-- | This module provides data definitions and functions for ledgers and
-- postings.

{-# LANGUAGE DataKinds   #-}
{-# LANGUAGE DerivingVia #-}

module Haspara.Accounting.Ledger where

import qualified Data.Aeson                 as Aeson
import qualified Data.Map.Strict            as HM
import           Data.Maybe                 (fromMaybe, listToMaybe)
import qualified Data.Text                  as T
import           Data.Time                  (Day)
import           GHC.Generics               (Generic)
import           GHC.TypeLits               (KnownNat, Nat)
import           Haspara.Accounting.Account (Account(accountKind))
import           Haspara.Accounting.Amount  (Amount, amountFromQuantity, amountFromValue)
import           Haspara.Accounting.Balance (Balance(Balance), updateBalance)
import           Haspara.Accounting.Journal (JournalEntry(..), JournalEntryItem(JournalEntryItem))
import           Haspara.Accounting.Side    (normalSideByAccountKind)
import           Haspara.Internal.Aeson     (commonAesonOptions)
import           Haspara.Quantity           (Quantity)


-- | Data definition for a general ledger.
newtype GeneralLedger (precision :: Nat) account event = GeneralLedger
  { generalLedgerLedgers :: [Ledger precision account event]
  }
  deriving (Eq, Generic, Show)


instance (KnownNat precision, Aeson.FromJSON account, Aeson.FromJSON event) => Aeson.FromJSON (GeneralLedger precision account event) where
  parseJSON = Aeson.genericParseJSON $ commonAesonOptions "generalLedger"


instance (KnownNat precision, Aeson.ToJSON account, Aeson.ToJSON event) => Aeson.ToJSON (GeneralLedger precision account event) where
  toJSON = Aeson.genericToJSON $ commonAesonOptions "generalLedger"


-- | Data definition for a ledger.
data Ledger (precision :: Nat) account event = Ledger
  { ledgerAccount :: !(Account account)
  , ledgerOpening :: !(Balance precision)
  , ledgerRunning :: ![LedgerEntry precision event]
  }
  deriving (Eq, Generic, Show)


instance (KnownNat precision, Aeson.FromJSON account, Aeson.FromJSON event) => Aeson.FromJSON (Ledger precision account event) where
  parseJSON = Aeson.genericParseJSON $ commonAesonOptions "ledger"


instance (KnownNat precision, Aeson.ToJSON account, Aeson.ToJSON event) => Aeson.ToJSON (Ledger precision account event) where
  -- TODO: Add ledgerClosing, too.
  toJSON = Aeson.genericToJSON $ commonAesonOptions "ledger"


-- | Returns the closing balance of a ledger.
ledgerClosing
  :: KnownNat precision
  => Ledger precision account event
  -> Balance precision
ledgerClosing ledger = maybe (ledgerOpening ledger) ledgerEntryBalance (listToMaybe (ledgerRunning ledger))


-- | Type encoding of a ledger item.
data LedgerEntry (precision :: Nat) event = LedgerEntry
  { ledgerEntryDate        :: !Day
  , ledgerEntryAmount      :: !(Amount precision)
  , ledgerEntryDescription :: !T.Text
  , ledgerEntryEvent       :: !event
  , ledgerEntryPostingId   :: !T.Text
  , ledgerEntryBalance     :: !(Balance precision)
  }
  deriving (Eq, Generic, Show)


instance (KnownNat precision, Aeson.FromJSON event) => Aeson.FromJSON (LedgerEntry precision event) where
  parseJSON = Aeson.genericParseJSON $ commonAesonOptions "ledgerEntry"


instance (KnownNat precision, Aeson.ToJSON event) => Aeson.ToJSON (LedgerEntry precision event) where
  toJSON = Aeson.genericToJSON $ commonAesonOptions "ledgerEntry"


-- | Initializes an empty ledger for a given account.
initLedger
  :: KnownNat precision
  => Account account
  -> Ledger precision account event
initLedger acc = Ledger acc balance []
  where
    balance = Balance (normalSideByAccountKind (accountKind acc)) 0


-- | Initializes a ledger with the given opening balance.
initLedgerWithOpeningBalance
  :: KnownNat precision
  => Account account
  -> Balance precision
  -> Ledger precision account event
initLedgerWithOpeningBalance acc balance = Ledger acc balance []


-- | Initializes a ledger with the given opening value.
--
-- See 'amountFromValue' for the meaning of the concept of value.
initLedgerWithOpeningValue
  :: KnownNat precision
  => Account account
  -> Quantity precision
  -> Ledger precision account event
initLedgerWithOpeningValue acc qty = initLedgerWithOpeningBalance acc balance
  where
    amount = amountFromValue (accountKind acc) qty
    balance0 = Balance (normalSideByAccountKind (accountKind acc)) 0
    balance = updateBalance balance0 amount


-- | Initializes a ledger with the given opening quantity.
--
-- See 'amountFromQuantity' for the meaning of the concept of quantity.
initLedgerWithOpeningQuantity
  :: KnownNat precision
  => Account account
  -> Quantity precision
  -> Ledger precision account event
initLedgerWithOpeningQuantity acc qty = initLedgerWithOpeningBalance acc balance
  where
    amount = amountFromQuantity (accountKind acc) qty
    balance0 = Balance (normalSideByAccountKind (accountKind acc)) 0
    balance = updateBalance balance0 amount


-- | Posts a given list of journal entries to the given general ledger and
-- returns the new general ledger.
postEntries
  :: KnownNat precision
  => Eq account
  => Ord account
  => GeneralLedger precision account event
  -> [JournalEntry precision account event]
  -> GeneralLedger precision account event
postEntries = foldl postEntry


-- | Posts a given journal entry to the given general ledger and returns the new
-- general ledger.
postEntry
  :: KnownNat precision
  => Eq account
  => Ord account
  => GeneralLedger precision account event
  -> JournalEntry precision account event
  -> GeneralLedger precision account event
postEntry gl je = foldl (`postEntryItem` je) gl (journalEntryItems je)


-- | Posts a given journal entry item of a given journal entry to the given
-- general ledger and returns the new general ledger.
postEntryItem
  :: KnownNat precision
  => Eq account
  => Ord account
  => GeneralLedger precision account event
  -> JournalEntry precision account event
  -> JournalEntryItem precision account event
  -> GeneralLedger precision account event
postEntryItem gl je (JournalEntryItem amt acc evt) =
  let
    ledgers = generalLedgerLedgers gl
    ledgersDb = HM.fromList $ zip (fmap ledgerAccount ledgers) ledgers
    ledgerCurr = fromMaybe (initLedger acc) $ HM.lookup acc ledgersDb
    ledgerNext = postItem ledgerCurr (journalEntryDate je) amt (journalEntryDescription je) evt (journalEntryId je)
    ledgersDbNext = HM.insert acc ledgerNext ledgersDb
  in
    GeneralLedger
      { generalLedgerLedgers = HM.elems ledgersDbNext
      }


-- | Performs a posting to the given ledger.
postItem
  :: KnownNat precision
  => Ledger precision account event
  -> Day
  -> Amount precision
  -> T.Text
  -> event
  -> T.Text
  -> Ledger precision account event
postItem ledger date amt dsc evt pid =
  let
    balanceLast = ledgerClosing ledger
    balanceNext = updateBalance balanceLast amt
    item = LedgerEntry
      { ledgerEntryDate = date
      , ledgerEntryAmount = amt
      , ledgerEntryDescription = dsc
      , ledgerEntryEvent = evt
      , ledgerEntryPostingId = pid
      , ledgerEntryBalance = balanceNext
      }
  in
    ledger
      { ledgerRunning = item : ledgerRunning ledger
      }