    {- arch-tag: Time utilities main file
    Copyright (C) 2004-2005 John Goerzen <jgoerzen@complete.org>

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
    -}

    {- |
       Module     : System.Time.Utils
       Copyright  : Copyright (C) 2004-2005 John Goerzen
       License    : GNU GPL, version 2 or above

       Maintainer : John Goerzen <jgoerzen@complete.org>
       Stability  : provisional
       Portability: portable

    This module provides various Haskell utilities for dealing with times and
    dates.

    Written by John Goerzen, jgoerzen\@complete.org
    -}

    module System.Time.Utils(
                         timelocal,
                         timegm,
                         timeDiffToSecs,
                         epoch,
                         epochToClockTime,
                         clockTimeToEpoch,
                         renderSecs, renderTD
                        )
    where
    import System.Time
    import Data.Ratio

    {- | January 1, 1970, midnight, UTC, represented as a CalendarTime. -}
    epoch :: CalendarTime
    epoch = CalendarTime { ctYear = 1970, ctMonth = January,
                           ctDay = 1, ctHour = 0, ctMin = 0, ctSec = 0,
                           ctPicosec = 0, ctWDay = Thursday, ctYDay = 0,
                           ctTZName = "UTC", ctTZ = 0, ctIsDST = False}

    {- | Converts the specified CalendarTime (see System.Time) to seconds-since-epoch time.

    This conversion does respect the timezone specified on the input object.
    If you want a conversion from UTC, specify ctTZ = 0 and ctIsDST = False.

    When called like that, the behavior is equivolent to the GNU C function
    timegm().  Unlike the C library, Haskell's CalendarTime supports
    timezone information, so if such information is specified, it will impact
    the result.
    -}

    timegm :: CalendarTime -> Integer
    timegm ct =
        timeDiffToSecs (diffClockTimes (toClockTime ct) (toClockTime epoch))

    {- | Converts the specified CalendarTime (see System.Time) to
    seconds-since-epoch format.

    The input CalendarTime is assumed to be the time as given in your local
    timezone.  All timezone and DST fields in the object are ignored.

    This behavior is equivolent to the timelocal() and mktime() functions that
    C programmers are accustomed to.

    Please note that the behavior for this function during the hour immediately
    before or after a DST switchover may produce a result with a different hour
    than you expect.
    -}

    timelocal :: CalendarTime -> IO Integer
    timelocal ct =
        do guessct <- toCalendarTime guesscl
           let newct = ct {ctTZ = ctTZ guessct}
           return $ timegm newct
        where guesscl = toClockTime ct

    {- | Converts the given timeDiff to the number of seconds it represents.

    Uses the same algorithm as normalizeTimeDiff in GHC. -}
    timeDiffToSecs :: TimeDiff -> Integer
    timeDiffToSecs td =
        (fromIntegral $ tdSec td) +
        60 * ((fromIntegral $ tdMin td) +
              60 * ((fromIntegral $ tdHour td) +
                    24 * ((fromIntegral $ tdDay td) +
                          30 * ((fromIntegral $ tdMonth td) +
                                365 * (fromIntegral $ tdYear td)))))

    {- | Converts an Epoch time represented with an arbitrary Real to a ClockTime.
    This input could be a CTime from Foreign.C.Types or an EpochTime from
    System.Posix.Types. -}
    epochToClockTime :: Real a => a -> ClockTime
    epochToClockTime x =
        TOD seconds secfrac
        where ratval = toRational x
              seconds = floor ratval
              secfrac = floor $ (ratval - (seconds % 1) ) * picosecondfactor
              picosecondfactor = 10 ^ 12

    {- | Converts a ClockTime to something represented with an arbitrary Real.
    The result could be treated as a CTime from Foreign.C.Types or EpochTime from
    System.Posix.Types.  The inverse of 'epochToClockTime'.

    Fractions of a second are not preserved by this function. -}
    clockTimeToEpoch :: Num a => ClockTime -> a
    clockTimeToEpoch (TOD sec _) = fromInteger sec

    {- | Render a number of seconds as a human-readable amount.  Shows the two
    most significant places.  For instance:

    >renderSecs 121 = "2m1s"

    See also 'renderTD' for a function that works on a TimeDiff.
    -}
    renderSecs :: Integer -> String
    renderSecs i = renderTD $ diffClockTimes (TOD i 0) (TOD 0 0)

    {- | Like 'renderSecs', but takes a TimeDiff instead of an integer second
    count. -}
    renderTD :: TimeDiff -> String
    renderTD itd =
        case workinglist of
          [] -> "0s"
          _ -> concat . map (\(q, s) -> show q ++ [s]) $ workinglist
        where td = normalizeTimeDiff itd
              suffixlist = "yMdhms"
              quantlist = (\(TimeDiff y mo d h m s _) -> [y, mo, d, h, m, s]) td
              zippedlist = zip quantlist suffixlist
              -- Drop all leading elements that are 0, then take at most 2
              workinglist = take 2 . dropWhile (\(q, _) -> q == 0) $ zippedlist