{-# LANGUAGE OverloadedStrings, RankNTypes, FlexibleContexts #-}

module Main where

import Types
import Hash
import Pty
import Memory
import CmdLine
import Log
import Graphviz
import Replay
import Session
import Crypto

import Control.Concurrent
import Control.Concurrent.Async
import Control.Concurrent.STM
import System.IO
import System.Process
import System.Exit
import qualified Data.ByteString as B
import Data.List
import Data.List.NonEmpty (NonEmpty(..), toList)
import Data.Monoid
import Data.Time.Clock.POSIX

main :: IO ()
main = do
	c <- getCmdLine
	case mode c of
		Test -> test
		Graphviz g -> graphviz g
		Replay r -> replay r
 
test :: IO ()
test = do
	exitstatus <- go startSession
	sessionDone
	exitWith exitstatus
  where
	go startmsg = runWithPty "dash" [] $ \(p, ph) -> do
		ichan <- newTChanIO
		ochan <- newTChanIO
		dthread <- async (developer ichan ochan)
		uthread <- async (user startmsg p ichan ochan)
		exitstatus <- waitForProcess ph
		cancel dthread
		cancel uthread
		return exitstatus

networkDelay :: IO ()
-- networkDelay = threadDelay 800000 -- 800 ms ; the latency to geosync orbit
networkDelay = threadDelay 150000 -- 150 ms ; transatlantic latency

developer :: TChan (Message Entered) -> TChan (Message Seen) -> IO ()
developer ichan ochan = withLogger "debug-me-developer.log" $ \logger -> do
	-- Start by reading the initial two messages from the user side,
	-- their session key and the startup message.
	sessionmsg <- atomically $ readTChan ochan
	logger $ User sessionmsg
	sigverifier <- case sessionmsg of
		ControlMessage c@(Control (SessionKey pk) _) ->
			let sv = mkSigVerifier pk
			in if verifySigned sv c
				then return sv
				else protocolError "Badly signed session initialization message"
		_ -> protocolError $ "Unexpected session initialization message: " ++ show sessionmsg
	startmsg <- atomically $ readTChan ochan
	logger $ User startmsg
	starthash <- case startmsg of
		ActivityMessage act@(Activity (Seen (Val b)) Nothing _)
			| verifySigned sigverifier act -> do
				B.hPut stdout b
				hFlush stdout	
				return (hash act)
		_ -> protocolError $ "Unexpected startup message: " ++ show startmsg

	sk <- genMySessionKey
	devstate <- newTVarIO $ DeveloperState
		{ lastSeen = starthash
		, sentSince = mempty
		, enteredSince = mempty
		, lastActivity = starthash
		, developerSessionKey = sk
		, developerSigVerifier = sigverifier
		}
	ok <- authUser ichan ochan devstate logger
	if ok
		then do
			_ <- sendTtyInput ichan devstate logger
				`concurrently` sendTtyOutput ochan devstate logger
			return ()
		else do
			hPutStrLn stderr "\nUser did not grant access to their terminal."

data DeveloperState = DeveloperState
	{ lastSeen :: Hash
	, sentSince :: [B.ByteString]
	, enteredSince :: [Hash]
	, lastActivity :: Hash
	, developerSessionKey :: MySessionKey
	, developerSigVerifier :: SigVerifier
	}

-- | Read things typed by the developer, and forward them to the TChan.
sendTtyInput :: TChan (Message Entered) -> TVar DeveloperState -> Logger -> IO ()
sendTtyInput ichan devstate logger = go
  where
	go = do
		b <- B.hGetSome stdin 1024
		if b == B.empty
			then return ()
			else send b
	send b = do
		act <- atomically $ do
			ds <- readTVar devstate
			let ed = if lastActivity ds == lastSeen ds
				then B.concat $ sentSince ds
				else case reverse (sentSince ds) of
					[] -> mempty
					(lb:_) -> lb
			let entered = Entered
				{ enteredData = Val b
				, echoData = Val ed
				}
			let act = mkSigned (developerSessionKey ds) $
				Activity entered (Just $ lastActivity ds)
			writeTChan ichan (ActivityMessage act)
			let acth = hash act
			let ds' = ds
				{ sentSince = sentSince ds ++ [b]
				, enteredSince = enteredSince ds ++ [acth]
				, lastActivity = acth
				}
			writeTVar devstate ds'
			return act
		logger $ Developer $ ActivityMessage act
		go

-- | Read activity from the TChan and display it to the developer.
sendTtyOutput :: TChan (Message Seen) -> TVar DeveloperState -> Logger -> IO ()
sendTtyOutput ochan devstate logger = go
  where
	go = do
		(o, msg) <- atomically $ getUserMessage ochan devstate
		logger $ User msg
		emitOutput o
		go

-- | Present our session key to the user. 
-- Wait for them to accept or reject it, while displaying any Seen data
-- in the meantime.
authUser :: TChan (Message Entered) -> TChan (Message Seen) -> TVar DeveloperState -> Logger -> IO Bool
authUser ichan ochan devstate logger = do
	ds <- atomically $ readTVar devstate
	pk <- myPublicKey (developerSessionKey ds)
	let msg = ControlMessage $ mkSigned (developerSessionKey ds) 
		(Control (SessionKey pk))
	atomically $ writeTChan ichan msg
	logger $ Developer msg
	waitresp pk
  where
	waitresp pk = do
		(o, msg) <- atomically $ getUserMessage ochan devstate
		logger $ User msg
		emitOutput o
		case o of
			GotControl (SessionKeyAccepted pk')
				| pk' == pk -> return True
			GotControl (SessionKeyRejected pk')
				| pk' == pk -> return False
			_ -> waitresp pk

data Output
	= TtyOutput B.ByteString 
	| Beep
	| ProtocolError String
	| GotControl ControlAction

emitOutput :: Output -> IO ()
emitOutput (ProtocolError e) =
	protocolError e
emitOutput (TtyOutput b) = do
	B.hPut stdout b
	hFlush stdout
emitOutput Beep = do
	B.hPut stdout "\a"
	hFlush stdout
emitOutput (GotControl _) =
	return ()

-- | Get messages from user, check their signature, and make sure that they
-- are properly chained from past messages, before returning.
getUserMessage :: TChan (Message Seen) -> TVar DeveloperState -> STM (Output, Message Seen)
getUserMessage ochan devstate = do
	msg <- readTChan ochan
	ds <- readTVar devstate
	-- Check signature before doing anything else.
	if verifySigned (developerSigVerifier ds) msg
		then do
			o <- process ds msg
			return (o, msg)
		else getUserMessage ochan devstate
  where
	process ds (ActivityMessage act@(Activity (Seen (Val b)) _ _)) = do
		let (legal, ds') = isLegalSeen act ds
		if legal
			then do
				writeTVar devstate ds'
				return (TtyOutput b)
			else return (ProtocolError $ "Illegal Seen value: " ++ show (act, hash act))
	process ds (ControlMessage (Control (Rejected _) _)) = do
		-- When they rejected a message we sent,
		-- anything we sent subsequently will
		-- also be rejected, so forget about it.
		let ds' = ds
			{ sentSince = mempty
			, enteredSince = mempty
			}
		writeTVar devstate ds'
		return Beep
	process _ (ControlMessage (Control c@(SessionKey _) _)) =
		return (GotControl c)
	process _ (ControlMessage (Control c@(SessionKeyAccepted _) _)) =
		return (GotControl c)
	process _ (ControlMessage (Control c@(SessionKeyRejected _) _)) =
		return (GotControl c)

-- | Check if the Seen activity is legal, forming a chain with previous
-- ones, and returns an updated DeveloperState.
--
-- Does not check the signature.
isLegalSeen :: Activity Seen -> DeveloperState -> (Bool, DeveloperState)
isLegalSeen act@(Activity (Seen (Val b)) (Just hp) _) ds
	-- Does it chain to the last Seen activity?
	| hp == lastSeen ds = 
		-- Trim sentSince and enteredSince to
		-- values after the Seen value.
		let ss = sentSince ds
		    es = enteredSince ds
		    n = B.length b
		    (ss', es') = if b `B.isPrefixOf` mconcat ss
		    	then (drop n ss, drop n es)
			else (mempty, mempty)
		in yes ds
			{ lastSeen = acth
			, sentSince = ss'
			, enteredSince = es'
			, lastActivity = acth
			}
	-- Does it chain to something we've entered since the last Seen
	-- value? Eg, user sent A, we replied B C, and the user has
	-- now replied to B.
	-- If so, we can drop B (and anything before it) from
	-- enteredSince and sentSince.
	| otherwise = case elemIndex hp (enteredSince ds) of
		Nothing -> (False, ds)
		Just i -> 
			let ss = sentSince ds
			    es = enteredSince ds
			    ss' = drop (i+1) ss
			    es' = drop (i+1) es
			in yes ds
				{ lastSeen = acth
				, sentSince = ss'
				, enteredSince = es'
				, lastActivity = acth
				}
  where
	acth = hash act
	yes ds' = (True, ds')
isLegalSeen (Activity _ Nothing _) ds = (False, ds)

user :: B.ByteString -> Pty -> TChan (Message Entered) -> TChan (Message Seen) -> IO ()
user starttxt p ichan ochan = withLogger "debug-me.log" $ \logger -> do
	-- Start by establishing our session key, and displaying the starttxt.
	let initialmessage msg = do
		atomically $ writeTChan ochan msg
		logger $ User msg
	sk <- genMySessionKey
	pk <- myPublicKey sk
	let c = mkSigned sk $ Control (SessionKey pk)
	initialmessage $ ControlMessage c
	let act = mkSigned sk $ Activity (Seen (Val (starttxt <> "\r\n"))) Nothing
	let startmsg = ActivityMessage act
	initialmessage startmsg
	l <- mkLog (User startmsg) <$> getPOSIXTime
	us <- newTVarIO $ UserState
		{ backLog = l :| []
		, userSessionKey = sk
		, userSigVerifier = mempty
		}
	_ <- sendPtyOutput p ochan us logger
		`concurrently` sendPtyInput ichan ochan p us logger
	return ()
  where

-- | Log of recent Activity, with the most recent first.
type Backlog = NonEmpty Log

data UserState = UserState
	{ backLog :: Backlog
	, userSessionKey :: MySessionKey
	, userSigVerifier :: SigVerifier
	}

-- | Forward things written to the Pty out the TChan.
sendPtyOutput :: Pty -> TChan (Message Seen) -> TVar UserState -> Logger -> IO ()
sendPtyOutput p ochan us logger = go
  where
	go = do
		b <- readPty p
		now <- getPOSIXTime
		l <- atomically $ do
			let seen = Seen (Val b)
			sendDeveloper ochan us seen now
		logger $ User l
		go

class SendableToDeveloper t where
	sendDeveloper :: TChan (Message Seen) -> TVar UserState -> t -> POSIXTime -> STM (Message Seen)

instance SendableToDeveloper Seen where
	sendDeveloper ochan us seen now = do
		st <- readTVar us
		let bl@(prev :| _) = backLog st
		let msg = ActivityMessage $
			mkSigned (userSessionKey st) $ 
				Activity seen (loggedHash prev)
		let l = mkLog (User msg) now
		writeTChan ochan msg
		writeTVar us $ st { backLog = l :| toList bl }
		return msg

instance SendableToDeveloper ControlAction where
	sendDeveloper ochan us c _now = do
		st <- readTVar us
		let msg = ControlMessage $
			mkSigned (userSessionKey st) (Control c)
		-- Control messages are not kept in the backlog.
		writeTChan ochan msg
		return msg

-- | Read things to be entered from the TChan, verify if they're legal,
-- and send them to the Pty.
sendPtyInput :: TChan (Message Entered) -> TChan (Message Seen) -> Pty -> TVar UserState -> Logger -> IO ()
sendPtyInput ichan ochan p us logger = go
  where
	go = do
		networkDelay
		now <- getPOSIXTime
		v <- atomically $ getDeveloperMessage ichan ochan us now
		case v of
			InputMessage (ActivityMessage entered) -> do
				logger $ Developer $ ActivityMessage entered
				writePty p $ val $ enteredData $ activity entered
				go
			InputMessage (ControlMessage (Control c _)) -> case c of
				SessionKey pk -> do
					checkDeveloperPublicKey ochan us logger pk
					go
				Rejected r -> protocolError $ "User side received a Rejected: " ++ show r
				SessionKeyAccepted _ -> protocolError "User side received a SessionKeyAccepted"
				SessionKeyRejected _ -> protocolError "User side received a SessionKeyRejected"
			RejectedMessage rej -> do
				logger $ User rej
				go
			BadlySignedMessage _ -> go

data Input
	= InputMessage (Message Entered)
	| RejectedMessage (Message Seen)
	| BadlySignedMessage (Message Entered)

-- Get message from developer, verify its signature is from a developer we
-- have allowed (unless it's a SessionKey control message, then the
-- signature of the message is only verified against the key in it), and
-- make sure it's legal before returning it. If it's not legal, sends a
-- Reject message.
getDeveloperMessage :: TChan (Message Entered) -> TChan (Message Seen) -> TVar UserState -> POSIXTime -> STM Input
getDeveloperMessage ichan ochan us now = do
	msg <- readTChan ichan
	st <- readTVar us
	case msg of
		ControlMessage (Control (SessionKey pk) _) -> do
			let sigverifier = mkSigVerifier pk
			if verifySigned sigverifier msg
				then return (InputMessage msg)
				else return (BadlySignedMessage msg)
		_ -> if verifySigned (userSigVerifier st) msg
			then case msg of
				ActivityMessage entered -> do
					-- Don't need to retain backlog
					-- before the Activity that entered
					-- references.
					let bl' = reduceBacklog $
						truncateBacklog (backLog st) entered
					if isLegalEntered entered (st { backLog = bl' })
						then do
							let l = mkLog (Developer msg) now
							writeTVar us (st { backLog = l :| toList bl' })
							return (InputMessage msg)
						else do
							let reject = Rejected entered
							RejectedMessage <$> sendDeveloper ochan us reject now
				ControlMessage (Control _ _) ->
					return (InputMessage msg)
			else return (BadlySignedMessage msg)

-- | Check if the public key a developer presented is one we want to use,
-- and if so, add it to the userSigVerifier.
checkDeveloperPublicKey :: TChan (Message Seen) -> TVar UserState -> Logger -> PublicKey -> IO ()
checkDeveloperPublicKey ochan us logger pk = do
	now <- getPOSIXTime
	-- TODO check gpg sig..
	msg <- atomically $ do
		st <- readTVar us
		let sv = userSigVerifier st
		let sv' = sv `mappend` mkSigVerifier pk
		let st' = st { userSigVerifier = sv' }
		writeTVar us st'
		sendDeveloper ochan us (SessionKeyAccepted pk) now
	logger $ User msg

-- | Truncate the Backlog to remove entries older than the one
-- that the Activity Entered refers to, but only if the referred
-- to Activity is an Activity Seen. 
--
-- Once the developer has referred to a given Activity Seen in
-- their Activity Entered, they cannot refer backwards to anything
-- that came before it.
--
-- If the Activity refers to an item not in the backlog, no truncation is
-- done.
truncateBacklog :: Backlog -> Activity Entered -> Backlog
truncateBacklog (b :| l) (Activity _ (Just hp) _)
	| truncationpoint b = b :| []
	| otherwise = b :| go [] l
  where
	go c [] = reverse c
	go c (x:xs)
		| truncationpoint x = reverse (x:c)
		| otherwise = go (x:c) xs
	truncationpoint x@(Log { loggedMessage = User {}}) = loggedHash x == Just hp
	truncationpoint _ = False
truncateBacklog bl (Activity _ Nothing _) = bl

-- | To avoid DOS attacks that try to fill up the backlog and so use all
-- memory, don't let the backlog contain more than 1000 items, or
-- more than 16 megabytes of total data. (Excluding the most recent
-- item).
reduceBacklog :: Backlog -> Backlog
reduceBacklog (b :| l) = b :| go 0 (take 1000 l)
  where
	go _ [] = []
	go n (x:xs)
		| n > 16777216 = []
		| otherwise = x : go (n + dataSize x) xs

-- | Entered activity is legal when it points to the last logged activity,
-- because this guarantees that the person who entered it saw
-- the current state of the system before manipulating it.
--
-- To support typeahead on slow links, some echoData may be provided
-- in the Entered activity. If the Entered activity points
-- to an older activity, then the echoData must match the
-- concatenation of all Seen activities after that one, up to the
-- last logged activity.
--
-- Activities that do not enter data point to the first message
-- sent in the debug-me session.
--
-- Does not check the signature.
isLegalEntered :: Activity Entered -> UserState -> Bool
isLegalEntered (Activity _ Nothing _) _ = False
isLegalEntered (Activity a (Just hp) _) us
	| loggedHash lastact == Just hp = True
	| B.null (val (echoData a)) = False -- optimisation
	| any (== Just hp) (map loggedHash bl) =
		let sincehp = reverse (lastact : takeWhile (\l -> loggedHash l /= Just hp) bl)
		in echoData a == mconcat (map (getseen . loggedMessage) sincehp)
	| otherwise = False
  where
	(lastact :| bl) = backLog us
	getseen (User (ActivityMessage as)) = seenData $ activity as
	getseen _ = mempty

-- | Temporary hack while user and developer share a process.
protocolError :: String -> IO a
protocolError e = do
	hPutStrLn stderr e
	hFlush stderr
	_ <- exitWith (ExitFailure 101)
	error e