{-# LANGUAGE
    OverloadedStrings
  , UnicodeSyntax
  #-}
module Data.Time.RFC822.Internal
    ( rfc822DateAndTime
    , rfc822time
    )
    where
import Control.Applicative
import Data.Attoparsec.Char8
import Data.Time
import Data.Time.HTTP.Common

-- |Parse an RFC 822 date and time string.
rfc822DateAndTime ∷ Parser ZonedTime
rfc822DateAndTime = dateTime

dateTime ∷ Parser ZonedTime
dateTime = do weekDay ← optionMaybe $
                        do w ← shortWeekDayNameP
                           _ ← string ", "
                           return w
              gregDay ← date
              case weekDay of
                Nothing
                    -> return ()
                Just givenWD
                    -> assertWeekDayIsGood givenWD gregDay
              (tod, timeZone) ← rfc822time
              let lt = LocalTime gregDay tod
                  zt = ZonedTime lt timeZone
              return zt

date ∷ Parser Day
date = do day   ← read2
          _     ← char ' '
          month ← shortMonthNameP
          _     ← char ' '
          year  ← (+ 1900) <$> read2
          _     ← char ' '
          assertGregorianDateIsGood year month day

-- |Parse the time and time zone of an RFC 822 date and time string.
rfc822time ∷ Parser (TimeOfDay, TimeZone)
rfc822time = do tod ← hms
                _   ← char ' '
                tz  ← zone
                return (tod, tz)

hms ∷ Parser TimeOfDay
hms = do hour   ← read2
         minute ← char ':' *> read2
         second ← option 0 (char ':' *> read2)
         assertTimeOfDayIsGood hour minute second

zone ∷ Parser TimeZone
zone = choice [ string "UT"  *> return (TimeZone 0 False "UT" )
              , string "GMT" *> return (TimeZone 0 False "GMT")
              , char 'E'
                *> choice [ string "ST" *> return (TimeZone ((-5) * 60) False "EST")
                          , string "DT" *> return (TimeZone ((-4) * 60) True  "EDT")
                          ]
              , char 'C'
                *> choice [ string "ST" *> return (TimeZone ((-6) * 60) False "CST")
                          , string "DT" *> return (TimeZone ((-5) * 60) True  "CDT")
                          ]
              , char 'M'
                *> choice [ string "ST" *> return (TimeZone ((-7) * 60) False "MST")
                          , string "DT" *> return (TimeZone ((-6) * 60) True  "MDT")
                          , return (TimeZone ((-12) * 60) False "M")
                          ]
              , char 'P'
                *> choice [ string "ST" *> return (TimeZone ((-8) * 60) False "PST")
                          , string "DT" *> return (TimeZone ((-7) * 60) True  "PDT")
                          ]
              , char 'Z' *> return (TimeZone 0           False "Z")
              , char 'A' *> return (TimeZone ((-1) * 60) False "A")
              , char 'N' *> return (TimeZone (  1  * 60) False "N")
              , char 'Y' *> return (TimeZone ( 12  * 60) False "Y")
              , read4digitsTZ
              ]