-- |This module provides functions to parse and format RFC 733 date
-- and time formats.
--
-- The syntax is as follows:
--
-- > date-time   ::= [ day-of-week ", " ] date SP time ("-" | SP) zone
-- > day-of-week ::= "Monday"    | "Mon" | "Tuesday"  | "Tue"
-- >               | "Wednesday" | "Wed" | "Thursday" | "Thu"
-- >               | "Friday"    | "Fri" | "Saturday" | "Sat"
-- >               | "Sunday"    | "Sun"
-- > date        ::= day ("-" | SP) month ("-" | SP) year
-- > day         ::= 2DIGIT
-- > year        ::= 2DIGIT | 4DIGIT
-- > month       ::= "January"   | "Jan" | "February" | "Feb"
-- >               | "March"     | "Mar" | "April"    | "Apr"
-- >               | "May"               | "June"     | "Jun"
-- >               | "July"      | "Jul" | "August"   | "Aug"
-- >               | "September" | "Sep" | "October"  | "Oct"
-- >               | "November"  | "Nov" | "December" | "Dec"
-- > time        ::= hour [ ":" ] minute [ [ ":" ] second ]
-- > hour        ::= 2DIGIT
-- > minute      ::= 2DIGIT
-- > second      ::= 2DIGIT
-- > zone        ::= "GMT"              ; Universal Time
-- >               | "NST"              ; Newfoundland: -3:30
-- >               | "AST" | "ADT"      ; Atlantic    :  -4 /  -3
-- >               | "EST" | "EDT"      ; Eastern     :  -5 /  -4
-- >               | "CST" | "CDT"      ; Central     :  -6 /  -5
-- >               | "MST" | "MDT"      ; Mountain    :  -7 /  -6
-- >               | "PST" | "PDT"      ; Pacific     :  -8 /  -7
-- >               | "YST" | "YDT"      ; Yukon       :  -9 /  -8
-- >               | "HST" | "HDT"      ; Haw/Ala     : -10 /  -9
-- >               | "BST" | "BDT"      ; Bering      : -11 / -10
-- >               | "Z"                ; GMT
-- >               | "A"                ;  -1
-- >               | "M"                ; -12
-- >               | "N"                ;  +1
-- >               | "Y"                ; +12
-- >               | ("+" | "-") 4DIGIT ; Local diff: HHMM
module Data.Time.RFC733
    ( format
    , parse
    )
    where
import Data.Time
import Data.Time.Calendar.WeekDate
import Data.Time.HTTP.Common
import Data.Time.RFC733.Internal

-- |Format a 'ZonedTime' in RFC 733.
format :: ZonedTime -> String
format zonedTime
    = let localTime          = zonedTimeToLocalTime zonedTime
          timeZone           = zonedTimeZone zonedTime
          (year, month, day) = toGregorian (localDay localTime)
          (_, _, week)       = toWeekDate  (localDay localTime)
          timeOfDay          = localTimeOfDay localTime
      in
        concat [ longWeekDayName week
               , ", "
               , show2 day
               , "-"
               , shortMonthName month
               , "-"
               , show4 year
               , " "
               , show2 (todHour timeOfDay)
               , ":"
               , show2 (todMin timeOfDay)
               , ":"
               , show2 (floor (todSec timeOfDay))
               , "-"
               , show4digitsTZ timeZone
               ]

-- |Parse an RFC 733 date and time string. When the string can't be
-- parsed, it returns 'Nothing'.
parse :: String -> Maybe ZonedTime
parse src = case P.parse p "" src of
              Right zt -> Just zt
              Left  _  -> Nothing
    where
      p = do zt <- rfc733DateAndTime
             _  <- P.eof
             return zt