-- | Query and update documents

{-# LANGUAGE OverloadedStrings, RecordWildCards, NamedFieldPuns, TupleSections, FlexibleContexts, FlexibleInstances, UndecidableInstances, MultiParamTypeClasses, TypeFamilies, CPP, DeriveDataTypeable, ScopedTypeVariables, BangPatterns #-}

module Database.MongoDB.Query (
    -- * Monad
    Action, access, Failure(..), ErrorCode,
    AccessMode(..), GetLastError, master, slaveOk, accessMode,
    liftDB,
    MongoContext(..), HasMongoContext(..),
    -- * Database
    Database, allDatabases, useDb, thisDatabase,
    -- ** Authentication
    Username, Password, auth, authMongoCR, authSCRAMSHA1,
    -- * Collection
    Collection, allCollections,
    -- ** Selection
    Selection(..), Selector, whereJS,
    Select(select),
    -- * Write
    -- ** Insert
    insert, insert_, insertMany, insertMany_, insertAll, insertAll_,
    -- ** Update
    save, replace, repsert, upsert, Modifier, modify, updateMany, updateAll,
    WriteResult(..), UpdateOption(..), Upserted(..),
    -- ** Delete
    delete, deleteOne, deleteMany, deleteAll, DeleteOption(..),
    -- * Read
    -- ** Query
    Query(..), QueryOption(NoCursorTimeout, TailableCursor, AwaitData, Partial),
    Projector, Limit, Order, BatchSize,
    explain, find, findCommand, findOne, fetch,
    findAndModify, findAndModifyOpts, FindAndModifyOpts(..), defFamUpdateOpts,
    count, distinct,
    -- *** Cursor
    Cursor, nextBatch, next, nextN, rest, closeCursor, isCursorClosed,
    -- ** Aggregate
    Pipeline, AggregateConfig(..), aggregate, aggregateCursor,
    -- ** Group
    Group(..), GroupKey(..), group,
    -- ** MapReduce
    MapReduce(..), MapFun, ReduceFun, FinalizeFun, MROut(..), MRMerge(..),
    MRResult, mapReduce, runMR, runMR',
    -- * Command
    Command, runCommand, runCommand1,
    eval, retrieveServerData, ServerData(..)
) where

import qualified Control.Concurrent.MVar as MV
import Control.Concurrent.MVar.Lifted
  ( MVar,
    readMVar,
  )
import Control.Exception (Exception, catch, throwIO)
import Control.Monad
  ( liftM2,
    replicateM,
    unless,
    void,
    when,
  )
import Control.Monad.Reader (MonadReader, ReaderT, ask, asks, local, runReaderT)
import Control.Monad.Trans (MonadIO, liftIO)
import qualified Crypto.Hash.MD5 as MD5
import qualified Crypto.Hash.SHA1 as SHA1
import qualified Crypto.MAC.HMAC as HMAC
import qualified Crypto.Nonce as Nonce
import Data.Binary.Put (runPut)
import Data.Bits (xor)
import Data.Bson
  ( Document,
    Field (..),
    Javascript,
    Label,
    ObjectId,
    Val (..),
    Value (..),
    at,
    genObjectId,
    look,
    lookup,
    valueAt,
    (!?),
    (=:),
    (=?),
    merge,
    cast
  )
import Data.Bson.Binary (putDocument)
import qualified Data.ByteString as BS
import qualified Data.ByteString.Base16 as B16
import qualified Data.ByteString.Base64 as B64
import qualified Data.ByteString.Char8 as B
import qualified Data.ByteString.Lazy as LBS
import Data.Default.Class (Default (..))
import Data.Either (lefts, rights)
import qualified Data.Either as E
import Data.Functor ((<&>))
import Data.Int (Int32, Int64)
import Data.List (foldl1')
import qualified Data.Map as Map
import Data.Maybe (catMaybes, fromMaybe, isNothing, listToMaybe, mapMaybe, maybeToList, fromJust)
import Data.Text (Text)
import qualified Data.Text as T
import Data.Typeable (Typeable)
import Data.Word (Word32)
import Database.MongoDB.Internal.Protocol
  ( CursorId,
    DeleteOption (..),
    FullCollection,
    InsertOption (..),
    Notice (..),
    Password,
    Pipe,
    QueryOption (..),
    Reply (..),
    Request
      ( GetMore,
        qBatchSize,
        qFullCollection,
        qOptions,
        qProjector,
        qSelector,
        qSkip
      ),
    ResponseFlag (..),
    ServerData (..),
    UpdateOption (..),
    Username,
    Cmd (..),
    pwKey,
    FlagBit (..)
  )
import qualified Database.MongoDB.Internal.Protocol as P
import Database.MongoDB.Internal.Util (liftIOE, loop, true1, (<.>))
import System.Mem.Weak (Weak)
import Text.Read (readMaybe)
import Prelude hiding (lookup)

-- * Monad

type Action = ReaderT MongoContext
-- ^ A monad on top of m (which must be a MonadIO) that may access the database and may fail with a DB 'Failure'

access :: (MonadIO m) => Pipe -> AccessMode -> Database -> Action m a -> m a
-- ^ Run action against database on server at other end of pipe. Use access mode for any reads and writes.
-- Throw 'Failure' in case of any error.
access mongoPipe mongoAccessMode mongoDatabase action = runReaderT action MongoContext{..}

-- | A connection failure, or a read or write exception like cursor expired or inserting a duplicate key.
-- Note, unexpected data from the server is not a Failure, rather it is a programming error (you should call 'error' in this case) because the client and server are incompatible and requires a programming change.
data Failure =
     ConnectionFailure IOError  -- ^ TCP connection ('Pipeline') failed. May work if you try again on the same Mongo 'Connection' which will create a new Pipe.
    | CursorNotFoundFailure CursorId  -- ^ Cursor expired because it wasn't accessed for over 10 minutes, or this cursor came from a different server that the one you are currently connected to (perhaps a fail over happen between servers in a replica set)
    | QueryFailure ErrorCode String  -- ^ Query failed for some reason as described in the string
    | WriteFailure Int ErrorCode String -- ^ Error observed by getLastError after a write, error description is in string, index of failed document is the first argument
    | WriteConcernFailure Int String  -- ^ Write concern error. It's reported only by insert, update, delete commands. Not by wire protocol.
    | DocNotFound Selection  -- ^ 'fetch' found no document matching selection
    | AggregateFailure String -- ^ 'aggregate' returned an error
    | CompoundFailure [Failure] -- ^ When we need to aggregate several failures and report them.
    | ProtocolFailure Int String -- ^ The structure of the returned documents doesn't match what we expected
    deriving (Show, Eq, Typeable)
instance Exception Failure

type ErrorCode = Int
-- ^ Error code from @getLastError@ or query failure.

-- | Type of reads and writes to perform.
data AccessMode =
     ReadStaleOk  -- ^ Read-only action, reading stale data from a slave is OK.
    | UnconfirmedWrites  -- ^ Read-write action, slave not OK, every write is fire & forget.
    | ConfirmWrites GetLastError  -- ^ Read-write action, slave not OK, every write is confirmed with @getLastError@.
    deriving Show

type GetLastError = Document
-- ^ Parameters for @getLastError@ command. For example @[\"w\" =: 2]@ tells the server to wait for the write to reach at least two servers in replica set before acknowledging. See <http://www.mongodb.org/display/DOCS/Last+Error+Commands> for more options.

class Result a where
  isFailed :: a -> Bool

data WriteResult = WriteResult
                  { failed      :: Bool
                  , nMatched    :: Int
                  , nModified   :: Maybe Int
                  , nRemoved    :: Int
                  -- ^ Mongodb server before 2.6 doesn't allow to calculate this value.
                  -- This field is meaningless if we can't calculate the number of modified documents.
                  , upserted    :: [Upserted]
                  , writeErrors :: [Failure]
                  , writeConcernErrors :: [Failure]
                  } deriving Show

instance Result WriteResult where
  isFailed = failed

instance Result (Either a b) where
  isFailed (Left _) = True
  isFailed _ = False

data Upserted = Upserted
              { upsertedIndex :: Int
              , upsertedId    :: ObjectId
              } deriving Show

master :: AccessMode
-- ^ Same as 'ConfirmWrites' []
master = ConfirmWrites []

slaveOk :: AccessMode
-- ^ Same as 'ReadStaleOk'
slaveOk = ReadStaleOk

accessMode :: (Monad m) => AccessMode -> Action m a -> Action m a
-- ^ Run action with given 'AccessMode'
accessMode mode = local (\ctx -> ctx {mongoAccessMode = mode})

readMode :: AccessMode -> ReadMode
readMode ReadStaleOk = StaleOk
readMode _ = Fresh

writeMode :: AccessMode -> WriteMode
writeMode ReadStaleOk = Confirm []
writeMode UnconfirmedWrites = NoConfirm
writeMode (ConfirmWrites z) = Confirm z

-- | Values needed when executing a db operation
data MongoContext = MongoContext {
    mongoPipe :: Pipe, -- ^ operations read/write to this pipelined TCP connection to a MongoDB server
    mongoAccessMode :: AccessMode, -- ^ read/write operation will use this access mode
    mongoDatabase :: Database -- ^ operations query/update this database
}

mongoReadMode :: MongoContext -> ReadMode
mongoReadMode = readMode . mongoAccessMode

mongoWriteMode :: MongoContext -> WriteMode
mongoWriteMode = writeMode . mongoAccessMode

class HasMongoContext env where
    mongoContext :: env -> MongoContext
instance HasMongoContext MongoContext where
    mongoContext = id

liftDB :: (MonadReader env m, HasMongoContext env, MonadIO m)
       => Action IO a
       -> m a
liftDB m = do
    env <- ask
    liftIO $ runReaderT m (mongoContext env)

-- * Database

type Database = Text

allDatabases :: (MonadIO m) => Action m [Database]
-- ^ List all databases residing on server
allDatabases = map (at "name") . at "databases" <$> useDb "admin" (runCommand1 "listDatabases")

thisDatabase :: (Monad m) => Action m Database
-- ^ Current database in use
thisDatabase = asks mongoDatabase

useDb :: (Monad m) => Database -> Action m a -> Action m a
-- ^ Run action against given database
useDb db = local (\ctx -> ctx {mongoDatabase = db})

-- * Authentication

auth :: MonadIO m => Username -> Password -> Action m Bool
-- ^ Authenticate with the current database (if server is running in secure mode). Return whether authentication was successful or not. Reauthentication is required for every new pipe. SCRAM-SHA-1 will be used for server versions 3.0+, MONGO-CR for lower versions.
auth un pw = do
    let serverVersion = fmap (at "version") $ useDb "admin" $ runCommand ["buildinfo" =: (1 :: Int)]
    mmv <- readMaybe . T.unpack . head . T.splitOn "." <$> serverVersion
    maybe (return False) performAuth mmv
    where
    performAuth majorVersion =
        if majorVersion >= (3 :: Int)
        then authSCRAMSHA1 un pw
        else authMongoCR un pw

authMongoCR :: (MonadIO m) => Username -> Password -> Action m Bool
-- ^ Authenticate with the current database, using the MongoDB-CR authentication mechanism (default in MongoDB server < 3.0)
authMongoCR usr pss = do
    n <- at "nonce" <$> runCommand ["getnonce" =: (1 :: Int)]
    true1 "ok" <$> runCommand ["authenticate" =: (1 :: Int), "user" =: usr, "nonce" =: n, "key" =: pwKey n usr pss]

authSCRAMSHA1 :: MonadIO m => Username -> Password -> Action m Bool
-- ^ Authenticate with the current database, using the SCRAM-SHA-1 authentication mechanism (default in MongoDB server >= 3.0)
authSCRAMSHA1 un pw = do
    let hmac = HMAC.hmac SHA1.hash 64
    nonce <- liftIO (Nonce.withGenerator Nonce.nonce128 <&> B64.encode)
    let firstBare = B.concat [B.pack $ "n=" ++ T.unpack un ++ ",r=", nonce]
    let client1 = ["saslStart" =: (1 :: Int), "mechanism" =: ("SCRAM-SHA-1" :: String), "payload" =: (B.unpack . B64.encode $ B.concat [B.pack "n,,", firstBare]), "autoAuthorize" =: (1 :: Int)]
    server1 <- runCommand client1

    shortcircuit (true1 "ok" server1) $ do
        let serverPayload1 = B64.decodeLenient . B.pack . at "payload" $ server1
        let serverData1 = parseSCRAM serverPayload1
        let iterations = read . B.unpack $ Map.findWithDefault "1" "i" serverData1
        let salt = B64.decodeLenient $ Map.findWithDefault "" "s" serverData1
        let snonce = Map.findWithDefault "" "r" serverData1

        shortcircuit (B.isInfixOf nonce snonce) $ do
            let withoutProof = B.concat [B.pack "c=biws,r=", snonce]
            let digestS = B.pack $ T.unpack un ++ ":mongo:" ++ T.unpack pw
            let digest = B16.encode $ MD5.hash digestS
            let saltedPass = scramHI digest salt iterations
            let clientKey = hmac saltedPass (B.pack "Client Key")
            let storedKey = SHA1.hash clientKey
            let authMsg = B.concat [firstBare, B.pack ",", serverPayload1, B.pack ",", withoutProof]
            let clientSig = hmac storedKey authMsg
            let pval = B64.encode . BS.pack $ BS.zipWith xor clientKey clientSig
            let clientFinal = B.concat [withoutProof, B.pack ",p=", pval]
            let serverKey = hmac saltedPass (B.pack "Server Key")
            let serverSig = B64.encode $ hmac serverKey authMsg
            let client2 = ["saslContinue" =: (1 :: Int), "conversationId" =: (at "conversationId" server1 :: Int), "payload" =: B.unpack (B64.encode clientFinal)]
            server2 <- runCommand client2

            shortcircuit (true1 "ok" server2) $ do
                let serverPayload2 = B64.decodeLenient . B.pack $ at "payload" server2
                let serverData2 = parseSCRAM serverPayload2
                let serverSigComp = Map.findWithDefault "" "v" serverData2

                shortcircuit (serverSig == serverSigComp) $ do
                  let done = true1 "done" server2
                  if done
                    then return True
                    else do
                      let client2Step2 = [ "saslContinue" =: (1 :: Int)
                                         , "conversationId" =: (at "conversationId" server1 :: Int)
                                         , "payload" =: String ""]
                      server3 <- runCommand client2Step2
                      shortcircuit (true1 "ok" server3) $ do
                        return True
    where
    shortcircuit True f = f
    shortcircuit False _ = return False

scramHI :: B.ByteString -> B.ByteString -> Int -> B.ByteString
scramHI digest salt iters = snd $ foldl com (u1, u1) [1..(iters-1)]
    where
    hmacd = HMAC.hmac SHA1.hash 64 digest
    u1 = hmacd (B.concat [salt, BS.pack [0, 0, 0, 1]])
    com (u,uc) _ = let u' = hmacd u in (u', BS.pack $ BS.zipWith xor uc u')

parseSCRAM :: B.ByteString -> Map.Map B.ByteString B.ByteString
parseSCRAM = Map.fromList . fmap (cleanup . T.breakOn "=") . T.splitOn "," . T.pack . B.unpack
    where cleanup (t1, t2) = (B.pack $ T.unpack t1, B.pack . T.unpack $ T.drop 1 t2)

-- As long as server api  is not requested OP_Query has to be used. See:
-- https://github.com/mongodb/specifications/blob/6dc6f80026f0f8d99a8c81f996389534b14f6602/source/mongodb-handshake/handshake.rst#specification
retrieveServerData :: (MonadIO m) => Action m ServerData
retrieveServerData = do
  d <- runCommand1 "isMaster"
  let newSd = ServerData
                { isMaster = fromMaybe False $ lookup "isMaster" d
                , minWireVersion = fromMaybe 0 $ lookup "minWireVersion" d
                , maxWireVersion = fromMaybe 0 $ lookup "maxWireVersion" d
                , maxMessageSizeBytes = fromMaybe 48000000 $ lookup "maxMessageSizeBytes" d
                , maxBsonObjectSize = fromMaybe (16 * 1024 * 1024) $ lookup "maxBsonObjectSize" d
                , maxWriteBatchSize = fromMaybe 1000 $ lookup "maxWriteBatchSize" d
                }
  return newSd

-- * Collection

type Collection = Text
-- ^ Collection name (not prefixed with database)

allCollections :: MonadIO m => Action m [Collection]
-- ^ List all collections in this database
allCollections = do
    p <- asks mongoPipe
    let sd = P.serverData p
    if maxWireVersion sd <= 2
      then do
        db <- thisDatabase
        docs <- rest =<< find (query [] "system.namespaces") {sort = ["name" =: (1 :: Int)]}
        (return . filter (not . isSpecial db)) (map (dropDbPrefix . at "name") docs)
      else
        if maxWireVersion sd < 17
          then do
            r <- runCommand1 "listCollections"
            let curData = do
                       (Doc curDoc) <- r !? "cursor"
                       (curId :: Int64) <- curDoc !? "id"
                       (curNs :: Text) <- curDoc !? "ns"
                       (firstBatch :: [Value]) <- curDoc !? "firstBatch"
                       return (curId, curNs, mapMaybe cast' firstBatch :: [Document])
            case curData of
              Nothing -> return []
              Just (curId, curNs, firstBatch) -> do
                db <- thisDatabase
                nc <- newCursor db curNs 0 $ return $ Batch Nothing curId firstBatch
                docs <- rest nc
                return $ mapMaybe (\d -> d !? "name") docs
          else do
            let q = Query [] (Select ["listCollections" =: (1 :: Int)] "$cmd") [] 0 0 [] False 0 []
            qr <- queryRequestOpMsg False q
            dBatch <- liftIO $ requestOpMsg p qr []
            db <- thisDatabase
            nc <- newCursor db "$cmd" 0 dBatch
            docs <- rest nc
            return $ mapMaybe (\d -> d !? "name") docs
 where
    dropDbPrefix = T.tail . T.dropWhile (/= '.')
    isSpecial db col = T.any (== '$') col && db <.> col /= "local.oplog.$main"

-- * Selection

data Selection = Select {selector :: Selector, coll :: Collection}  deriving (Show, Eq)
-- ^ Selects documents in collection that match selector

type Selector = Document
-- ^ Filter for a query, analogous to the where clause in SQL. @[]@ matches all documents in collection. @[\"x\" =: a, \"y\" =: b]@ is analogous to @where x = a and y = b@ in SQL. See <http://www.mongodb.org/display/DOCS/Querying> for full selector syntax.

whereJS :: Selector -> Javascript -> Selector
-- ^ Add Javascript predicate to selector, in which case a document must match both selector and predicate
whereJS sel js = ("$where" =: js) : sel

class Select aQueryOrSelection where
    select :: Selector -> Collection -> aQueryOrSelection
    -- ^ 'Query' or 'Selection' that selects documents in collection that match selector. The choice of type depends on use, for example, in @'find' (select sel col)@ it is a Query, and in @'delete' (select sel col)@ it is a Selection.

instance Select Selection where
    select = Select

instance Select Query where
    select = query

-- * Write

data WriteMode =
      NoConfirm  -- ^ Submit writes without receiving acknowledgments. Fast. Assumes writes succeed even though they may not.
    | Confirm GetLastError  -- ^ Receive an acknowledgment after every write, and raise exception if one says the write failed. This is acomplished by sending the getLastError command, with given 'GetLastError' parameters, after every write.
    deriving (Show, Eq)

write :: Notice -> Action IO (Maybe Document)
-- ^ Send write to server, and if write-mode is 'Safe' then include getLastError request and raise 'WriteFailure' if it reports an error.
write notice = asks mongoWriteMode >>= \mode -> case mode of
    NoConfirm -> do
      pipe <- asks mongoPipe
      liftIOE ConnectionFailure $ P.send pipe [notice]
      return Nothing
    Confirm params -> do
        let q = query (("getlasterror" =: (1 :: Int)) : params) "$cmd"
        pipe <- asks mongoPipe
        Batch _ _ [doc] <- do
          r <- queryRequest False q {limit = 1}
          rr <- liftIO $ request pipe [notice] r
          fulfill rr
        return $ Just doc

-- ** Insert

insert :: (MonadIO m) => Collection -> Document -> Action m Value
-- ^ Insert document into collection and return its \"_id\" value, which is created automatically if not supplied
insert col doc = do
  doc' <- liftIO $ assignId doc
  res <- insertBlock [] col (0, [doc'])
  case res of
    Left failure -> liftIO $ throwIO failure
    Right r -> return $ head r

insert_ :: (MonadIO m) => Collection -> Document -> Action m ()
-- ^ Same as 'insert' except don't return _id
insert_ col doc = insert col doc >> return ()

insertMany :: (MonadIO m) => Collection -> [Document] -> Action m [Value]
-- ^ Insert documents into collection and return their \"_id\" values,
-- which are created automatically if not supplied.
-- If a document fails to be inserted (eg. due to duplicate key)
-- then remaining docs are aborted, and @LastError@ is set.
-- An exception will be throw if any error occurs.
insertMany = insert' []

insertMany_ :: (MonadIO m) => Collection -> [Document] -> Action m ()
-- ^ Same as 'insertMany' except don't return _ids
insertMany_ col docs = insertMany col docs >> return ()

insertAll :: (MonadIO m) => Collection -> [Document] -> Action m [Value]
-- ^ Insert documents into collection and return their \"_id\" values,
-- which are created automatically if not supplied. If a document fails
-- to be inserted (eg. due to duplicate key) then remaining docs
-- are still inserted.
insertAll = insert' [KeepGoing]

insertAll_ :: (MonadIO m) => Collection -> [Document] -> Action m ()
-- ^ Same as 'insertAll' except don't return _ids
insertAll_ col docs = insertAll col docs >> return ()

insertCommandDocument :: [InsertOption] -> Collection -> [Document] -> Document -> Document
insertCommandDocument opts col docs writeConcern =
          [ "insert" =: col
          , "ordered" =: (KeepGoing `notElem` opts)
          , "documents" =: docs
          , "writeConcern" =: writeConcern
          ]

takeRightsUpToLeft :: [Either a b] -> [b]
takeRightsUpToLeft l = E.rights $ takeWhile E.isRight l

insert' :: (MonadIO m)
        => [InsertOption] -> Collection -> [Document] -> Action m [Value]
-- ^ Insert documents into collection and return their \"_id\" values, which are created automatically if not supplied
insert' opts col docs = do
  p <- asks mongoPipe
  let sd = P.serverData p
  docs' <- liftIO $ mapM assignId docs
  mode <- asks mongoWriteMode
  let writeConcern = case mode of
                        NoConfirm -> ["w" =: (0 :: Int32)]
                        Confirm params -> params
  let docSize = sizeOfDocument $ insertCommandDocument opts col [] writeConcern
  let ordered = KeepGoing `notElem` opts
  let preChunks = splitAtLimit
                      (maxBsonObjectSize sd - docSize)
                                           -- size of auxiliary part of insert
                                           -- document should be subtracted from
                                           -- the overall size
                      (maxWriteBatchSize sd)
                      docs'
  let chunks =
        if ordered
            then takeRightsUpToLeft preChunks
            else rights preChunks

  let lens = map length chunks
  let lSums = 0 : zipWith (+) lSums lens

  chunkResults <- interruptibleFor ordered (zip lSums chunks) $ insertBlock opts col

  let lchunks = lefts preChunks
  when (not $ null lchunks) $ do
    liftIO $ throwIO $ head lchunks

  let lresults = lefts chunkResults
  when (not $ null lresults) $ liftIO $ throwIO $ head lresults
  return $ concat $ rights chunkResults

insertBlock :: (MonadIO m)
            => [InsertOption] -> Collection -> (Int, [Document]) -> Action m (Either Failure [Value])
-- ^ This will fail if the list of documents is bigger than restrictions
insertBlock _ _ (_, []) = return $ Right []
insertBlock opts col (prevCount, docs) = do
    db <- thisDatabase

    p <- asks mongoPipe
    let sd = P.serverData p
    if maxWireVersion sd < 2
      then do
        res <- liftDB $ write (Insert (db <.> col) opts docs)
        let errorMessage = do
              jRes <- res
              em <- lookup "err" jRes
              return $ WriteFailure prevCount (fromMaybe 0 $ lookup "code" jRes)  em
              -- In older versions of ^^ the protocol we can't really say which document failed.
              -- So we just report the accumulated number of documents in the previous blocks.

        case errorMessage of
          Just failure -> return $ Left failure
          Nothing -> return $ Right $ map (valueAt "_id") docs
      else if maxWireVersion sd == 2 && maxWireVersion sd < 17 then do
        mode <- asks mongoWriteMode
        let writeConcern = case mode of
                              NoConfirm -> ["w" =: (0 :: Int32)]
                              Confirm params -> params
        doc <- runCommand $ insertCommandDocument opts col docs writeConcern
        case (look "writeErrors" doc, look "writeConcernError" doc) of
          (Nothing, Nothing) -> return $ Right $ map (valueAt "_id") docs
          (Just (Array errs), Nothing) -> do
            let writeErrors = map (anyToWriteError prevCount) errs
            let errorsWithFailureIndex = map (addFailureIndex prevCount) writeErrors
            return $ Left $ CompoundFailure errorsWithFailureIndex
          (Nothing, Just err) -> do
            return $ Left $ WriteFailure
                                    prevCount
                                    (fromMaybe 0 $ lookup "ok" doc)
                                    (show err)
          (Just (Array errs), Just writeConcernErr) -> do
            let writeErrors = map (anyToWriteError prevCount) errs
            let errorsWithFailureIndex = map (addFailureIndex prevCount) writeErrors
            return $ Left $ CompoundFailure $ WriteFailure
                                    prevCount
                                    (fromMaybe 0 $ lookup "ok" doc)
                                    (show writeConcernErr) : errorsWithFailureIndex
          (Just unknownValue, Nothing) -> do
            return $ Left $ ProtocolFailure prevCount $ "Expected array of errors. Received: " ++ show unknownValue
          (Just unknownValue, Just writeConcernErr) -> do
            return $ Left $ CompoundFailure [ ProtocolFailure prevCount $ "Expected array of errors. Received: " ++ show unknownValue
                                              , WriteFailure prevCount (fromMaybe 0 $ lookup "ok" doc) $ show writeConcernErr]
      else do
        mode <- asks mongoWriteMode
        let writeConcern = case mode of
                              NoConfirm -> ["w" =: (0 :: Int32)]
                              Confirm params -> merge params ["w" =: (1 :: Int32)]
        doc <- runCommand $ insertCommandDocument opts col docs writeConcern
        case (look "writeErrors" doc, look "writeConcernError" doc) of
          (Nothing, Nothing) -> return $ Right $ map (valueAt "_id") docs
          (Just (Array errs), Nothing) -> do
            let writeErrors = map (anyToWriteError prevCount) errs
            let errorsWithFailureIndex = map (addFailureIndex prevCount) writeErrors
            return $ Left $ CompoundFailure errorsWithFailureIndex
          (Nothing, Just err) -> do
            return $ Left $ WriteFailure
                                    prevCount
                                    (fromMaybe 0 $ lookup "ok" doc)
                                    (show err)
          (Just (Array errs), Just writeConcernErr) -> do
            let writeErrors = map (anyToWriteError prevCount) errs
            let errorsWithFailureIndex = map (addFailureIndex prevCount) writeErrors
            return $ Left $ CompoundFailure $ WriteFailure
                                    prevCount
                                    (fromMaybe 0 $ lookup "ok" doc)
                                    (show writeConcernErr) : errorsWithFailureIndex
          (Just unknownValue, Nothing) -> do
            return $ Left $ ProtocolFailure prevCount $ "Expected array of errors. Received: " ++ show unknownValue
          (Just unknownValue, Just writeConcernErr) -> do
            return $ Left $ CompoundFailure [ ProtocolFailure prevCount $ "Expected array of errors. Received: " ++ show unknownValue
                                              , WriteFailure prevCount (fromMaybe 0 $ lookup "ok" doc) $ show writeConcernErr]

splitAtLimit :: Int -> Int -> [Document] -> [Either Failure [Document]]
splitAtLimit maxSize maxCount list = chop (go 0 0 []) list
  where
    go :: Int -> Int -> [Document] -> [Document] -> (Either Failure [Document], [Document])
    go _ _ res [] = (Right $ reverse res, [])
    go curSize curCount [] (x:xs) |
      (curSize + sizeOfDocument x + 2 + curCount) > maxSize =
        (Left $ WriteFailure 0 0 "One document is too big for the message", xs)
    go curSize curCount res (x:xs) =
      if ((curSize + sizeOfDocument x + 2 + curCount) > maxSize)
                                 -- we have ^ 2 brackets and curCount commas in
                                 -- the document that we need to take into
                                 -- account
          || ((curCount + 1) > maxCount)
        then
          (Right $ reverse res, x:xs)
        else
          go (curSize + sizeOfDocument x) (curCount + 1) (x:res) xs

    chop :: ([a] -> (b, [a])) -> [a] -> [b]
    chop _ [] = []
    chop f as = let (b, as') = f as in b : chop f as'

sizeOfDocument :: Document -> Int
sizeOfDocument d = fromIntegral $ LBS.length $ runPut $ putDocument d

assignId :: Document -> IO Document
-- ^ Assign a unique value to _id field if missing
assignId doc = if any (("_id" ==) . label) doc
    then return doc
    else (\oid -> ("_id" =: oid) : doc) <$> genObjectId

-- ** Update

save :: (MonadIO m)
     => Collection -> Document -> Action m ()
-- ^ Save document to collection, meaning insert it if its new (has no \"_id\" field) or upsert it if its not new (has \"_id\" field)
save col doc = case look "_id" doc of
    Nothing -> insert_ col doc
    Just i -> upsert (Select ["_id" := i] col) doc

replace :: (MonadIO m)
        => Selection -> Document -> Action m ()
-- ^ Replace first document in selection with given document
replace = update []

repsert :: (MonadIO m)
        => Selection -> Document -> Action m ()
-- ^ Replace first document in selection with given document, or insert document if selection is empty
repsert = update [Upsert]
{-# DEPRECATED repsert "use upsert instead" #-}

upsert :: (MonadIO m)
       => Selection -> Document -> Action m ()
-- ^ Update first document in selection with given document, or insert document if selection is empty
upsert = update [Upsert]

type Modifier = Document
-- ^ Update operations on fields in a document. See <https://docs.mongodb.com/manual/reference/operator/update/>

modify :: (MonadIO m)
       => Selection -> Modifier -> Action m ()
-- ^ Update all documents in selection using given modifier
modify = update [MultiUpdate]

update :: (MonadIO m)
       => [UpdateOption] -> Selection -> Document -> Action m ()
-- ^ Update first document in selection using updater document, unless 'MultiUpdate' option is supplied then update all documents in selection. If 'Upsert' option is supplied then treat updater as document and insert it if selection is empty.
update opts (Select sel col) up = do
    pipe <- asks mongoPipe
    db <- thisDatabase
    let sd = P.serverData pipe
    if maxWireVersion sd < 17
      then do
        ctx <- ask
        liftIO $ runReaderT (void $ write (Update (db <.> col) opts sel up)) ctx
      else do
        liftIOE ConnectionFailure $
          P.sendOpMsg
            pipe
            [Nc (Update (db <.> col) opts sel up)]
            (Just P.MoreToCome)
            ["writeConcern" =: ["w" =: (0 :: Int32)]]

updateCommandDocument :: Collection -> Bool -> [Document] -> Document -> Document
updateCommandDocument col ordered updates writeConcern =
  [ "update"  =: col
  , "ordered" =: ordered
  , "updates" =: updates
  , "writeConcern" =: writeConcern
  ]

{-| Bulk update operation. If one update fails it will not update the remaining
    documents. Current returned value is only a place holder. With mongodb server
    before 2.6 it will send update requests one by one. In order to receive
    error messages in versions under 2.6 you need to user confirmed writes.
    Otherwise even if the errors had place the list of errors will be empty and
    the result will be success. After 2.6 it will use bulk update feature in
    mongodb.
 -}
updateMany :: (MonadIO m)
           => Collection
           -> [(Selector, Document, [UpdateOption])]
           -> Action m WriteResult
updateMany = update' True

{-| Bulk update operation. If one update fails it will proceed with the
    remaining documents. With mongodb server before 2.6 it will send update
    requests one by one. In order to receive error messages in versions under
    2.6 you need to use confirmed writes.  Otherwise even if the errors had
    place the list of errors will be empty and the result will be success.
    After 2.6 it will use bulk update feature in mongodb.
 -}
updateAll :: (MonadIO m)
           => Collection
           -> [(Selector, Document, [UpdateOption])]
           -> Action m WriteResult
updateAll = update' False

update' :: (MonadIO m)
        => Bool
        -> Collection
        -> [(Selector, Document, [UpdateOption])]
        -> Action m WriteResult
update' ordered col updateDocs = do
  p <- asks mongoPipe
  let sd = P.serverData p
  let updates = map (\(s, d, os) -> [ "q" =: s
                                    , "u" =: d
                                    , "upsert" =: (Upsert `elem` os)
                                    , "multi" =: (MultiUpdate `elem` os)])
                updateDocs

  mode <- asks mongoWriteMode
  ctx <- ask
  liftIO $ do
    let writeConcern = case mode of
                          NoConfirm -> ["w" =: (0 :: Int32)]
                          Confirm params -> params
    let docSize = sizeOfDocument $ updateCommandDocument
                                                      col
                                                      ordered
                                                      []
                                                      writeConcern
    let preChunks = splitAtLimit
                        (maxBsonObjectSize sd - docSize)
                                             -- size of auxiliary part of update
                                             -- document should be subtracted from
                                             -- the overall size
                        (maxWriteBatchSize sd)
                        updates
    let chunks =
          if ordered
              then takeRightsUpToLeft preChunks
              else rights preChunks
    let lens = map length chunks
    let lSums = 0 : zipWith (+) lSums lens
    blocks <- interruptibleFor ordered (zip lSums chunks) $ \b -> do
      runReaderT (updateBlock ordered col b) ctx
      `catch` \(e :: Failure) -> do
        return $ WriteResult True 0 Nothing 0 [] [e] []
    let failedTotal = any failed blocks
    let updatedTotal = sum $ map nMatched blocks
    let modifiedTotal =
          if all (isNothing . nModified) blocks
            then Nothing
            else Just $ sum $ mapMaybe nModified blocks
    let totalWriteErrors = concatMap writeErrors blocks
    let totalWriteConcernErrors = concatMap writeConcernErrors blocks

    let upsertedTotal = concatMap upserted blocks
    return $ WriteResult
                  failedTotal
                  updatedTotal
                  modifiedTotal
                  0 -- nRemoved
                  upsertedTotal
                  totalWriteErrors
                  totalWriteConcernErrors

    `catch` \(e :: Failure) -> return $ WriteResult True 0 Nothing 0 [] [e] []

updateBlock :: (MonadIO m)
            => Bool -> Collection -> (Int, [Document]) -> Action m WriteResult
updateBlock ordered col (prevCount, docs) = do
  p <- asks mongoPipe
  let sd = P.serverData p
  if maxWireVersion sd < 2
    then liftIO $ ioError $ userError "updateMany doesn't support mongodb older than 2.6"
    else if maxWireVersion sd == 2 && maxWireVersion sd < 17 then do
      mode <- asks mongoWriteMode
      let writeConcern = case mode of
                          NoConfirm -> ["w" =: (0 :: Int32)]
                          Confirm params -> params
      doc <- runCommand $ updateCommandDocument col ordered docs writeConcern

      let n = fromMaybe 0 $ doc !? "n"
      let writeErrorsResults =
            case look "writeErrors" doc of
              Nothing -> WriteResult False 0 (Just 0) 0 [] [] []
              Just (Array err) -> WriteResult True 0 (Just 0) 0 [] (map (anyToWriteError prevCount) err) []
              Just unknownErr -> WriteResult
                                      True
                                      0
                                      (Just 0)
                                      0
                                      []
                                      [ ProtocolFailure
                                            prevCount
                                          $ "Expected array of error docs, but received: "
                                              ++ show unknownErr]
                                      []

      let writeConcernResults =
            case look "writeConcernError" doc of
              Nothing ->  WriteResult False 0 (Just 0) 0 [] [] []
              Just (Doc err) -> WriteResult
                                    True
                                    0
                                    (Just 0)
                                    0
                                    []
                                    []
                                    [ WriteConcernFailure
                                        (fromMaybe (-1) $ err !? "code")
                                        (fromMaybe "" $ err !? "errmsg")
                                    ]
              Just unknownErr -> WriteResult
                                      True
                                      0
                                      (Just 0)
                                      0
                                      []
                                      []
                                      [ ProtocolFailure
                                            prevCount
                                          $ "Expected doc in writeConcernError, but received: "
                                              ++ show unknownErr]

      let upsertedList = maybe [] (map docToUpserted) (doc !? "upserted")
      let successResults = WriteResult False n (doc !? "nModified") 0 upsertedList [] []
      return $ foldl1' mergeWriteResults [writeErrorsResults, writeConcernResults, successResults]
    else do
      mode <- asks mongoWriteMode
      let writeConcern = case mode of
                          NoConfirm -> ["w" =: (0 :: Int32)]
                          Confirm params -> merge params ["w" =: (1 :: Int32)]
      doc <- runCommand $ updateCommandDocument col ordered docs writeConcern

      let n = fromMaybe 0 $ doc !? "n"
      let writeErrorsResults =
            case look "writeErrors" doc of
              Nothing -> WriteResult False 0 (Just 0) 0 [] [] []
              Just (Array err) -> WriteResult True 0 (Just 0) 0 [] (map (anyToWriteError prevCount) err) []
              Just unknownErr -> WriteResult
                                      True
                                      0
                                      (Just 0)
                                      0
                                      []
                                      [ ProtocolFailure
                                            prevCount
                                          $ "Expected array of error docs, but received: "
                                              ++ show unknownErr]
                                      []

      let writeConcernResults =
            case look "writeConcernError" doc of
              Nothing ->  WriteResult False 0 (Just 0) 0 [] [] []
              Just (Doc err) -> WriteResult
                                    True
                                    0
                                    (Just 0)
                                    0
                                    []
                                    []
                                    [ WriteConcernFailure
                                        (fromMaybe (-1) $ err !? "code")
                                        (fromMaybe "" $ err !? "errmsg")
                                    ]
              Just unknownErr -> WriteResult
                                      True
                                      0
                                      (Just 0)
                                      0
                                      []
                                      []
                                      [ ProtocolFailure
                                            prevCount
                                          $ "Expected doc in writeConcernError, but received: "
                                              ++ show unknownErr]

      let upsertedList = maybe [] (map docToUpserted) (doc !? "upserted")
      let successResults = WriteResult False n (doc !? "nModified") 0 upsertedList [] []
      return $ foldl1' mergeWriteResults [writeErrorsResults, writeConcernResults, successResults]

interruptibleFor :: (Monad m, Result b) => Bool -> [a] -> (a -> m b) -> m [b]
interruptibleFor ordered = go []
  where
    go !res [] _ = return $ reverse res
    go !res (x:xs) f = do
      y <- f x
      if isFailed y && ordered
        then return $ reverse (y:res)
        else go (y:res) xs f

mergeWriteResults :: WriteResult -> WriteResult -> WriteResult
mergeWriteResults
  (WriteResult failed1 nMatched1 nModified1 nDeleted1 upserted1 writeErrors1 writeConcernErrors1)
  (WriteResult failed2 nMatched2 nModified2 nDeleted2 upserted2 writeErrors2 writeConcernErrors2) =
    WriteResult
        (failed1 || failed2)
        (nMatched1 + nMatched2)
        (liftM2 (+) nModified1 nModified2)
        (nDeleted1 + nDeleted2)
        -- This function is used in foldl1' function. The first argument is the accumulator.
        -- The list in the accumulator is usually longer than the subsequent value which goes in the second argument.
        -- So, changing the order of list concatenation allows us to keep linear complexity of the
        -- whole list accumulation process.
        (upserted2 ++ upserted1)
        (writeErrors2 ++ writeErrors1)
        (writeConcernErrors2 ++ writeConcernErrors1)


docToUpserted :: Document -> Upserted
docToUpserted doc = Upserted ind uid
  where
    ind = at "index" doc
    uid = at "_id"   doc

docToWriteError :: Document -> Failure
docToWriteError doc = WriteFailure ind code msg
  where
    ind  = at "index"  doc
    code = at "code"   doc
    msg  = at "errmsg" doc

-- ** Delete

delete :: (MonadIO m)
       => Selection -> Action m ()
-- ^ Delete all documents in selection
delete s = do
    pipe <- asks mongoPipe
    let sd = P.serverData pipe
    if maxWireVersion sd < 17
      then deleteHelper [] s
      else deleteMany (coll s) [([], [])] >> return ()

deleteOne :: (MonadIO m)
          => Selection -> Action m ()
-- ^ Delete first document in selection
deleteOne sel@((Select sel' col)) = do
    pipe <- asks mongoPipe
    let sd = P.serverData pipe
    if maxWireVersion sd < 17
      then deleteHelper [SingleRemove] sel
      else do
        -- Starting with v6 confirming writes via getLastError as it is
        -- performed in the deleteHelper call via its call to write is
        -- deprecated. To confirm writes now an appropriate writeConcern has to be
        -- set. These confirmations were discarded in deleteHelper anyway so no
        -- need to dispatch on the writeConcern as it is currently done in deleteHelper
        -- via write for older versions
        db <- thisDatabase
        liftIOE ConnectionFailure $
          P.sendOpMsg
            pipe
            [Nc (Delete (db <.> col) [] sel')]
            (Just P.MoreToCome)
            ["writeConcern" =: ["w" =: (0 :: Int32)]]

deleteHelper :: (MonadIO m)
             => [DeleteOption] -> Selection -> Action m ()
deleteHelper opts (Select sel col) = do
    ctx <- ask
    db <- thisDatabase
    liftIO $ runReaderT (void $ write (Delete (db <.> col) opts sel)) ctx

{-| Bulk delete operation. If one delete fails it will not delete the remaining
    documents. Current returned value is only a place holder. With mongodb server
    before 2.6 it will send delete requests one by one. After 2.6 it will use
    bulk delete feature in mongodb.
 -}
deleteMany :: (MonadIO m)
           => Collection
           -> [(Selector, [DeleteOption])]
           -> Action m WriteResult
deleteMany = delete' True

{-| Bulk delete operation. If one delete fails it will proceed with the
    remaining documents. Current returned value is only a place holder. With
    mongodb server before 2.6 it will send delete requests one by one. After 2.6
    it will use bulk delete feature in mongodb.
 -}
deleteAll :: (MonadIO m)
          => Collection
          -> [(Selector, [DeleteOption])]
          -> Action m WriteResult
deleteAll = delete' False

deleteCommandDocument :: Collection -> Bool -> [Document] -> Document -> Document
deleteCommandDocument col ordered deletes writeConcern =
  [ "delete"       =: col
  , "ordered"      =: ordered
  , "deletes"      =: deletes
  , "writeConcern" =: writeConcern
  ]

delete' :: (MonadIO m)
        => Bool
        -> Collection
        -> [(Selector, [DeleteOption])]
        -> Action m WriteResult
delete' ordered col deleteDocs = do
  p <- asks mongoPipe
  let sd = P.serverData p
  let deletes = map (\(s, os) -> [ "q"     =: s
                                 , "limit" =: if SingleRemove `elem` os
                                               then (1 :: Int) -- Remove only one matching
                                               else (0 :: Int) -- Remove all matching
                                 ])
                    deleteDocs

  mode <- asks mongoWriteMode
  let writeConcern = case mode of
                        NoConfirm -> ["w" =: (0 :: Int32)]
                        Confirm params -> params
  let docSize = sizeOfDocument $ deleteCommandDocument col ordered [] writeConcern
  let chunks = splitAtLimit
                      (maxBsonObjectSize sd - docSize)
                                           -- size of auxiliary part of delete
                                           -- document should be subtracted from
                                           -- the overall size
                      (maxWriteBatchSize sd)
                      deletes
  ctx <- ask
  let lens = map (either (const 1) length) chunks
  let lSums = 0 : zipWith (+) lSums lens
  let failureResult e = return $ WriteResult True 0 Nothing 0 [] [e] []
  let doChunk b = runReaderT (deleteBlock ordered col b) ctx `catch` failureResult
  blockResult <- liftIO $ interruptibleFor ordered (zip lSums chunks) $ \(n, c) ->
    case c of
      Left e -> failureResult e
      Right b -> doChunk (n, b)
  return $ foldl1' mergeWriteResults blockResult


addFailureIndex :: Int -> Failure -> Failure
addFailureIndex i (WriteFailure ind code s) = WriteFailure (ind + i) code s
addFailureIndex _ f = f

deleteBlock :: (MonadIO m)
            => Bool -> Collection -> (Int, [Document]) -> Action m WriteResult
deleteBlock ordered col (prevCount, docs) = do
  p <- asks mongoPipe
  let sd = P.serverData p
  if maxWireVersion sd < 2
    then liftIO $ ioError $ userError "deleteMany doesn't support mongodb older than 2.6"
    else if maxWireVersion sd == 2 && maxWireVersion sd < 17 then do
      mode <- asks mongoWriteMode
      let writeConcern = case mode of
                          NoConfirm -> ["w" =: (0 :: Int32)]
                          Confirm params -> params
      doc <- runCommand $ deleteCommandDocument col ordered docs writeConcern
      let n = fromMaybe 0 $ doc !? "n"

      let successResults = WriteResult False 0 Nothing n [] [] []
      let writeErrorsResults =
            case look "writeErrors" doc of
              Nothing ->  WriteResult False 0 Nothing 0 [] [] []
              Just (Array err) -> WriteResult True 0 Nothing 0 [] (map (anyToWriteError prevCount) err) []
              Just unknownErr -> WriteResult
                                      True
                                      0
                                      Nothing
                                      0
                                      []
                                      [ ProtocolFailure
                                            prevCount
                                          $ "Expected array of error docs, but received: "
                                              ++ show unknownErr]
                                      []
      let writeConcernResults =
            case look "writeConcernError" doc of
              Nothing ->  WriteResult False 0 Nothing 0 [] [] []
              Just (Doc err) -> WriteResult
                                    True
                                    0
                                    Nothing
                                    0
                                    []
                                    []
                                    [ WriteConcernFailure
                                        (fromMaybe (-1) $ err !? "code")
                                        (fromMaybe "" $ err !? "errmsg")
                                    ]
              Just unknownErr -> WriteResult
                                      True
                                      0
                                      Nothing
                                      0
                                      []
                                      []
                                      [ ProtocolFailure
                                            prevCount
                                          $ "Expected doc in writeConcernError, but received: "
                                              ++ show unknownErr]
      return $ foldl1' mergeWriteResults [successResults, writeErrorsResults, writeConcernResults]
    else do
      mode <- asks mongoWriteMode
      let writeConcern = case mode of
                          NoConfirm -> ["w" =: (0 :: Int32)]
                          Confirm params -> merge params ["w" =: (1 :: Int32)]
      doc <- runCommand $ deleteCommandDocument col ordered docs writeConcern
      let n = fromMaybe 0 $ doc !? "n"

      let successResults = WriteResult False 0 Nothing n [] [] []
      let writeErrorsResults =
            case look "writeErrors" doc of
              Nothing ->  WriteResult False 0 Nothing 0 [] [] []
              Just (Array err) -> WriteResult True 0 Nothing 0 [] (map (anyToWriteError prevCount) err) []
              Just unknownErr -> WriteResult
                                      True
                                      0
                                      Nothing
                                      0
                                      []
                                      [ ProtocolFailure
                                            prevCount
                                          $ "Expected array of error docs, but received: "
                                              ++ show unknownErr]
                                      []
      let writeConcernResults =
            case look "writeConcernError" doc of
              Nothing ->  WriteResult False 0 Nothing 0 [] [] []
              Just (Doc err) -> WriteResult
                                    True
                                    0
                                    Nothing
                                    0
                                    []
                                    []
                                    [ WriteConcernFailure
                                        (fromMaybe (-1) $ err !? "code")
                                        (fromMaybe "" $ err !? "errmsg")
                                    ]
              Just unknownErr -> WriteResult
                                      True
                                      0
                                      Nothing
                                      0
                                      []
                                      []
                                      [ ProtocolFailure
                                            prevCount
                                          $ "Expected doc in writeConcernError, but received: "
                                              ++ show unknownErr]
      return $ foldl1' mergeWriteResults [successResults, writeErrorsResults, writeConcernResults]

anyToWriteError :: Int -> Value -> Failure
anyToWriteError _ (Doc d) = docToWriteError d
anyToWriteError ind _ = ProtocolFailure ind "Unknown bson value"

-- * Read

data ReadMode =
      Fresh  -- ^ read from master only
    | StaleOk  -- ^ read from slave ok
    deriving (Show, Eq)

readModeOption :: ReadMode -> [QueryOption]
readModeOption Fresh = []
readModeOption StaleOk = [SlaveOK]

-- ** Query

-- | Use 'select' to create a basic query with defaults, then modify if desired. For example, @(select sel col) {limit = 10}@
data Query = Query {
    options :: [QueryOption],  -- ^ Default = @[]@
    selection :: Selection,
    project :: Projector,  -- ^ @[]@ = all fields. Default = @[]@
    skip :: Word32,  -- ^ Number of initial matching documents to skip. Default = 0
    limit :: Limit, -- ^ Maximum number of documents to return, 0 = no limit. Default = 0
    sort :: Order,  -- ^ Sort results by this order, @[]@ = no sort. Default = @[]@
    snapshot :: Bool,  -- ^ If true assures no duplicates are returned, or objects missed, which were present at both the start and end of the query's execution (even if the object were updated). If an object is new during the query, or deleted during the query, it may or may not be returned, even with snapshot mode. Note that short query responses (less than 1MB) are always effectively snapshotted. Default = @False@
    batchSize :: BatchSize,  -- ^ The number of document to return in each batch response from the server. 0 means use Mongo default. Default = 0
    hint :: Order  -- ^ Force MongoDB to use this index, @[]@ = no hint. Default = @[]@
    } deriving (Show, Eq)

type Projector = Document
-- ^ Fields to return, analogous to the select clause in SQL. @[]@ means return whole document (analogous to * in SQL). @[\"x\" =: 1, \"y\" =: 1]@ means return only @x@ and @y@ fields of each document. @[\"x\" =: 0]@ means return all fields except @x@.

type Limit = Word32
-- ^ Maximum number of documents to return, i.e. cursor will close after iterating over this number of documents. 0 means no limit.

type Order = Document
-- ^ Fields to sort by. Each one is associated with 1 or -1. Eg. @[\"x\" =: 1, \"y\" =: -1]@ means sort by @x@ ascending then @y@ descending

type BatchSize = Word32
-- ^ The number of document to return in each batch response from the server. 0 means use Mongo default.

-- noticeCommands and adminCommands are needed to identify whether
-- queryRequestOpMsg is called via runCommand or not. If not it will
-- behave like being called by a "find"-like command and add additional fields
-- specific to the find command into the selector, such as "filter", "projection" etc.
noticeCommands :: [Text]
noticeCommands = [ "aggregate"
                 , "count"
                 , "delete"
                 , "findAndModify"
                 , "insert"
                 , "listCollections"
                 , "update"
                 ]

adminCommands :: [Text]
adminCommands = [ "buildinfo"
                , "clone"
                , "collstats"
                , "copydb"
                , "copydbgetnonce"
                , "create"
                , "dbstats"
                , "deleteIndexes"
                , "drop"
                , "dropDatabase"
                , "renameCollection"
                , "repairDatabase"
                , "serverStatus"
                , "validate"
                ]

query :: Selector -> Collection -> Query
-- ^ Selects documents in collection that match selector. It uses no query options, projects all fields, does not skip any documents, does not limit result size, uses default batch size, does not sort, does not hint, and does not snapshot.
query sel col = Query [] (Select sel col) [] 0 0 [] False 0 []

find :: MonadIO m => Query -> Action m Cursor
-- ^ Fetch documents satisfying query
find q@Query{selection, batchSize} = do
    pipe <- asks mongoPipe
    db <- thisDatabase
    let sd = P.serverData pipe
    if maxWireVersion sd < 17
      then do
        qr <- queryRequest False q
        dBatch <- liftIO $ request pipe [] qr
        newCursor db (coll selection) batchSize dBatch
      else do
        qr <- queryRequestOpMsg False q
        let newQr =
              case fst qr of
                Req qry ->
                  let coll = last $ T.splitOn "." (qFullCollection qry)
                  in (Req $ qry {qSelector = merge (qSelector qry) [ "find" =: coll ]}, snd qr)
                -- queryRequestOpMsg only returns Cmd types constructed via Req
                _ -> error "impossible"
        dBatch <- liftIO $ requestOpMsg pipe newQr []
        newCursor db (coll selection) batchSize dBatch

findCommand :: (MonadIO m, MonadFail m) => Query -> Action m Cursor
-- ^ Fetch documents satisfying query using the command "find"
findCommand q@Query{..} = do
    pipe <- asks mongoPipe
    let sd = P.serverData pipe
    if maxWireVersion sd < 17
      then do
        let aColl = coll selection
        response <- runCommand $
          [ "find"        =: aColl
          , "filter"      =: selector selection
          , "sort"        =: sort
          , "projection"  =: project
          , "hint"        =: hint
          , "skip"        =: toInt32 skip
          ]
          ++ mconcat -- optional fields. They should not be present if set to 0 and mongo will use defaults
             [ "batchSize" =? toMaybe (/= 0) toInt32 batchSize
             , "limit"     =? toMaybe (/= 0) toInt32 limit
             ]
        getCursorFromResponse aColl response
          >>= either (liftIO . throwIO . QueryFailure (at "code" response)) return
      else find q
    where
      toInt32 :: Integral a => a -> Int32
      toInt32 = fromIntegral

      toMaybe :: (a -> Bool) -> (a -> b) -> a -> Maybe b
      toMaybe predicate f a
        | predicate a = Just (f a)
        | otherwise   = Nothing

findOne :: (MonadIO m) => Query -> Action m (Maybe Document)
-- ^ Fetch first document satisfying query or @Nothing@ if none satisfy it
findOne q = do
    pipe <- asks mongoPipe
    let legacyQuery = do
            qr <- queryRequest False q {limit = 1}
            rq <- liftIO $ request pipe [] qr
            Batch _ _ docs <- liftDB $ fulfill rq
            return (listToMaybe docs)
        isHandshake = (== ["isMaster" =: (1 :: Int32)])  $ selector $ selection q :: Bool
    if isHandshake
      then legacyQuery
      else do
        let sd = P.serverData pipe
        if (maxWireVersion sd < 17)
          then legacyQuery
          else do
            qr <- queryRequestOpMsg False q {limit = 1}
            let newQr =
                  case fst qr of
                    Req qry ->
                      let coll = last $ T.splitOn "." (qFullCollection qry)
                          -- We have to understand whether findOne is called as
                          -- command directly. This is necessary since findOne is used via
                          -- runCommand as a vehicle to execute any type of commands and notices.
                          labels = catMaybes $ map (\f -> look f $ qSelector qry) (noticeCommands ++ adminCommands) :: [Value]
                      in if null labels
                           then (Req $ qry {qSelector = merge (qSelector qry) [ "find" =: coll ]}, snd qr)
                           else qr
                    _ -> error "impossible"
            rq <- liftIO $ requestOpMsg pipe newQr []
            Batch _ _ docs <- liftDB $ fulfill rq
            return (listToMaybe docs)

fetch :: (MonadIO m) => Query -> Action m Document
-- ^ Same as 'findOne' except throw 'DocNotFound' if none match
fetch q = findOne q >>= maybe (liftIO $ throwIO $ DocNotFound $ selection q) return

-- | Options for @findAndModify@
data FindAndModifyOpts
  = FamRemove Bool -- ^ remove the selected document when the boolean is @True@
  | FamUpdate
    { famUpdate :: Document -- ^ the update instructions, or a replacement document
    , famNew :: Bool -- ^ return the document with the modifications made on the update
    , famUpsert :: Bool -- ^ create a new document if no documents match the query
    }
  deriving Show

-- | Default options used by 'findAndModify'.
defFamUpdateOpts :: Document -> FindAndModifyOpts
defFamUpdateOpts ups = FamUpdate
                       { famNew = True
                       , famUpsert = False
                       , famUpdate = ups
                       }

-- | Run the @findAndModify@ command as an update without an upsert and new set to @True@.
-- Return a single updated document (@new@ option is set to @True@).
--
-- See 'findAndModifyOpts' for more options.
findAndModify :: (MonadIO m, MonadFail m)
              => Query
              -> Document -- ^ updates
              -> Action m (Either String Document)
findAndModify q ups = do
  eres <- findAndModifyOpts q (defFamUpdateOpts ups)
  return $ case eres of
    Left l -> Left l
    Right r -> case r of
      -- only possible when upsert is True and new is False
      Nothing  -> Left "findAndModify: impossible null result"
      Just doc -> Right doc

-- | Run the @findAndModify@ command
-- (allows more options than 'findAndModify')
findAndModifyOpts :: (MonadIO m, MonadFail m)
                  => Query
                  -> FindAndModifyOpts
                  -> Action m (Either String (Maybe Document))
findAndModifyOpts Query {
    selection = Select sel collection
  , project = project
  , sort = sort
  } famOpts = do
    result <- runCommand
       ([ "findAndModify" := String collection
        , "query"  := Doc sel
        , "fields" := Doc project
        , "sort"   := Doc sort
        ] ++
            case famOpts of
              FamRemove shouldRemove -> [ "remove" := Bool shouldRemove ]
              FamUpdate {..} ->
                [ "update" := Doc famUpdate
                , "new"    := Bool famNew    -- return updated document, not original document
                , "upsert" := Bool famUpsert -- insert if nothing is found
                ])
    return $ case lookupErr result of
        Just e -> leftErr e
        Nothing -> case lookup "value" result of
            Nothing   -> leftErr "no document found"
            Just mdoc -> case mdoc of
                Just doc@(_:_) -> Right (Just doc)
                Just [] -> case famOpts of
                    FamUpdate { famUpsert = True, famNew = False } -> Right Nothing
                    _ -> leftErr $ show result
                _  -> leftErr $ show result
  where
    leftErr err = Left $ "findAndModify " `mappend` show collection
        `mappend` "\nfrom query: " `mappend` show sel
        `mappend` "\nerror: " `mappend` err

    -- return Nothing means ok, Just is the error message
    lookupErr :: Document -> Maybe String
    lookupErr result = do
        errObject <- lookup "lastErrorObject" result
        lookup "err" errObject

explain :: (MonadIO m) => Query -> Action m Document
-- ^ Return performance stats of query execution
explain q = do  -- same as findOne but with explain set to true
    pipe <- asks mongoPipe
    qr <- queryRequest True q {limit = 1}
    r <- liftIO $ request pipe [] qr
    Batch _ _ docs <- liftDB $ fulfill r
    return $ if null docs then error ("no explain: " ++ show q) else head docs

count :: (MonadIO m) => Query -> Action m Int
-- ^ Fetch number of documents satisfying query (including effect of skip and/or limit if present)
count Query{selection = Select sel col, skip, limit} = at "n" <$> runCommand
    (["count" =: col, "query" =: sel, "skip" =: (fromIntegral skip :: Int32)]
        ++ ("limit" =? if limit == 0 then Nothing else Just (fromIntegral limit :: Int32)))

distinct :: (MonadIO m) => Label -> Selection -> Action m [Value]
-- ^ Fetch distinct values of field in selected documents
distinct k (Select sel col) = at "values" <$> runCommand ["distinct" =: col, "key" =: k, "query" =: sel]

queryRequest :: (Monad m, MonadIO m) => Bool -> Query -> Action m (Request, Maybe Limit)
-- ^ Translate Query to Protocol.Query. If first arg is true then add special $explain attribute.
queryRequest isExplain Query{..} = do
    ctx <- ask
    return $ queryRequest' (mongoReadMode ctx) (mongoDatabase ctx)
 where
    queryRequest' rm db = (P.Query{..}, remainingLimit) where
        qOptions = readModeOption rm ++ options
        qFullCollection = db <.> coll selection
        qSkip = fromIntegral skip
        (qBatchSize, remainingLimit) = batchSizeRemainingLimit batchSize (if limit == 0 then Nothing else Just limit)
        qProjector = project
        mOrder = if null sort then Nothing else Just ("$orderby" =: sort)
        mSnapshot = if snapshot then Just ("$snapshot" =: True) else Nothing
        mHint = if null hint then Nothing else Just ("$hint" =: hint)
        mExplain = if isExplain then Just ("$explain" =: True) else Nothing
        special = catMaybes [mOrder, mSnapshot, mHint, mExplain]
        qSelector = if null special then s else ("$query" =: s) : special where s = selector selection

queryRequestOpMsg :: (Monad m, MonadIO m) => Bool -> Query -> Action m (Cmd, Maybe Limit)
-- ^ Translate Query to Protocol.Query. If first arg is true then add special $explain attribute.
queryRequestOpMsg isExplain Query{..} = do
    ctx <- ask
    return $ queryRequest' (mongoReadMode ctx) (mongoDatabase ctx)
 where
    queryRequest' rm db = (Req P.Query{..}, remainingLimit) where
        qOptions = readModeOption rm ++ options
        qFullCollection = db <.> coll selection
        qSkip = fromIntegral skip
        (qBatchSize, remainingLimit) = batchSizeRemainingLimit batchSize (if limit == 0 then Nothing else Just limit)
        -- Check whether this query is not a command in disguise. If
        -- isNotCommand is true, then we treat this as a find command and add
        -- the relevant fields to the selector
        isNotCommand = null $ catMaybes $ map (\l -> look l (selector selection)) (noticeCommands ++ adminCommands)
        mOrder = if null sort then Nothing else Just ("sort" =: sort)
        mSnapshot = if snapshot then Just ("snapshot" =: True) else Nothing
        mHint = if null hint then Nothing else Just ("hint" =: hint)
        mExplain = if isExplain then Just ("$explain" =: True) else Nothing
        special = catMaybes [mOrder, mSnapshot, mHint, mExplain]
        qProjector = if isNotCommand then ["projection" =: project] else project
        qSelector = if isNotCommand then c else s
          where s = selector selection
                bSize = if qBatchSize == 0 then Nothing else Just ("batchSize" =: qBatchSize)
                mLimit = if limit == 0 then Nothing else maybe Nothing (\rL -> Just ("limit" =: (fromIntegral rL :: Int32))) remainingLimit
                c = ("filter" =: s) : special ++ maybeToList bSize ++ maybeToList mLimit

batchSizeRemainingLimit :: BatchSize -> Maybe Limit -> (Int32, Maybe Limit)
-- ^ Given batchSize and limit return P.qBatchSize and remaining limit
batchSizeRemainingLimit batchSize mLimit =
  let remaining =
        case mLimit of
          Nothing    -> batchSize
          Just limit ->
            if 0 < batchSize && batchSize < limit
              then batchSize
              else limit
  in (fromIntegral remaining, mLimit)

type DelayedBatch = IO Batch
-- ^ A promised batch which may fail

data Batch = Batch (Maybe Limit) CursorId [Document]
-- ^ CursorId = 0 means cursor is finished. Documents is remaining documents to serve in current batch. Limit is number of documents to return. Nothing means no limit.

request :: Pipe -> [Notice] -> (Request, Maybe Limit) -> IO DelayedBatch
-- ^ Send notices and request and return promised batch
request pipe ns (req, remainingLimit) = do
    promise <- liftIOE ConnectionFailure $ P.call pipe ns req
    let protectedPromise = liftIOE ConnectionFailure promise
    return $ fromReply remainingLimit =<< protectedPromise

requestOpMsg :: Pipe -> (Cmd, Maybe Limit) -> Document -> IO DelayedBatch
-- ^ Send notices and request and return promised batch
requestOpMsg pipe (Req r, remainingLimit) params = do
  promise <- liftIOE ConnectionFailure $ P.callOpMsg pipe r Nothing params
  let protectedPromise = liftIOE ConnectionFailure promise
  return $ fromReply remainingLimit =<< protectedPromise
requestOpMsg _ (Nc _, _) _ = error "requestOpMsg: Only messages of type Query are supported"

fromReply :: Maybe Limit -> Reply -> DelayedBatch
-- ^ Convert Reply to Batch or Failure
fromReply limit Reply{..} = do
    mapM_ checkResponseFlag rResponseFlags
    return (Batch limit rCursorId rDocuments)
 where
    -- If response flag indicates failure then throw it, otherwise do nothing
    checkResponseFlag flag = case flag of
        AwaitCapable -> return ()
        CursorNotFound -> throwIO $ CursorNotFoundFailure rCursorId
        QueryError -> throwIO $ QueryFailure (at "code" $ head rDocuments) (at "$err" $ head rDocuments)
fromReply limit ReplyOpMsg{..} = do
    let section = head sections
        cur = maybe Nothing cast $ look "cursor" section
    case cur of
      Nothing -> return (Batch limit 0 sections)
      Just doc ->
          case look "firstBatch" doc of
            Just ar -> do
              let docs = fromJust $ cast ar
                  id' = fromJust $ cast $ valueAt "id" doc
              return (Batch limit id' docs)
            -- A cursor without a firstBatch field, should be a reply to a
            -- getMore query and thus have a nextBatch key
            Nothing -> do
              let docs = fromJust $ cast $ valueAt "nextBatch" doc
                  id' = fromJust $ cast $ valueAt "id" doc
              return (Batch limit id' docs)

fulfill :: DelayedBatch -> Action IO Batch
-- ^ Demand and wait for result, raise failure if exception
fulfill = liftIO

-- *** Cursor

data Cursor = Cursor FullCollection BatchSize (MVar DelayedBatch)
-- ^ Iterator over results of a query. Use 'next' to iterate or 'rest' to get all results. A cursor is closed when it is explicitly closed, all results have been read from it, garbage collected, or not used for over 10 minutes (unless 'NoCursorTimeout' option was specified in 'Query'). Reading from a closed cursor raises a 'CursorNotFoundFailure'. Note, a cursor is not closed when the pipe is closed, so you can open another pipe to the same server and continue using the cursor.

newCursor :: MonadIO m => Database -> Collection -> BatchSize -> DelayedBatch -> Action m Cursor
-- ^ Create new cursor. If you don't read all results then close it. Cursor will be closed automatically when all results are read from it or when eventually garbage collected.
newCursor db col batchSize dBatch = do
    var <- liftIO $ MV.newMVar dBatch
    let cursor = Cursor (db <.> col) batchSize var
    _ <- liftDB $ mkWeakMVar var (closeCursor cursor)
    return cursor

nextBatch :: MonadIO m => Cursor -> Action m [Document]
-- ^ Return next batch of documents in query result, which will be empty if finished.
nextBatch (Cursor fcol batchSize var) = liftDB $ modifyMVar var $ \dBatch -> do
    -- Pre-fetch next batch promise from server and return current batch.
    Batch mLimit cid docs <- liftDB $ fulfill' fcol batchSize dBatch
    let newLimit = do
              limit <- mLimit
              return $ limit - min limit (fromIntegral $ length docs)
    let emptyBatch = return $ Batch (Just 0) 0 []
    let getNextBatch = nextBatch' fcol batchSize newLimit cid
    let resultDocs = maybe id (take . fromIntegral) mLimit docs
    case (cid, newLimit) of
      (0, _)      -> return (emptyBatch, resultDocs)
      (_, Just 0) -> do
        pipe <- asks mongoPipe
        liftIOE ConnectionFailure $ P.send pipe [KillCursors [cid]]
        return (emptyBatch, resultDocs)
      (_, _)      -> (, resultDocs) <$> getNextBatch

fulfill' :: FullCollection -> BatchSize -> DelayedBatch -> Action IO Batch
-- Discard pre-fetched batch if empty with nonzero cid.
fulfill' fcol batchSize dBatch = do
    b@(Batch limit cid docs) <- fulfill dBatch
    if cid /= 0 && null docs && (limit > Just 0)
        then nextBatch' fcol batchSize limit cid >>= fulfill
        else return b

nextBatch' :: (MonadIO m) => FullCollection -> BatchSize -> Maybe Limit -> CursorId -> Action m DelayedBatch
nextBatch' fcol batchSize limit cid = do
    pipe <- asks mongoPipe
    let sd = P.serverData pipe
    if maxWireVersion sd < 17
      then liftIO $ request pipe [] (GetMore fcol batchSize' cid, remLimit)
      else liftIO $ requestOpMsg pipe (Req $ GetMore fcol batchSize' cid, remLimit) []
    where (batchSize', remLimit) = batchSizeRemainingLimit batchSize limit

next :: MonadIO m => Cursor -> Action m (Maybe Document)
-- ^ Return next document in query result, or Nothing if finished.
next (Cursor fcol batchSize var) = liftDB $ modifyMVar var nextState where
    -- Pre-fetch next batch promise from server when last one in current batch is returned.
    -- nextState:: DelayedBatch -> Action m (DelayedBatch, Maybe Document)
    nextState dBatch = do
        Batch mLimit cid docs <- liftDB $ fulfill' fcol batchSize dBatch
        if mLimit == Just 0
          then return (return $ Batch (Just 0) 0 [], Nothing)
          else
            case docs of
                doc : docs' -> do
                    let newLimit = do
                              limit <- mLimit
                              return $ limit - 1
                    dBatch' <- if null docs' && cid /= 0 && ((newLimit > Just 0) || isNothing newLimit)
                        then nextBatch' fcol batchSize newLimit cid
                        else return $ return (Batch newLimit cid docs')
                    when (newLimit == Just 0) $ unless (cid == 0) $ do
                      pipe <- asks mongoPipe
                      let sd = P.serverData pipe
                      if maxWireVersion sd < 17
                        then liftIOE ConnectionFailure $ P.send pipe [KillCursors [cid]]
                        else liftIOE ConnectionFailure $ P.sendOpMsg pipe [Kc (P.KillC (KillCursors [cid]) fcol)] (Just MoreToCome) []
                    return (dBatch', Just doc)
                [] -> if cid == 0
                    then return (return $ Batch (Just 0) 0 [], Nothing)  -- finished
                    else do
                      nb <- nextBatch' fcol batchSize mLimit cid
                      return (nb, Nothing)

nextN :: MonadIO m => Int -> Cursor -> Action m [Document]
-- ^ Return next N documents or less if end is reached
nextN n c = catMaybes <$> replicateM n (next c)

rest :: MonadIO m => Cursor -> Action m [Document]
-- ^ Return remaining documents in query result
rest c = loop (next c)

closeCursor :: MonadIO m => Cursor -> Action m ()
closeCursor (Cursor _ _ var) = liftDB $ modifyMVar var $ \dBatch -> do
    Batch _ cid _ <- fulfill dBatch
    unless (cid == 0) $ do
      pipe <- asks mongoPipe
      liftIOE ConnectionFailure $ P.send pipe [KillCursors [cid]]
    return (return $ Batch (Just 0) 0 [], ())

isCursorClosed :: MonadIO m => Cursor -> Action m Bool
isCursorClosed (Cursor _ _ var) = do
        Batch _ cid docs <- liftDB $ fulfill =<< readMVar var
        return (cid == 0 && null docs)

-- ** Aggregate

type Pipeline = [Document]
-- ^ The Aggregate Pipeline

aggregate :: (MonadIO m, MonadFail m) => Collection -> Pipeline -> Action m [Document]
-- ^ Runs an aggregate and unpacks the result. See <http://docs.mongodb.org/manual/core/aggregation/> for details.
aggregate aColl agg = do
    aggregateCursor aColl agg def >>= rest

data AggregateConfig = AggregateConfig
  { allowDiskUse :: Bool -- ^ Enable writing to temporary files (aggregations have a 100Mb RAM limit)
  }
  deriving Show

instance Default AggregateConfig where
  def = AggregateConfig
    { allowDiskUse = False
    }

aggregateCommand :: Collection -> Pipeline -> AggregateConfig -> Document
aggregateCommand aColl agg AggregateConfig {..} =
  [ "aggregate" =: aColl
  , "pipeline" =: agg
  , "cursor" =: ([] :: Document)
  , "allowDiskUse" =: allowDiskUse
  ]

aggregateCursor :: (MonadIO m, MonadFail m) => Collection -> Pipeline -> AggregateConfig -> Action m Cursor
-- ^ Runs an aggregate and unpacks the result. See <http://docs.mongodb.org/manual/core/aggregation/> for details.
aggregateCursor aColl agg cfg = do
    pipe <- asks mongoPipe
    let sd = P.serverData pipe
    if maxWireVersion sd < 17
      then do
        response <- runCommand (aggregateCommand aColl agg cfg)
        getCursorFromResponse aColl response
           >>= either (liftIO . throwIO . AggregateFailure) return
      else do
        let q = select (aggregateCommand aColl agg cfg) aColl
        qr <- queryRequestOpMsg False q
        dBatch <- liftIO $ requestOpMsg pipe qr []
        db <- thisDatabase
        Right <$> newCursor db aColl 0 dBatch
           >>= either (liftIO . throwIO . AggregateFailure) return

getCursorFromResponse
  :: (MonadIO m, MonadFail m)
  => Collection
  -> Document
  -> Action m (Either String Cursor)
getCursorFromResponse aColl response
  | true1 "ok" response = do
      cursor     <- lookup "cursor" response
      firstBatch <- lookup "firstBatch" cursor
      cursorId   <- lookup "id" cursor
      db         <- thisDatabase
      Right <$> newCursor db aColl 0 (return $ Batch Nothing cursorId firstBatch)
  | otherwise = return $ Left $ at "errmsg" response

-- ** Group

-- | Groups documents in collection by key then reduces (aggregates) each group
data Group = Group {
    gColl :: Collection,
    gKey :: GroupKey,  -- ^ Fields to group by
    gReduce :: Javascript,  -- ^ @(doc, agg) -> ()@. The reduce function reduces (aggregates) the objects iterated. Typical operations of a reduce function include summing and counting. It takes two arguments, the current document being iterated over and the aggregation value, and updates the aggregate value.
    gInitial :: Document,  -- ^ @agg@. Initial aggregation value supplied to reduce
    gCond :: Selector,  -- ^ Condition that must be true for a row to be considered. @[]@ means always true.
    gFinalize :: Maybe Javascript  -- ^ @agg -> () | result@. An optional function to be run on each item in the result set just before the item is returned. Can either modify the item (e.g., add an average field given a count and a total) or return a replacement object (returning a new object with just @_id@ and average fields).
    } deriving (Show, Eq)

data GroupKey = Key [Label] | KeyF Javascript  deriving (Show, Eq)
-- ^ Fields to group by, or function (@doc -> key@) returning a "key object" to be used as the grouping key. Use 'KeyF' instead of 'Key' to specify a key that is not an existing member of the object (or, to access embedded members).

groupDocument :: Group -> Document
-- ^ Translate Group data into expected document form
groupDocument Group{..} =
    ("finalize" =? gFinalize) ++ [
    "ns" =: gColl,
    case gKey of Key k -> "key" =: map (=: True) k; KeyF f -> "$keyf" =: f,
    "$reduce" =: gReduce,
    "initial" =: gInitial,
    "cond" =: gCond ]

group :: (MonadIO m) => Group -> Action m [Document]
-- ^ Execute group query and return resulting aggregate value for each distinct key
group g = at "retval" <$> runCommand ["group" =: groupDocument g]

-- ** MapReduce

-- | Maps every document in collection to a list of (key, value) pairs, then for each unique key reduces all its associated values to a single result. There are additional parameters that may be set to tweak this basic operation.
-- This implements the latest version of map-reduce that requires MongoDB 1.7.4 or greater. To map-reduce against an older server use 'runCommand' directly as described in http://www.mongodb.org/display/DOCS/MapReduce.
data MapReduce = MapReduce {
    rColl :: Collection,
    rMap :: MapFun,
    rReduce :: ReduceFun,
    rSelect :: Selector,  -- ^ Operate on only those documents selected. Default is @[]@ meaning all documents.
    rSort :: Order,  -- ^ Default is @[]@ meaning no sort
    rLimit :: Limit,  -- ^ Default is 0 meaning no limit
    rOut :: MROut,  -- ^ Output to a collection with a certain merge policy. Default is no collection ('Inline'). Note, you don't want this default if your result set is large.
    rFinalize :: Maybe FinalizeFun,  -- ^ Function to apply to all the results when finished. Default is Nothing.
    rScope :: Document,  -- ^ Variables (environment) that can be accessed from map/reduce/finalize. Default is @[]@.
    rVerbose :: Bool  -- ^ Provide statistics on job execution time. Default is False.
    } deriving (Show, Eq)

type MapFun = Javascript
-- ^ @() -> void@. The map function references the variable @this@ to inspect the current object under consideration. The function must call @emit(key,value)@ at least once, but may be invoked any number of times, as may be appropriate.

type ReduceFun = Javascript
-- ^ @(key, [value]) -> value@. The reduce function receives a key and an array of values and returns an aggregate result value. The MapReduce engine may invoke reduce functions iteratively; thus, these functions must be idempotent.  That is, the following must hold for your reduce function: @reduce(k, [reduce(k,vs)]) == reduce(k,vs)@. If you need to perform an operation only once, use a finalize function. The output of emit (the 2nd param) and reduce should be the same format to make iterative reduce possible.

type FinalizeFun = Javascript
-- ^ @(key, value) -> final_value@. A finalize function may be run after reduction.  Such a function is optional and is not necessary for many map/reduce cases.  The finalize function takes a key and a value, and returns a finalized value.

data MROut =
      Inline -- ^ Return results directly instead of writing them to an output collection. Results must fit within 16MB limit of a single document
    | Output MRMerge Collection (Maybe Database) -- ^ Write results to given collection, in other database if specified. Follow merge policy when entry already exists
    deriving (Show, Eq)

data MRMerge =
      Replace  -- ^ Clear all old data and replace it with new data
    | Merge  -- ^ Leave old data but overwrite entries with the same key with new data
    | Reduce  -- ^ Leave old data but combine entries with the same key via MR's reduce function
    deriving (Show, Eq)

type MRResult = Document
-- ^ Result of running a MapReduce has some stats besides the output. See http://www.mongodb.org/display/DOCS/MapReduce#MapReduce-Resultobject

mrDocument :: MapReduce -> Document
-- ^ Translate MapReduce data into expected document form
mrDocument MapReduce{..} =
    ("mapreduce" =: rColl) :
    ("out" =: mrOutDoc rOut) :
    ("finalize" =? rFinalize) ++ [
    "map" =: rMap,
    "reduce" =: rReduce,
    "query" =: rSelect,
    "sort" =: rSort,
    "limit" =: (fromIntegral rLimit :: Int),
    "scope" =: rScope,
    "verbose" =: rVerbose ]

mrOutDoc :: MROut -> Document
-- ^ Translate MROut into expected document form
mrOutDoc Inline = ["inline" =: (1 :: Int)]
mrOutDoc (Output mrMerge coll mDB) = (mergeName mrMerge =: coll) : mdb mDB where
    mergeName Replace = "replace"
    mergeName Merge = "merge"
    mergeName Reduce = "reduce"
    mdb Nothing = []
    mdb (Just db) = ["db" =: db]

mapReduce :: Collection -> MapFun -> ReduceFun -> MapReduce
-- ^ MapReduce on collection with given map and reduce functions. Remaining attributes are set to their defaults, which are stated in their comments.
mapReduce col map' red = MapReduce col map' red [] [] 0 Inline Nothing [] False

runMR :: MonadIO m => MapReduce -> Action m Cursor
-- ^ Run MapReduce and return cursor of results. Error if map/reduce fails (because of bad Javascript)
runMR mr = do
    res <- runMR' mr
    case look "result" res of
        Just (String coll) -> find $ query [] coll
        Just (Doc doc) -> useDb (at "db" doc) $ find $ query [] (at "collection" doc)
        Just x -> error $ "unexpected map-reduce result field: " ++ show x
        Nothing -> newCursor "" "" 0 $ return $ Batch (Just 0) 0 (at "results" res)

runMR' :: (MonadIO m) => MapReduce -> Action m MRResult
-- ^ Run MapReduce and return a MR result document containing stats and the results if Inlined. Error if the map/reduce failed (because of bad Javascript).
runMR' mr = do
    doc <- runCommand (mrDocument mr)
    return $ if true1 "ok" doc then doc else error $ "mapReduce error:\n" ++ show doc ++ "\nin:\n" ++ show mr

-- * Command

type Command = Document
-- ^ A command is a special query or action against the database. See <http://www.mongodb.org/display/DOCS/Commands> for details.

runCommand :: (MonadIO m) => Command -> Action m Document
-- ^ Run command against the database and return its result
runCommand c = fromMaybe err <$> findOne (query c "$cmd") where
    err = error $ "Nothing returned for command: " ++ show c

runCommand1 :: (MonadIO m) => Text -> Action m Document
-- ^ @runCommand1 foo = runCommand [foo =: 1]@
runCommand1 c = runCommand [c =: (1 :: Int)]

eval :: (MonadIO m, Val v) => Javascript -> Action m v
-- ^ Run code on server
eval code = do
    p <- asks mongoPipe
    let sd = P.serverData p
    if maxWireVersion sd <= 7
      then at "retval" <$> runCommand ["$eval" =: code]
      else error "The command db.eval() has been removed since MongoDB 4.2"

modifyMVar :: MVar a -> (a -> Action IO (a, b)) -> Action IO b
modifyMVar v f = do
  ctx <- ask
  liftIO $ MV.modifyMVar v (\x -> runReaderT (f x) ctx)

mkWeakMVar :: MVar a -> Action IO () -> Action IO (Weak (MVar a))
mkWeakMVar m closing = do
  ctx <- ask

#if MIN_VERSION_base(4,6,0)
  liftIO $ MV.mkWeakMVar m $ runReaderT closing ctx
#else
  liftIO $ MV.addMVarFinalizer m $ runReaderT closing ctx
#endif


{- Authors: Tony Hannan <tony@10gen.com>
   Copyright 2011 10gen Inc.
   Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at: http://www.apache.org/licenses/LICENSE-2.0. Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. -}