Map           as M
import qualified Data.Maybe         as Maybe
import qualified Data.Time.Calendar as Calendar

import           Common.Model       (Income (..), MonthStats (..), Payment (..),
                                     Stats)

paymentsAndIncomes :: [Payment] -> [Income] -> Stats
paymentsAndIncomes payments incomes =

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

  where

    toMonthStat (start, paymentsByCategory) =
      MonthStats start paymentsByCategory (incomesAt start)

    incomesAt day =
      M.map (incomeAt day) lastToFirstIncomesByUser

    incomeAt day lastToFirstIncome =
      Maybe.maybe 0 _income_amount
      . Maybe.listToMaybe
      . dropWhile (\i -> _income_date i > day)
      $ lastToFirstIncome

    lastToFirstIncomesByUser =
      M.map (reverse . L.sortOn _income_date)
      . groupBy _income_userId
      $ incomes

    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

groupBy :: Ord k => (a -> k) -> [a] -> Map k [a]
groupBy key =
  M.fromListWith (++) . map (key &&& pure)