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module Main where
import Data.Time
monthBeginning :: TimeZone -> Integer -> Int -> UTCTime
monthBeginning zone year month = localTimeToUTC zone
(LocalTime (fromGregorian year month 1) midnight)
findTransition :: UTCTime -> UTCTime -> IO [(UTCTime,TimeZone,TimeZone)]
findTransition a b = do
za <- getTimeZone a
zb <- getTimeZone b
if za == zb then return [] else do
let c = addUTCTime ((diffUTCTime b a) / 2) a
if a == c then return [(b,za,zb)] else do
tp <- findTransition a c
tq <- findTransition c b
return (tp ++ tq)
showZoneTime :: TimeZone -> UTCTime -> String
showZoneTime zone time = show (utcToZonedTime zone time)
showTransition :: (UTCTime,TimeZone,TimeZone) -> String
showTransition (time,zone1,zone2) = (showZoneTime zone1 time) ++ " => " ++ (showZoneTime zone2 time)
main :: IO ()
main = do
now <- getCurrentTime
zone <- getTimeZone now
let (year,_,_) = toGregorian (localDay (utcToLocalTime zone now))
putStrLn ("DST adjustments for " ++ show year ++ ":")
let t0 = monthBeginning zone year 1
let t1 = monthBeginning zone year 4
let t2 = monthBeginning zone year 7
let t3 = monthBeginning zone year 10
let t4 = monthBeginning zone (year + 1) 1
tr1 <- findTransition t0 t1
tr2 <- findTransition t1 t2
tr3 <- findTransition t2 t3
tr4 <- findTransition t3 t4
mapM_ (putStrLn . showTransition) (tr1 ++ tr2 ++ tr3 ++ tr4)
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