    {-# LANGUAGE CPP #-}

    #include "HsTimeConfig.h"

    #if defined(mingw32_HOST_OS) || !defined(HAVE_CLOCK_GETTIME)
    {-# LANGUAGE Safe #-}
    #else
    {-# LANGUAGE Trustworthy #-}
    #endif

    module Data.Time.Clock.Internal.SystemTime (
        SystemTime (..),
        getSystemTime,
        getTime_resolution,
        getTAISystemTime,
    ) where

    import Control.DeepSeq
    import Data.Data
    import Data.Int (Int64)
    import Data.Time.Clock.Internal.DiffTime
    import Data.Word

    #ifdef mingw32_HOST_OS
    import qualified System.Win32.Time as Win32
    #elif defined(HAVE_CLOCK_GETTIME)
    import Data.Time.Clock.Internal.CTimespec
    import Foreign.C.Types (CLong(..), CTime(..))
    #else
    import Data.Time.Clock.Internal.CTimeval
    import Foreign.C.Types (CLong(..))
    #endif
    --------------------------------------------------------------------------------

    -- | 'SystemTime' is time returned by system clock functions.
    -- Its semantics depends on the clock function, but the epoch is typically the beginning of 1970.
    -- Note that 'systemNanoseconds' of 1E9 to 2E9-1 can be used to represent leap seconds.
    data SystemTime = MkSystemTime
        { SystemTime -> Int64
    systemSeconds :: {-# UNPACK #-} !Int64
        , SystemTime -> Word32
    systemNanoseconds :: {-# UNPACK #-} !Word32
        }
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        rnf :: SystemTime -> ()
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    -- | Get the system time, epoch start of 1970 UTC, leap-seconds ignored.
    -- 'getSystemTime' is typically much faster than 'getCurrentTime'.
    getSystemTime :: IO SystemTime

    -- | The resolution of 'getSystemTime', 'getCurrentTime', 'getPOSIXTime'.
    -- On UNIX systems this uses @clock_getres@, which may be <https://github.com/microsoft/WSL/issues/6029 wrong on WSL2>.
    getTime_resolution :: DiffTime

    -- | If supported, get TAI time, epoch start of 1970 TAI, with resolution.
    -- This is supported only on UNIX systems, and only those with CLOCK_TAI available at run-time.
    getTAISystemTime :: Maybe (DiffTime, IO SystemTime)

    #ifdef mingw32_HOST_OS
    -- On Windows, the equivalent of POSIX time is "file time", defined as
    -- the number of 100-nanosecond intervals that have elapsed since
    -- 12:00 A.M. January 1, 1601 (UTC).  We can convert this into a POSIX
    -- time by adjusting the offset to be relative to the POSIX epoch.
    getSystemTime = do
        Win32.FILETIME ft <- Win32.getSystemTimeAsFileTime
        let (s, us) = (ft - win32_epoch_adjust) `divMod` 10000000
        return (MkSystemTime (fromIntegral s) (fromIntegral us * 100))
      where
        win32_epoch_adjust :: Word64
        win32_epoch_adjust = 116444736000000000

    getTime_resolution = 100E-9 -- 100ns

    getTAISystemTime = Nothing
    #elif defined(HAVE_CLOCK_GETTIME)
    -- Use hi-res clock_gettime
    timespecToSystemTime :: CTimespec -> SystemTime
    timespecToSystemTime (MkCTimespec (CTime s) (CLong ns)) = (MkSystemTime (fromIntegral s) (fromIntegral ns))

    timespecToDiffTime :: CTimespec -> DiffTime
    timespecToDiffTime (MkCTimespec (CTime s) ns) = (fromIntegral s) + (fromIntegral ns) * 1E-9

    clockGetSystemTime :: ClockID -> IO SystemTime
    clockGetSystemTime clock = fmap timespecToSystemTime $ clockGetTime clock

    getSystemTime = clockGetSystemTime clock_REALTIME

    getTime_resolution = timespecToDiffTime realtimeRes

    getTAISystemTime = do
        clockID <- clock_TAI
        resolution <- clockResolution clockID
        return $ (timespecToDiffTime resolution, clockGetSystemTime clockID)
    #else
    -- Use gettimeofday
    getSystemTime :: IO SystemTime
    getSystemTime = do
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    getTime_resolution :: DiffTime
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    #endif