{-# LANGUAGE Safe #-}
{-# LANGUAGE ForeignFunctionInterface #-}

module Data.Time.LocalTime.Internal.TimeZone
    (
    -- * Time zones
      TimeZone(..)
    , timeZoneOffsetString
    , timeZoneOffsetString'
    , timeZoneOffsetString''
    , minutesToTimeZone
    , hoursToTimeZone
    , utc
    -- getting the locale time zone
    , getTimeZone
    , getCurrentTimeZone
    ) where

import Control.DeepSeq
import Data.Data
import Data.Time.Calendar.Private
import Data.Time.Clock.Internal.UTCTime
import Data.Time.Clock.POSIX
import Data.Time.Clock.System
import Foreign
import Foreign.C

-- | A TimeZone is a whole number of minutes offset from UTC, together with a name and a \"just for summer\" flag.
data TimeZone = TimeZone
    { timeZoneMinutes :: Int
    -- ^ The number of minutes offset from UTC. Positive means local time will be later in the day than UTC.
    , timeZoneSummerOnly :: Bool
    -- ^ Is this time zone just persisting for the summer?
    , timeZoneName :: String
    -- ^ The name of the zone, typically a three- or four-letter acronym.
    } deriving (Eq, Ord, Data, Typeable)

instance NFData TimeZone where
    rnf (TimeZone m so n) = rnf m `seq` rnf so `seq` rnf n `seq` ()

-- | Create a nameless non-summer timezone for this number of minutes.
minutesToTimeZone :: Int -> TimeZone
minutesToTimeZone m = TimeZone m False ""

-- | Create a nameless non-summer timezone for this number of hours.
hoursToTimeZone :: Int -> TimeZone
hoursToTimeZone i = minutesToTimeZone (60 * i)

showT :: Bool -> PadOption -> Int -> String
showT False opt t = showPaddedNum opt ((div t 60) * 100 + (mod t 60))
showT True opt t = let
    opt' =
        case opt of
            NoPad -> NoPad
            Pad i c -> Pad (max 0 $ i - 3) c
    in showPaddedNum opt' (div t 60) ++ ":" ++ show2 (mod t 60)

timeZoneOffsetString'' :: Bool -> PadOption -> TimeZone -> String
timeZoneOffsetString'' colon opt (TimeZone t _ _)
    | t < 0 = '-' : (showT colon opt (negate t))
timeZoneOffsetString'' colon opt (TimeZone t _ _) = '+' : (showT colon opt t)

-- | Text representing the offset of this timezone, such as \"-0800\" or \"+0400\" (like @%z@ in formatTime), with arbitrary padding.
timeZoneOffsetString' :: Maybe Char -> TimeZone -> String
timeZoneOffsetString' Nothing = timeZoneOffsetString'' False NoPad
timeZoneOffsetString' (Just c) = timeZoneOffsetString'' False $ Pad 4 c

-- | Text representing the offset of this timezone, such as \"-0800\" or \"+0400\" (like @%z@ in formatTime).
timeZoneOffsetString :: TimeZone -> String
timeZoneOffsetString = timeZoneOffsetString'' False (Pad 4 '0')

-- | This only shows the time zone name, or offset if the name is empty.
instance Show TimeZone where
    show zone@(TimeZone _ _ "") = timeZoneOffsetString zone
    show (TimeZone _ _ name) = name

-- | The UTC time zone.
utc :: TimeZone
utc = TimeZone 0 False "UTC"

{-# CFILES cbits/HsTime.c #-}
foreign import ccall unsafe "HsTime.h get_current_timezone_seconds" get_current_timezone_seconds
    :: CTime -> Ptr CInt -> Ptr CString -> IO CLong

getTimeZoneCTime :: CTime -> IO TimeZone
getTimeZoneCTime ctime =
    with
        0
        (\pdst ->
             with
                 nullPtr
                 (\pcname -> do
                      secs <- get_current_timezone_seconds ctime pdst pcname
                      case secs of
                          0x80000000 -> fail "localtime_r failed"
                          _ -> do
                              dst <- peek pdst
                              cname <- peek pcname
                              name <- peekCString cname
                              return (TimeZone (div (fromIntegral secs) 60) (dst == 1) name)))

toCTime :: Int64 -> IO CTime
toCTime t = let
    tt = fromIntegral t
    t' = fromIntegral tt
    -- there's no instance Bounded CTime, so this is the easiest way to check for overflow
    in if t' == t
           then return $ CTime tt
           else fail "Data.Time.LocalTime.Internal.TimeZone.toCTime: Overflow"

-- | Get the local time-zone for a given time (varying as per summertime adjustments).
getTimeZoneSystem :: SystemTime -> IO TimeZone
getTimeZoneSystem t = do
    ctime <- toCTime $ systemSeconds t
    getTimeZoneCTime ctime

-- | Get the local time-zone for a given time (varying as per summertime adjustments).
getTimeZone :: UTCTime -> IO TimeZone
getTimeZone t = do
    ctime <- toCTime $ floor $ utcTimeToPOSIXSeconds t
    getTimeZoneCTime ctime

-- | Get the current time-zone.
getCurrentTimeZone :: IO TimeZone
getCurrentTimeZone = getSystemTime >>= getTimeZoneSystem
