{- This file is part of Vervis.
 -
 - Written in 2019 by fr33domlover <fr33domlover@riseup.net>.
 -
 - ♡ Copying is an act of love. Please copy, reuse and share.
 -
 - The author(s) have dedicated all copyright and related and neighboring
 - rights to this software to the public domain worldwide. This software is
 - distributed without any warranty.
 -
 - You should have received a copy of the CC0 Public Domain Dedication along
 - with this software. If not, see
 - <http://creativecommons.org/publicdomain/zero/1.0/>.
 -}

module Web.ActivityPub
    ( -- * Actor
      --
      -- ActivityPub actor document including a public key, with a 'FromJSON'
      -- instance for fetching and a 'ToJSON' instance for publishing.
      ActorType (..)
    , Algorithm (..)
    , PublicKey (..)
    , Actor (..)

      -- * Activity
      --
      -- Very basic activity document which is just general JSON with some
      -- basic checks. 'FromJSON' instance for receiving POSTs, and 'ToJSON'
      -- instance for delivering to other servers.
    , Activity (..)

      -- * Utilities
    , provideAP
    , APGetError (..)
    , httpGetAP
    , httpPostAP
    )
where

import Prelude

import Control.Applicative ((<|>))
import Control.Exception (Exception, try)
import Control.Monad.IO.Class
import Control.Monad.Trans.Writer (Writer)
import Data.Aeson
import Data.Aeson.Types (Parser)
import Data.ByteString (ByteString)
import Data.List.NonEmpty (NonEmpty)
import Data.PEM
import Data.Semigroup (Endo)
import Data.Text (Text)
import Data.Text.Encoding (encodeUtf8, decodeUtf8)
import Network.HTTP.Client
import Network.HTTP.Client.Conduit.ActivityPub (httpAPEither)
import Network.HTTP.Client.Signature (signRequest)
import Network.HTTP.Signature (KeyId, Signature)
import Network.HTTP.Simple (JSONException)
import Network.HTTP.Types.Header (HeaderName, hContentType)
import Network.URI
import Yesod.Core.Content (ContentType)
import Yesod.Core.Handler (ProvidedRep, provideRepType)

import qualified Data.HashMap.Strict as M (lookup)
import qualified Data.Text as T (unpack)
import qualified Data.Vector as V (fromList)

frg :: Text
frg = "https://forgefed.angeley.es/ns#"

as2context :: Text
as2context = "https://www.w3.org/ns/activitystreams"

actorContext :: Value
actorContext = Array $ V.fromList
    [ String as2context
    , String "https://w3id.org/security/v1"
    ]

parseURI' :: Text -> Parser URI
parseURI' t =
    case parseURI $ T.unpack t of
        Nothing -> fail "Invalid absolute URI"
        Just u ->
            if uriScheme u == "https:"
                then return u
                else fail "URI scheme isn't https"

renderURI :: URI -> String
renderURI u = uriToString id u ""

data ActorType = ActorTypePerson | ActorTypeOther Text

instance FromJSON ActorType where
    parseJSON = withText "ActorType" $ \ t ->
        pure $ case t of
            "Person" -> ActorTypePerson
            _        -> ActorTypeOther t

instance ToJSON ActorType where
    toJSON = error "toJSON ActorType"
    toEncoding at =
        toEncoding $ case at of
            ActorTypePerson  -> "Person"
            ActorTypeOther t -> t

data Algorithm = AlgorithmEd25519 | AlgorithmOther Text

instance FromJSON Algorithm where
    parseJSON = withText "Algorithm" $ \ t ->
        pure $ if t == frg <> "ed25519"
            then AlgorithmEd25519
            else AlgorithmOther t

instance ToJSON Algorithm where
    toJSON = error "toJSON Algorithm"
    toEncoding algo =
        toEncoding $ case algo of
            AlgorithmEd25519 -> frg <> "ed25519"
            AlgorithmOther t -> t

data PublicKey = PublicKey
    { publicKeyId    :: URI
    , publicKeyOwner :: URI
    , publicKeyPem   :: PEM
    , publicKeyAlgo  :: Maybe Algorithm
    }

instance FromJSON PublicKey where
    parseJSON = withObject "PublicKey" $ \ o ->
        PublicKey
            <$> (parseURI' =<< o .: "id")
            <*> (parseURI' =<< o .: "owner")
            <*> (parsePEM =<< o .: "publicKeyPem")
            <*> o .:? (frg <> "algorithm")
        where
        parsePEM t =
            case pemParseBS $ encodeUtf8 t of
                Left e   -> fail $ "PEM parsing failed: " ++ e
                Right xs ->
                    case xs of
                        []  -> fail "Empty PEM"
                        [x] -> pure x
                        _   -> fail "Multiple PEM sections"

instance ToJSON PublicKey where
    toJSON = error "toJSON PublicKey"
    toEncoding (PublicKey id_ owner pem malgo) =
        pairs
            $  "id"           .= renderURI id_
            <> "owner"        .= renderURI owner
            <> "publicKeyPem" .= decodeUtf8 (pemWriteBS pem)
            <> maybe mempty ((frg <> "algorithm") .=) malgo

data Actor = Actor
    { actorId        :: URI
    , actorType      :: ActorType
    , actorUsername  :: Text
    , actorInbox     :: URI
    , actorPublicKey :: PublicKey
    }

instance FromJSON Actor where
    parseJSON = withObject "Actor" $ \ o ->
        Actor
            <$> (parseURI' =<< o .: "id")
            <*> o .: "type"
            <*> o .: "preferredUsername"
            <*> (parseURI' =<< o .: "inbox")
            <*> o .: "publicKey"

instance ToJSON Actor where
    toJSON = error "toJSON Actor"
    toEncoding (Actor id_ typ username inbox pkey) =
        pairs
            $  "@context"          .= actorContext
            <> "id"                .= renderURI id_
            <> "type"              .= typ
            <> "preferredUsername" .= username
            <> "inbox"             .= renderURI inbox
            <> "publicKey"         .= pkey

-- | This may seem trivial, but it exists for a good reason: In the 'FromJSON'
-- instance we perform sanity checks. We just don't need to remember the fields
-- after checking, so we don't unnecessarily add them as fields. We just keep
-- the _to_ field, which tells us who the target actor is (we currently support
-- only the _to_ field, and it has to be a single URI, and that URI has to be
-- an actor, not a collection). The 'Object' we keep is simply for encoding
-- back to JSON. I suppose that's actually silly, we could just keep the actual
-- ByteString, but I guess it's okay for now, and it happens to guarantee the
-- JSON we POST has no extra whitespace.
data Activity = Activity
    { activityTo   :: URI
    , activityJSON :: Object
    }

instance FromJSON Activity where
    parseJSON = withObject "Activity" $ \ o -> do
        c <- o .: "@context"
        if c == as2context
            then return ()
            else fail "@context isn't the AS2 context URI"
        case M.lookup "id" o of
            Nothing -> return ()
            Just _  -> fail "id is provided; let the server set it"
        case M.lookup "type" o of
            Nothing         -> fail "Activity type missing"
            Just (String _) -> return ()
            Just _          -> fail "Activity type isn't a string"
        case M.lookup "actor" o of
            Nothing -> return ()
            Just _  -> fail "actor is provided; let the server set it"
        mto <- case M.lookup "object" o of
            Nothing -> return Nothing
            Just v -> case v of
                String _   -> return Nothing
                Object obj -> do
                    case M.lookup "id" obj of
                        Nothing -> return ()
                        Just _  -> fail "object's id is provided; let the server set it"
                    case M.lookup "type" obj of
                        Nothing         -> fail "Activity object type missing"
                        Just (String _) -> return ()
                        Just _          -> fail "Activity object type isn't a string"
                    case M.lookup "actor" o <|> M.lookup "attributedTo" o of
                        Nothing -> return ()
                        Just _  -> fail "attribution is provided; let the server set it"
                    obj .:? "to"
                _          -> fail "Activity object isn't JSON string or object"
        mto2 <- o .:? "to"
        to <- case mto <|> mto2 of
            Nothing -> fail "to not provided"
            Just t  -> parseURI' t
        return $ Activity to o

instance ToJSON Activity where
    toJSON = error "toJSON Activity"
    toEncoding = toEncoding . activityJSON

typeActivityStreams2 :: ContentType
typeActivityStreams2 = "application/activity+json"

typeActivityStreams2LD :: ContentType
typeActivityStreams2LD =
    "application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\""

provideAP :: (Monad m, ToJSON a) => a -> Writer (Endo [ProvidedRep m]) ()
provideAP v = do
    let enc = toEncoding v
    -- provideRepType typeActivityStreams2   $ return enc
    provideRepType typeActivityStreams2LD $ return enc

data APGetError
    = APGetErrorHTTP HttpException
    | APGetErrorJSON JSONException
    | APGetErrorContentType Text
    deriving Show

instance Exception APGetError

-- | Perform an HTTP GET request to fetch an ActivityPub object.
--
-- * Verify the URI scheme is _https:_ and authority part is present
-- * Set _Accept_ request header
-- * Perform the GET request
-- * Verify the _Content-Type_ response header
-- * Parse the JSON response body
httpGetAP
    :: (MonadIO m, FromJSON a)
    => Manager
    -> URI
    -> m (Either APGetError (Response a))
httpGetAP manager uri =
    if uriScheme uri /= "https:"
        then return $ Left $ APGetErrorHTTP $ InvalidUrlException (show uri) "Scheme isn't https"
        else liftIO $ mkResult <$> try (httpAPEither manager =<< requestFromURI uri)
    where
    lookup' x = map snd . filter ((== x) . fst)
    mkResult (Left e)  = Left $ APGetErrorHTTP e
    mkResult (Right r) =
        case lookup' hContentType $ responseHeaders r of
            []  -> Left $ APGetErrorContentType "No Content-Type"
            [b] -> if b == typeActivityStreams2LD || b == typeActivityStreams2
                        then case responseBody r of
                            Left e  -> Left $ APGetErrorJSON e
                            Right v -> Right $ v <$ r
                        else Left $ APGetErrorContentType $ "Non-AP Content-Type: " <> decodeUtf8 b
            _   -> Left $ APGetErrorContentType "Multiple Content-Type"

-- Set method to POST, Set Content-Type, make HTTP signature, set response to throw on non-2xx
-- status

-- | Perform an HTTP POST request to submit an ActivityPub object.
--
-- * Verify the URI scheme is _https:_ and authority part is present
-- * Set _Content-Type_ request header
-- * Compute HTTP signature and add _Signature_ request header
-- * Perform the POST request
-- * Verify the response status is 2xx
httpPostAP
    :: (MonadIO m, ToJSON a)
    => Manager
    -> URI
    -> NonEmpty HeaderName
    -> (ByteString -> (KeyId, Signature))
    -> a
    -> m (Either HttpException (Response ()))
httpPostAP manager uri headers sign value =
    if uriScheme uri /= "https:"
        then return $ Left $ InvalidUrlException (show uri) "Scheme isn't https"
        else liftIO $ try $ do
            req <- requestFromURI uri
            let req' =
                    setRequestCheckStatus $
                    consHeader hContentType typeActivityStreams2LD $
                    req { method      = "POST"
                        , requestBody = RequestBodyLBS $ encode value
                        }
                sign' b =
                    let (k, s) = sign b
                    in  (Nothing, k, s)
            req'' <- signRequest headers sign' Nothing req'
            httpNoBody req'' manager
    where
    consHeader n b r = r { requestHeaders = (n, b) : requestHeaders r }