{-|
  Top-Level Pier Management

  This is the code that starts the IO drivers and deals with communication
  between the serf, the event log, and the IO drivers.
-}
module Urbit.Vere.Pier
  ( booted
  , runSerf
  , resumed
  , getSnapshot
  , pier
  , runPersist
  , runCompute
  , genBootSeq
  )
where

import Urbit.Prelude

import Control.Monad.Trans.Maybe
import RIO.Directory
import Urbit.Arvo
import Urbit.King.App
import Urbit.Vere.Pier.Types
import Urbit.Vere.Stat

import Control.Monad.STM      (retry)
import System.Environment     (getExecutablePath)
import System.FilePath        (splitFileName)
import System.Posix.Files     (ownerModes, setFileMode)
import Urbit.EventLog.LMDB    (EventLog)
import Urbit.EventLog.Event   (buildLogEvent)
import Urbit.King.API         (TermConn)
import Urbit.Noun.Time        (Wen)
import Urbit.TermSize         (TermSize(..), termSize)
import Urbit.Vere.Serf        (Serf)

import qualified Data.Text                   as T
import qualified System.Entropy              as Ent
import qualified Urbit.EventLog.LMDB         as Log
import qualified Urbit.King.API              as King
import qualified Urbit.Noun.Time             as Time
import qualified Urbit.Vere.Ames             as Ames
import qualified Urbit.Vere.Behn             as Behn
import qualified Urbit.Vere.Clay             as Clay
import qualified Urbit.Vere.Eyre             as Eyre
import qualified Urbit.Vere.Eyre.KingSubsite as Site
import qualified Urbit.Vere.Http.Client      as Iris
import qualified Urbit.Vere.Serf             as Serf
import qualified Urbit.Vere.Term             as Term
import qualified Urbit.Vere.Term.API         as Term
import qualified Urbit.Vere.Term.Demux       as Term


-- Initialize pier directory. --------------------------------------------------

data PierDirectoryAlreadyExists = PierDirectoryAlreadyExists
 deriving (Show, Exception)

setupPierDirectory :: FilePath -> RIO e ()
setupPierDirectory shipPath = do
  -- shipPath will already exist because we put a lock file there.
  alreadyExists <- doesPathExist (shipPath </> ".urb")
  when alreadyExists $ do
    throwIO PierDirectoryAlreadyExists
  for_ ["put", "get", "log", "chk"] $ \seg -> do
    let pax = shipPath </> ".urb" </> seg
    createDirectoryIfMissing True pax
    io $ setFileMode pax ownerModes


-- Load pill into boot sequence. -----------------------------------------------

genEntropy :: MonadIO m => m Entropy
genEntropy = Entropy . fromIntegral . bytesAtom <$> io (Ent.getEntropy 64)

genBootSeq :: MonadIO m => Ship -> Pill -> Bool -> LegacyBootEvent -> m BootSeq
genBootSeq ship Pill {..} lite boot = io $ do
  ent <- genEntropy
  let ovums = preKern ent <> pKernelOvums <> postKern <> pUserspaceOvums
  pure $ BootSeq ident pBootFormulas ovums
 where
  ident = LogIdentity ship isFake (fromIntegral $ length pBootFormulas)
  preKern ent =
    [ EvBlip $ BlipEvArvo $ ArvoEvWhom () ship
    , EvBlip $ BlipEvArvo $ ArvoEvWack () ent
    ]
  postKern = [EvBlip $ BlipEvTerm $ TermEvBoot (1, ()) lite boot]
  isFake   = case boot of
    Fake _ -> True
    _      -> False


-- Write to the log. -----------------------------------------------------------

-- | Write a batch of jobs to the event log.
writeJobs :: EventLog -> Vector Job -> RIO e ()
writeJobs log !jobs = do
  expect <- atomically (Log.nextEv log)
  events <- fmap fromList $ traverse fromJob (zip [expect ..] $ toList jobs)
  Log.appendEvents log events
 where
  fromJob :: (EventId, Job) -> RIO e ByteString
  fromJob (expectedId, job) = do
    unless (expectedId == jobId job) $ error $ show
      ("bad job id!", expectedId, jobId job)
    pure $ buildLogEvent (jobMug job) (jobPayload job)

  jobMug :: Job -> Mug
  jobMug (RunNok (LifeCyc _ m _)) = m
  jobMug (DoWork (Work _ m _ _)) = m

  jobPayload :: Job -> Noun
  jobPayload (RunNok (LifeCyc _ _ n)) = toNoun n
  jobPayload (DoWork (Work _ _ d o))  = toNoun (d, o)


-- Acquire a running serf. -----------------------------------------------------

runSerf
  :: HasPierEnv e
  => TVar ((Atom, Tank) -> IO ())
  -> FilePath
  -> RAcquire e Serf
runSerf vSlog pax = do
  env <- ask
  serfProg <- io getSerfProg
  Serf.withSerf (config env serfProg)
 where
  slog s = atomically (readTVar vSlog) >>= (\f -> f s)
  config env serfProg = Serf.Config
    { scSerf = env ^. pierConfigL . pcSerfExe . to (maybe serfProg unpack)
    , scPier = pax
    , scFlag = env ^. pierConfigL . pcSerfFlags
    , scSlog = slog
    , scStdr = \txt -> slog (0, (textToTank txt))
    , scDead = pure () -- TODO: What can be done?
    }
  getSerfProg :: IO FilePath
  getSerfProg = do
    (path, filename) <- splitFileName <$> getExecutablePath
    pure $ case filename of
      "urbit"      -> path </> "urbit-worker"
      "urbit-king" -> path </> "urbit-worker"
      _            -> "urbit-worker"


-- Boot a new ship. ------------------------------------------------------------

booted
  :: TVar ((Atom, Tank) -> IO ())
  -> Pill
  -> Bool
  -> Ship
  -> LegacyBootEvent
  -> RAcquire PierEnv (Serf, EventLog)
booted vSlog pill lite ship boot = do
  rio $ bootNewShip pill lite ship boot
  resumed vSlog Nothing

bootSeqJobs :: Time.Wen -> BootSeq -> [Job]
bootSeqJobs now (BootSeq ident nocks ovums) = zipWith ($) bootSeqFns [1 ..]
 where
  wen :: EventId -> Time.Wen
  wen off = Time.addGap now ((fromIntegral off - 1) ^. from Time.microSecs)

  bootSeqFns :: [EventId -> Job]
  bootSeqFns = fmap nockJob nocks <> fmap ovumJob ovums
   where
    nockJob nok eId = RunNok $ LifeCyc eId 0 nok
    ovumJob ov eId = DoWork $ Work eId 0 (wen eId) ov

bootNewShip
  :: HasPierEnv e
  => Pill
  -> Bool
  -> Ship
  -> LegacyBootEvent
  -> RIO e ()
bootNewShip pill lite ship bootEv = do
  seq@(BootSeq ident x y) <- genBootSeq ship pill lite bootEv
  logInfo "BootSeq Computed"

  pierPath <- view pierPathL

  rio (setupPierDirectory pierPath)
  logInfo "Directory setup."

  let logPath = (pierPath </> ".urb/log")

  rwith (Log.new logPath ident) $ \log -> do
    logInfo "Event log initialized."
    jobs <- (\now -> bootSeqJobs now seq) <$> io Time.now
    writeJobs log (fromList jobs)

  logInfo "Finsihed populating event log with boot sequence"


-- Resume an existing ship. ----------------------------------------------------

resumed
  :: TVar ((Atom, Tank) -> IO ())
  -> Maybe Word64
  -> RAcquire PierEnv (Serf, EventLog)
resumed vSlog replayUntil = do
  rio $ logTrace "Resuming ship"
  top <- view pierPathL
  tap <- fmap (fromMaybe top) $ rio $ runMaybeT $ do
    ev <- MaybeT (pure replayUntil)
    MaybeT (getSnapshot top ev)

  rio $ do
    logTrace $ display @Text ("pier: " <> pack top)
    logTrace $ display @Text ("running serf in: " <> pack tap)

  log  <- Log.existing (top </> ".urb/log")
  serf <- runSerf vSlog tap

  rio $ do
    logInfo "Replaying events"
    Serf.execReplay serf log replayUntil >>= \case
      Left err -> error (show err)
      Right 0  -> do
        logInfo "No work during replay so no snapshot"
        pure ()
      Right _  -> do
        logInfo "Taking snapshot"
        io (Serf.snapshot serf)
        logInfo "SNAPSHOT TAKEN"

  pure (serf, log)

-- | Get a fake pier directory for partial snapshots.
getSnapshot :: forall e . FilePath -> Word64 -> RIO e (Maybe FilePath)
getSnapshot top last = do
  lastSnapshot <- lastMay <$> listReplays
  pure (replayToPath <$> lastSnapshot)
 where
  replayDir = top </> ".partial-replay"
  replayToPath eId = replayDir </> show eId

  listReplays :: RIO e [Word64]
  listReplays = do
    createDirectoryIfMissing True replayDir
    snapshotNums <- mapMaybe readMay <$> listDirectory replayDir
    pure $ sort (filter (<= fromIntegral last) snapshotNums)


-- Run Pier --------------------------------------------------------------------

pier
  :: (Serf, EventLog)
  -> TVar ((Atom, Tank) -> IO ())
  -> MVar ()
  -> [Ev]
  -> RAcquire PierEnv ()
pier (serf, log) vSlog startedSig injected = do
  let logId = Log.identity log :: LogIdentity
  let ship  = who logId :: Ship

  -- TODO Instead of using a TMVar, pull directly from the IO driver
  -- event sources.
  computeQ :: TMVar RunReq      <- newEmptyTMVarIO
  persistQ :: TQueue (Fact, FX) <- newTQueueIO
  executeQ :: TQueue FX         <- newTQueueIO
  saveSig :: TMVar ()           <- newEmptyTMVarIO
  kingApi :: King.King          <- King.kingAPI

  termApiQ :: TQueue TermConn <- atomically $ do
    q <- newTQueue
    writeTVar (King.kTermConn kingApi) (Just $ writeTQueue q)
    pure q

  initialTermSize <- io $ termSize

  (demux :: Term.Demux, muxed :: Term.Client) <- atomically $ do
    res <- Term.mkDemux initialTermSize
    pure (res, Term.useDemux res)

  void $ acquireWorker "TERMSERV Listener" $ forever $ do
    logInfo "TERMSERV Waiting for external terminal."
    atomically $ do
      ext <- Term.connClient <$> readTQueue termApiQ
      Term.addDemux ext demux
    logInfo "TERMSERV External terminal connected."

  scryQ <- newTQueueIO
  onKill  <- view onKillPierSigL

  -- Our call above to set the logging function which echos errors from the
  -- Serf doesn't have the appended \r\n because those \r\n s are added in
  -- the c serf code. Logging output from our haskell process must manually
  -- add them.
  let compute = putTMVar computeQ
  let execute = writeTQueue executeQ
  let persist = writeTQueue persistQ
  let sigint  = Serf.sendSIGINT serf
  let scry    = \w b g -> do
        res <- newEmptyMVar
        atomically $ writeTQueue scryQ (w, b, g, putMVar res)
        takeMVar res

  -- Set up the runtime stat counters.
  stat <- newStat

  -- Set up the runtime subsite server and its capability to slog
  -- and display stats.
  siteSlog <- newTVarIO (const $ pure ())
  runtimeSubsite <- Site.kingSubsite ship scry (renderStat stat) siteSlog

  --  Slogs go to stderr, to the runtime subsite, and to the terminal.
  env <- ask
  atomically $ writeTVar vSlog $ \s@(_, tank) -> runRIO env $ do
      atomically $ Term.slog muxed s
      io $ readTVarIO siteSlog >>= ($ s)
      logOther "serf" (display $ T.strip $ tankToText tank)

  (bootEvents, startDrivers) <- do
    env <- ask
    let err = atomically . Term.trace muxed . (<> "\r\n")
    siz <- atomically $ Term.curDemuxSize demux
    let fak = isFake logId
    drivers env ship fak compute scry (siz, muxed) err sigint stat runtimeSubsite

  let computeConfig = ComputeConfig { ccOnWork      = takeTMVar computeQ
                                    , ccOnKill      = onKill
                                    , ccOnSave      = takeTMVar saveSig
                                    , ccOnScry      = readTQueue scryQ
                                    , ccPutResult   = persist
                                    , ccShowSpinner = Term.spin muxed
                                    , ccHideSpinner = Term.stopSpin muxed
                                    , ccLastEvInLog = Log.lastEv log
                                    }

  tSerf <- acquireWorker "Serf" (runCompute serf computeConfig)

  -- TODO: Go through the version negotionation with the serf. Before we start
  -- the drivers, we send a %wyrd event and wait for a %wynn
  doVersionNegotiation compute

  -- Run all born events and retry them until they succeed.
  wackEv <- EvBlip . BlipEvArvo . ArvoEvWack () <$> genEntropy
  rio $ for_ (wackEv : bootEvents) $ \ev -> do
    okaySig <- newEmptyMVar

    let inject n = atomically $ compute $ RRWork $ EvErr ev $ cb n

        --  TODO Make sure this dies cleanly.
        cb :: Int -> WorkError -> IO ()
        cb n | n >= 3 = error ("boot event failed: " <> show ev)
        cb n          = \case
          RunOkay _ _       -> putMVar okaySig ()
          RunSwap _ _ _ _ _ -> putMVar okaySig ()
          RunBail _         -> inject (n + 1)

    -- logTrace ("[BOOT EVENT]: " <> display (summarizeEvent ev))
    io (inject 0)

  let slog :: Text -> IO ()
      slog txt = do
        fn <- atomically (readTVar vSlog)
        fn (0, textToTank txt)

  drivz <- startDrivers
  tExec <- acquireWorker "Effects" (router slog (readTQueue executeQ) drivz)
  tDisk <- acquireWorkerBound "Persist" (runPersist log persistQ execute)

  -- Now that the Serf is configured, the IO drivers are hooked up, their
  -- starting events have been dispatched, and the terminal is live, we can now
  -- handle injecting events requested from the command line.
  for_ (zip [1..] injected) $ \(num, ev) -> rio $ do
    logTrace $ display @Text ("Injecting event " ++ (tshow num) ++ " of " ++
                              (tshow $ length injected) ++ "...")
    okaySig :: MVar (Either [Goof] ()) <- newEmptyMVar

    let inject = atomically $ compute $ RRWork $ EvErr ev $ cb
        cb :: WorkError -> IO ()
        cb = \case
          RunOkay _ _       -> putMVar okaySig (Right ())
          RunSwap _ _ _ _ _ -> putMVar okaySig (Right ())
          RunBail goofs     -> putMVar okaySig (Left goofs)

    io inject

    takeMVar okaySig >>= \case
      Left goof -> logError $ display @Text ("Goof in injected event: " <>
                                             tshow goof)
      Right ()  -> pure ()


  let snapshotEverySecs = 120

  void $ acquireWorker "Save" $ forever $ do
    threadDelay (snapshotEverySecs * 1_000_000)
    void $ atomically $ tryPutTMVar saveSig ()

  putMVar startedSig ()

  -- Wait for something to die.

  let ded = asum
        [ death "effects thread" tExec
        , death "persist thread" tDisk
        , death "compute thread" tSerf
        ]

  atomically ded >>= \case
    Left  (tag, exn)       -> logError $ displayShow (tag, "crashed", exn)
    Right "compute thread" -> pure ()
    Right tag              -> logError $ displayShow (tag, "exited unexpectly")

  atomically $ (Term.spin muxed) (Just "shutdown")

death :: Text -> Async () -> STM (Either (Text, SomeException) Text)
death tag tid = do
  waitCatchSTM tid <&> \case
    Left  exn -> Left (tag, exn)
    Right ()  -> Right tag

-- %wyrd version negotiation ---------------------------------------------------

doVersionNegotiation
  :: HasPierEnv e
  => (RunReq -> STM ())
  -> RAcquire e ()
doVersionNegotiation compute = do
  -- What we want to do is actually inspect the effects here.
  let k   = Wynn [("zuse", 420),
                  ("lull", 330),
                  ("arvo", 240),
                  ("hoon", 140),
                  ("nock", 4)]
      sen = MkTerm "121331"  -- TODO: I can just generate a nonce here.
      v   = Vere sen [Cord "kh", Cord "1.0"] k
      ev  = EvBlip $ BlipEvArvo $ ArvoEvWyrd () v

  okaySig :: MVar (Either [Goof] FX) <- newEmptyMVar
  let inject = atomically $ compute $ RRWork $ EvErr ev $ cb
      cb :: WorkError -> IO ()
      cb = \case
        RunOkay _ fx       -> putMVar okaySig (Right fx)
        RunSwap _ _ _ _ fx -> putMVar okaySig (Right fx)
        RunBail goofs      -> putMVar okaySig (Left goofs)

  -- OK, we are actually getting an exception from the remote side here.
  logDebug "About to inject wyrd"
  io inject
  logDebug "Injected wyrd"

  takeMVar okaySig >>= \case
    Left goof -> logError $ display @Text ("Goof in wyrd event: " <>
                                           tshow goof)
    Right fx  -> do
      -- TODO: We need to actually iterate over the fx list to search for
      -- version negotiation events.
      logDebug $ display @Text ("FX list: " <> tshow fx)


-- Start All Drivers -----------------------------------------------------------

data Drivers = Drivers
  { dBehn :: BehnEf -> IO ()
  , dIris :: HttpClientEf -> IO ()
  , dEyre :: HttpServerEf -> IO ()
  , dNewt :: NewtEf -> IO ()
  , dSync :: SyncEf -> IO ()
  , dTerm :: TermEf -> IO ()
  }

drivers
  :: HasPierEnv e
  => e
  -> Ship
  -> Bool
  -> (RunReq -> STM ())
  -> (Wen -> Gang -> Path -> IO (Maybe (Term, Noun)))
  -> (TermSize, Term.Client)
  -> (Text -> RIO e ())
  -> IO ()
  -> Stat
  -> Site.KingSubsite
  -> RAcquire e ([Ev], RAcquire e Drivers)
drivers env who isFake plan scry termSys stderr serfSIGINT stat sub = do
  let Stat{..} = stat

  (behnBorn, runBehn) <- rio Behn.behn'
  (termBorn, runTerm) <- rio (Term.term' termSys (renderStat stat) serfSIGINT)
  (amesBorn, runAmes) <- rio (Ames.ames' who isFake statAmes scry stderr)
  (httpBorn, runEyre) <- rio (Eyre.eyre' who isFake stderr sub)
  (clayBorn, runClay) <- rio Clay.clay'
  (irisBorn, runIris) <- rio Iris.client'

  putStrLn ("ship is " <> tshow who)

  let initialEvents = mconcat [behnBorn,clayBorn,amesBorn,httpBorn,irisBorn,termBorn]

  let runDrivers = do
        behn <- runBehn
        term <- runTerm
        ames <- runAmes
        iris <- runIris
        eyre <- runEyre
        clay <- runClay

        -- Sources lower in the list are starved until sources above them
        -- have no events to offer.
        acquireWorker "Event Prioritization" $ forever $ atomically $ do
          let x = diEventSource
          let eventSources = [x term, x clay, x behn, x iris, x eyre, x ames]
          pullEvent eventSources >>= \case
            Nothing -> retry
            Just rr -> plan rr

        pure $ Drivers
          { dTerm = diOnEffect term
          , dBehn = diOnEffect behn
          , dNewt = diOnEffect ames
          , dIris = diOnEffect iris
          , dEyre = diOnEffect eyre
          , dSync = diOnEffect clay
          }

  pure (initialEvents, runDrivers)
 where
  pullEvent :: [STM (Maybe a)] -> STM (Maybe a)
  pullEvent []     = pure Nothing
  pullEvent (d:ds) = d >>= \case
    Just r  -> pure (Just r)
    Nothing -> pullEvent ds


-- Route Effects to Drivers ----------------------------------------------------

router :: HasPierEnv e => (Text -> IO ()) -> STM FX -> Drivers -> RIO e ()
router slog waitFx Drivers {..} = do
  kill <- view killPierActionL
  let exit = io (slog "<<<shutdown>>>\r\n") >> atomically kill
  let vega = io (slog "<<<reset>>>\r\n")
  forever $ do
    fx <- atomically waitFx
    for_ fx $ \ef -> do
      logEffect ef
      case ef of
        GoodParse (EfVega _ _              ) -> vega
        GoodParse (EfExit _ _              ) -> exit
        GoodParse (EfVane (VEBehn       ef)) -> io (dBehn ef)
        GoodParse (EfVane (VEBoat       ef)) -> io (dSync ef)
        GoodParse (EfVane (VEClay       ef)) -> io (dSync ef)
        GoodParse (EfVane (VEHttpClient ef)) -> io (dIris ef)
        GoodParse (EfVane (VEHttpServer ef)) -> io (dEyre ef)
        GoodParse (EfVane (VENewt       ef)) -> io (dNewt ef)
        GoodParse (EfVane (VESync       ef)) -> io (dSync ef)
        GoodParse (EfVane (VETerm       ef)) -> io (dTerm ef)
        FailParse n -> logError $ display $ pack @Text (ppShow n)


-- Compute (Serf) Thread -------------------------------------------------------

logEvent :: HasLogFunc e => Ev -> RIO e ()
logEvent ev = do
  --logInfo  $ "<- " <> display (summarizeEvent ev)
  logDebug $ "[EVENT]\n" <> display pretty
 where
  pretty :: Text
  pretty = pack $ unlines $ fmap ("\t" <>) $ lines $ ppShow ev

logEffect :: HasLogFunc e => Lenient Ef -> RIO e ()
logEffect ef = do
  --logInfo  $ "  -> " <> display (summarizeEffect ef)
  logDebug $ display $ "[EFFECT]\n" <> pretty ef
 where
  pretty :: Lenient Ef -> Text
  pretty = \case
    GoodParse e -> pack $ unlines $ fmap ("\t" <>) $ lines $ ppShow e
    FailParse n -> pack $ unlines $ fmap ("\t" <>) $ lines $ ppShow n

data ComputeConfig = ComputeConfig
  { ccOnWork      :: STM RunReq
  , ccOnKill      :: STM ()
  , ccOnSave      :: STM ()
  , ccOnScry      :: STM (Wen, Gang, Path, Maybe (Term, Noun) -> IO ())
  , ccPutResult   :: (Fact, FX) -> STM ()
  , ccShowSpinner :: Maybe Text -> STM ()
  , ccHideSpinner :: STM ()
  , ccLastEvInLog :: STM EventId
  }

runCompute :: forall e . HasKingEnv e => Serf.Serf -> ComputeConfig -> RIO e ()
runCompute serf ComputeConfig {..} = do
  logDebug "runCompute"

  let onRR = asum [ ccOnKill <&> Serf.RRKill
                  , ccOnSave <&> Serf.RRSave
                  , ccOnWork
                  , ccOnScry <&> \(w,g,p,k) -> Serf.RRScry w g p k
                  ]

  vEvProcessing :: TMVar Ev <- newEmptyTMVarIO

  void $ async $ forever (atomically (takeTMVar vEvProcessing) >>= logEvent)

  let onSpin :: Maybe Ev -> STM ()
      onSpin = \case
        Nothing -> ccHideSpinner
        Just ev -> do
          ccShowSpinner (getSpinnerNameForEvent ev)
          putTMVar vEvProcessing ev

  let maxBatchSize = 10

  io (Serf.run serf maxBatchSize ccLastEvInLog onRR ccPutResult onSpin)


-- Event-Log Persistence Thread ------------------------------------------------

data PersistExn = BadEventId EventId EventId
  deriving Show

instance Exception PersistExn where
  displayException (BadEventId expected got) =
    unlines [ "Out-of-order event id send to persist thread."
            , "\tExpected " <> show expected <> " but got " <> show got
            ]

runPersist
  :: forall e
   . HasPierEnv e
  => EventLog
  -> TQueue (Fact, FX)
  -> (FX -> STM ())
  -> RIO e ()
runPersist log inpQ out = do
  dryRun <- view dryRunL
  forever $ do
    writs  <- atomically getBatchFromQueue
    events <- validateFactsAndGetBytes (fst <$> toNullable writs)
    unless dryRun (Log.appendEvents log events)
    atomically $ for_ writs $ \(_, fx) -> do
      out fx

 where
  validateFactsAndGetBytes :: [Fact] -> RIO e (Vector ByteString)
  validateFactsAndGetBytes facts = do
    expect <- atomically (Log.nextEv log)
    lis <- for (zip [expect ..] facts) $ \(expectedId, Fact eve mug wen non) ->
      do
        unless (expectedId == eve) $ do
          throwIO (BadEventId expectedId eve)
        pure $ buildLogEvent mug $ toNoun (wen, non)
    pure (fromList lis)

  getBatchFromQueue :: STM (NonNull [(Fact, FX)])
  getBatchFromQueue = readTQueue inpQ >>= go . singleton
   where
    go acc = tryReadTQueue inpQ >>= \case
      Nothing   -> pure (reverse acc)
      Just item -> go (item <| acc)