{-|

This module provides additional German public holidays that are not
covered by the [bank holidays](Data.Time.Calendar.BankHoliday.Germany).

Public holidays – except for
'Data.Time.Calendar.BankHoliday.Germany.GermanUnityDay' – are under
federal obligations in Germany („Ländersache“).

Most bank holidays are also federal public holidays
(see 'Data.Time.Calendar.BankHoliday.Germany.isPublicHoliday').
But there are some additional extra holidays which may differ between
federal states.

For example Heilige Drei Könige is not a bank holiday but it is a
public holiday in Bavaria.

Note: The extra holidays are currently only implemented for Bavaria.

Example for computing all public holidays in Bavaria (Landkreis
Miesbach, Oberbayern) in the next couple years:

@
  import Data.List
  import Data.Time
  import qualified Data.Time.Calendar.BankHoliday.Germany as BH
  import qualified Data.Time.Calendar.BankHoliday.Germany.ExtraHolidays as EH

  start = fromGregorian 2024 1 1

  end = fromGregorian 2026 12 31

  holidays :: [[String]]
  holidays = map (\(x,y) -> [show x, BH.germanHolidayName y]) (filter (BH.isPublicHoliday . snd) $ BH.holidaysBetween start end)
          ++ map (\(x,y) -> [show x, EH.germanHolidayName y]) (filter ((/=EH.Friedensfest) . snd) $ EH.holidaysBetween EH.Bayern start end)

  putStrLn $ unlines $ sort $ map unwords holidays
@

Resources:

 - Bayern: https://www.stmi.bayern.de/suv/feiertage/

-}

module Data.Time.Calendar.BankHoliday.Germany.ExtraHolidays (
    ExtraHoliday(..),
    FederalState(..),
    holidaysBetween,
    fromDay,
    toDay,
    germanHolidayName,
    isHolidayInState
) where

import Prelude
import Data.Maybe
import Data.Time.Calendar
import Data.Time.Calendar.BankHoliday.Germany (calculateEasterSunday, yearFromDay)

-- | Germany's federal states – Deutsche Bundesländer.
data FederalState
  = BadenWuerttemberg
  | Bayern
  | Berlin
  | Brandenburg
  | Bremen
  | Hamburg
  | Hessen
  | MecklenburgVorpommern
  | Niedersachsen
  | NordrheinWestfalen
  | RheinlandPfalz
  | Saarland
  | Sachsen
  | SachsenAnhalt
  | SchleswigHolstein
  | Thueringen
  deriving (Enum, Eq, Bounded, Show, Read)

-- | Extra federal holidays, no overlap with
-- 'Data.Time.Calendar.BankHoliday.Germany.BankHoliday'.
-- Spezielle Feiertage der Bundesländer.
--
-- Note: Currently, only Bavaria's extra holidays are implemented.
data ExtraHoliday
  = HeiligeDreiKoenige     -- ^ Heilige Drei Könige (Bayern, …)
  | Fronleichnam           -- ^ Fronleichnam (Bayern, …)
  | Friedensfest           -- ^ Friedensfest (Bayern (Augsburg), …)
  | MariaeHimmelfahrt      -- ^ Mariä Himmelfahrt (Bayern (regional), …)
  | Allerheiligen          -- ^ Allerheiligen (Bayern, …)
  deriving (Enum, Eq, Bounded, Show, Read)

-- | Compute the date for a given year and extra holiday.
--
-- >>> toDay 2024 HeiligeDreiKoenige
-- 2024-01-06
toDay :: Year -> ExtraHoliday -> Day
toDay year HeiligeDreiKoenige      = fromGregorian year 1 6
toDay year Fronleichnam            = addDays 60 $ calculateEasterSunday year
toDay year Friedensfest            = fromGregorian year 8 8
toDay year MariaeHimmelfahrt       = fromGregorian year 8 15
toDay year Allerheiligen           = fromGregorian year 11 1

-- | Compute 'Maybe' the holiday for a given date.
--
-- >>> fromDay (fromGregorian 2024 11 1)
-- Just Allerheiligen
--
-- >>> fromDay (fromGregorian 2024 5 5)
-- Nothing
fromDay :: Day -> Maybe ExtraHoliday
fromDay day = listToMaybe $ filter (\d -> day == toDay (yearFromDay day) d) [minBound..maxBound]

-- | Compute pairs of date and holiday from start to end for the given federal state.
--
-- >>> map snd $ holidaysBetween Bayern (fromGregorian 2024 8 8) (fromGregorian 2024 8 15)
-- [Friedensfest,MariaeHimmelfahrt]
holidaysBetween :: FederalState -> Day -> Day -> [(Day, ExtraHoliday)]
holidaysBetween state start end = filter (isHolidayInState state . snd) $ catMaybes $ map (\d -> (d,) <$> fromDay d) [start..end]

-- | Translate the holiday name to German.
germanHolidayName :: ExtraHoliday -> String
germanHolidayName d = case d of
  HeiligeDreiKoenige     -> "Heilige Drei Könige"
  Fronleichnam           -> "Fronleichnam"
  Friedensfest           -> "Friedensfest"
  MariaeHimmelfahrt      -> "Mariä Himmelfahrt"
  Allerheiligen          -> "Allerheiligen"

-- | Check if 'ExtraHoliday' is a holiday in the given federal state.
--
-- >>> isHolidayInState Bayern Allerheiligen
-- True
--
-- >>> isHolidayInState Berlin Allerheiligen
-- False
isHolidayInState :: FederalState -> ExtraHoliday -> Bool
isHolidayInState Bayern HeiligeDreiKoenige = True
isHolidayInState Bayern Fronleichnam = True
isHolidayInState Bayern Friedensfest = True
isHolidayInState Bayern MariaeHimmelfahrt = True
isHolidayInState Bayern Allerheiligen = True
isHolidayInState _ _ = False