{-# LANGUAGE TupleSections #-}
module DNSCache
    ( DNSCache
    , reverseResolve
    , forwardResolve
    , newDNSCache
    , parseAddress
    , strip_brackets
    , withPort
    ) where

import Control.Concurrent
import Control.Concurrent.STM
import Data.Text       ( Text )
import Network.Socket  ( SockAddr(..), AddrInfoFlag(..), defaultHints, getAddrInfo, AddrInfo(..) )
import Data.Time.Clock ( UTCTime, getCurrentTime, diffUTCTime )
import System.IO.Error ( isDoesNotExistError )
import System.Endian   ( fromBE32, toBE32 )
import Control.Exception ( handle, ErrorCall(..) )
import Data.Map        ( Map )
import qualified Data.Map    as Map
import qualified Network.BSD as BSD
import qualified Data.Text   as Text
import Control.Monad
import Data.List
import Data.Ord
import Data.Maybe

import SockAddr      ()
import ControlMaybe  ( handleIO_ )
import GetHostByAddr ( getHostByAddr )

type TimeStamp = UTCTime

data DNSCache =
    DNSCache
    { fcache :: TVar (Map Text [(TimeStamp, SockAddr)])
    , rcache :: TVar (Map SockAddr [(TimeStamp, Text)])
    }


newDNSCache :: IO DNSCache
newDNSCache = do
        fcache <- newTVarIO Map.empty
        rcache <- newTVarIO Map.empty
        return DNSCache { fcache=fcache, rcache=rcache }

equivBy f a b = f a == f b

updateCache :: Eq x =>
    Bool -> TimeStamp -> [x] -> Maybe [(TimeStamp,x)] -> Maybe [(TimeStamp,x)]
updateCache withScrub utc xs mys = do
    let ys = maybe [] id mys
        ys' = filter scrub ys
        ys'' = map (utc,) xs ++ ys'
        minute = 60
        scrub (t,x) | withScrub && diffUTCTime utc t < minute = False
        scrub (t,x) | x `elem` xs = False
        scrub _ = True
    guard $ not (null ys'')
    return ys''

dnsObserve :: DNSCache -> Bool -> TimeStamp -> [(Text,SockAddr)] -> STM ()
dnsObserve dns withScrub utc obs = do
    f <- readTVar $ fcache dns
    r <- readTVar $ rcache dns
    let obs' = map (\(n,a)->(n,a `withPort` 0)) obs
        gs = do
            g <- groupBy (equivBy fst) $ sortBy (comparing fst) obs'
            (n,_) <- take 1 g
            return (n,map snd g)
        f' = foldl' updatef f gs
        hs = do
            h <- groupBy (equivBy snd) $ sortBy (comparing snd) obs'
            (_,a) <- take 1 h
            return (a,map fst h)
        r' = foldl' updater r hs
    writeTVar (fcache dns) f'
    writeTVar (rcache dns) r'
 where
    updatef f (n,addrs) = Map.alter (updateCache withScrub utc addrs) n f
    updater r (a,ns) = Map.alter (updateCache withScrub utc ns) a r

make6mapped4 addr@(SockAddrInet6 {}) = addr
make6mapped4 addr@(SockAddrInet port a) = SockAddrInet6 port 0 (0,0,0xFFFF,fromBE32 a) 0

rawForwardResolve dns fail timeout addrtext = do
    r <- atomically newEmptyTMVar
    mvar <- atomically newEmptyTMVar
    rt <- forkOS $ resolver r mvar
    tt <- forkIO $ timer (fail addrtext) timeout r rt
    atomically $ putTMVar mvar tt
    atomically $ readTMVar r
 where
    resolver r mvar = do
      xs <- handle (\e -> let _ = isDoesNotExistError e in return [])
            $ do fmap (nub . map (make6mapped4 . addrAddress)) $ 
                  getAddrInfo (Just $ defaultHints { addrFlags = [ AI_CANONNAME, AI_V4MAPPED ]})
                    (Just $ Text.unpack $ strip_brackets addrtext)
                    (Just "5269")
      did <- atomically $ tryPutTMVar r xs
      when did $ do
      tt <- atomically $ readTMVar mvar
      throwTo tt (ErrorCall "Interrupted delay")
      utc <- getCurrentTime
      atomically $ dnsObserve dns True utc $ map (addrtext,) xs
      return ()

strip_brackets s = 
      case Text.uncons s of
          Just ('[',t) -> Text.takeWhile (/=']') t
          _            -> s


reportTimeout addrtext = do
      putStrLn $ "timeout resolving: "++show addrtext
      -- killThread rt

timer fail timeout r rt = do
      handle (\(ErrorCall _)-> return ()) $ do
      threadDelay timeout
      did <- atomically $ tryPutTMVar r []
      when did fail

unmap6mapped4 addr@(SockAddrInet6 port _ (0,0,0xFFFF,a) _) =
    SockAddrInet port (toBE32 a)
unmap6mapped4 addr = addr

rawReverseResolve dns fail timeout addr = do
    r <- atomically newEmptyTMVar
    mvar <- atomically newEmptyTMVar
    rt <- forkOS $ resolver r mvar
    tt <- forkIO $ timer (fail addr) timeout r rt
    atomically $ putTMVar mvar tt
    atomically $ readTMVar r
 where
    resolver r mvar = 
      handleIO_ (return ()) $ do
        ent <- getHostByAddr (unmap6mapped4 addr) --  AF_UNSPEC addr
        let names = BSD.hostName ent : BSD.hostAliases ent
            xs = map Text.pack $ nub names
        forkIO $ do
            utc <- getCurrentTime
            atomically $ dnsObserve dns False utc $ map (,addr) xs
        atomically $ putTMVar r  xs

expiredReverse dns addr = do
    utc <- getCurrentTime
    addr <- return $ addr `withPort` 0
    es <- atomically $ do
        r <- readTVar $ rcache dns
        let ns = maybe [] id $ Map.lookup addr r
            minute = 60 -- seconds
            (es0,ns') = partition ( (>=minute) . flip diffUTCTime utc . fst ) ns
            es = map snd es0
        modifyTVar' (rcache dns) $ Map.insert addr ns'
        f <- readTVar $ fcache dns
        let f' = foldl' (flip $ Map.alter (expire utc)) f es
            expire utc Nothing   = Nothing
            expire utc (Just as) = if null as' then Nothing else Just as'
                where as' = filter ( (<minute) . flip diffUTCTime utc . fst) as
        writeTVar (fcache dns) f'
        return es
    return es

cachedReverse dns addr = do
    utc <- getCurrentTime
    addr <- return $ addr `withPort` 0
    atomically $ do
        r <- readTVar (rcache dns)
        let ns = maybe [] id $ Map.lookup addr r
        {-
            ns' = filter ( (<minute) . flip diffUTCTime utc . fst) ns
            minute = 60 -- seconds
        modifyTVar' (rcache dns) $ Map.insert addr ns'
        return $ map snd ns'
        -}
        return $ map snd ns

cachedForward dns n = do
    utc <- getCurrentTime
    atomically $ do
        f <- readTVar (fcache dns)
        let as = maybe [] id $ Map.lookup n f
            as' = filter ( (<minute) . flip diffUTCTime utc . fst) as
            minute = 60 -- seconds
        modifyTVar' (fcache dns) $ Map.insert n as'
        return $ map snd as'

reverseResolve dns addr = do
    expired <- expiredReverse dns addr
    forM_ expired $ \n -> forkIO $ do
        rawForwardResolve dns (const $ return ()) 1000000 n
        return ()
    xs <- rawReverseResolve dns (const $ return ()) 1000000 addr
    cs <- cachedReverse dns addr
    return $ xs ++ filter (not . flip elem xs) cs

forwardResolve dns n = do
    as <- rawForwardResolve dns (const $ return ()) 1000000 n
    if null as
      then cachedForward dns n
      else return as

parseAddress :: Text -> IO (Maybe SockAddr)
parseAddress addr_str = do
    info <- getAddrInfo (Just $ defaultHints { addrFlags = [ AI_NUMERICHOST ] }) 
                        (Just . Text.unpack $ addr_str) 
                        (Just "0")
    return . listToMaybe $ map addrAddress info

withPort :: SockAddr -> Int -> SockAddr
withPort (SockAddrInet _ a)      port = SockAddrInet (toEnum port) a
withPort (SockAddrInet6 _ a b c) port = SockAddrInet6 (toEnum port) a b c