-- | This module provides type classes for encoding and decoding protocol
-- buffers message, as well as a safer alternative to the raw 'Proto3.Wire'
-- library based on 'GHC.Generics'.
--
-- = Classes
--
-- The 'Primitive' class captures those types which correspond to primitive field
-- types, as defined by the protocol buffers specification. A 'Primitive' type is
-- one which can always be encoded as a single key/value pair in the wire format.
--
-- The 'MessageField' class captures those types which are encoded under a single
-- key in the wire format, i.e. primitives, packed and unpacked lists, and
-- embedded messages.
--
-- The 'Message' class captures types which correspond to protocol buffers messages.
-- Instances of 'Message' can be written by hand for your types by using the
-- functions in the 'Proto3.Suite.Encode' and 'Proto3.Suite.Decode'
-- modules. In the case where the message format is determined by your Haskell code,
-- you might prefer to derive your 'Message' instances using generic deriving.
--
-- = Generic Instances
--
-- Using the 'GHC.Generics' approach, instead of generating Haskell code from a
-- .proto file, we write our message formats as Haskell types, and generate a
-- serializer/deserializer pair.
--
-- To use this library, simply derive a 'Generic' instance for your type(s), and
-- use the default `Message` instance.
--
-- For generic 'Message' instances, field numbers are automatically generated,
-- starting at 1. Therefore, adding new fields is a compatible change only at the
-- end of a record. Renaming fields is also safe. You should not use the generic
-- instances if you are starting from an existing .proto file.
--
-- = Strings
--
-- Use 'TL.Text' instead of 'String' for string types inside messages.
--
-- = Example
--
-- > data MultipleFields =
-- >   MultipleFields { multiFieldDouble :: Double
-- >                  , multiFieldFloat  :: Float
-- >                  , multiFieldInt32  :: Int32
-- >                  , multiFieldInt64  :: Int64
-- >                  , multiFieldString :: TL.Text
-- >                  , multiFieldBool   :: Bool
-- >                  } deriving (Show, Generic, Eq)
-- >
-- > instance Message MultipleFields
-- >
-- > serialized = toLazyByteString $ MultipleFields 1.0 1.0 1 1 "hi" True
-- >
-- > deserialized :: MultipleFields
-- > deserialized = case parse (toStrict serialized) of
-- >                  Left e -> error e
-- >                  Right msg -> msg

{-# LANGUAGE DataKinds                  #-}
{-# LANGUAGE DefaultSignatures          #-}
{-# LANGUAGE FlexibleContexts           #-}
{-# LANGUAGE FlexibleInstances          #-}
{-# LANGUAGE MagicHash                  #-}
{-# LANGUAGE PolyKinds                  #-}
{-# LANGUAGE ScopedTypeVariables        #-}
{-# LANGUAGE TypeApplications           #-}
{-# LANGUAGE TypeFamilies               #-}
{-# LANGUAGE TypeOperators              #-}
{-# LANGUAGE UndecidableInstances       #-}

module Proto3.Suite.Class
  ( Primitive(..)
  , MessageField(..)
  , Message(..)

  -- * Encoding
  , toLazyByteString

  -- * Decoding
  , HasDefault(..)
  , fromByteString
  , fromB64

  -- * Documentation
  , Named(..)
  , Finite(..)
  , message
  , Proto3.Suite.Class.enum

  -- * Generic Classes
  , GenericMessage(..)
  ) where

import           Control.Applicative
import           Control.Monad
import qualified Data.ByteString        as B
import qualified Data.ByteString.Base64 as B64
import qualified Data.ByteString.Lazy   as BL
import           Data.Coerce            (coerce)
import qualified Data.Foldable          as Foldable
import           Data.Functor           (($>))
import           Data.Int               (Int32, Int64)
import qualified Data.Map               as M
import           Data.Maybe             (fromMaybe, isNothing)
import           Data.Monoid            ((<>))
import           Data.Proxy             (Proxy (..))
import           Data.String            (IsString (..))
import qualified Data.Text              as T
import qualified Data.Text.Lazy         as TL
import qualified Data.Traversable       as TR
import qualified Data.Vector            as Vector
import           Data.Vector            (Vector)
import           Data.Word              (Word32, Word64)
import           GHC.Exts               (fromList, Proxy#, proxy#)
import           GHC.Generics
import           GHC.TypeLits
import           Proto3.Suite.DotProto  as DotProto
import           Proto3.Suite.Types     as Wire
import           Proto3.Wire
import           Proto3.Wire.Class      (ProtoEnum(..))
import           Proto3.Wire.Decode     (ParseError, Parser (..), RawField,
                                         RawMessage, RawPrimitive, runParser)
import qualified Proto3.Wire.Decode     as Decode
import qualified Proto3.Wire.Encode     as Encode

-- | A class for types with default values per the protocol buffers spec.
class HasDefault a where
  -- | The default value for this type.
  def :: a

  default def :: (Generic a, GenericHasDefault (Rep a)) => a
  def = to (genericDef @(Rep a))

  isDefault :: a -> Bool

  default isDefault :: Eq a => a -> Bool
  isDefault = (== def)

-- | Do not encode the default value
omittingDefault
  :: HasDefault a
  => (a -> Encode.MessageBuilder)
  -> a
  -> Encode.MessageBuilder
omittingDefault f p
  | isDefault p = mempty
  | otherwise = f p

-- -- | Numeric types default to zero
-- instance Num a => HasDefault a where def = 0

instance HasDefault Int where def = 0
instance HasDefault Integer where def = 0

instance HasDefault Int32 where def = 0
instance HasDefault Int64 where def = 0
instance HasDefault Word32 where def = 0
instance HasDefault Word64 where def = 0
instance HasDefault (Signed Int32) where def = 0
instance HasDefault (Signed Int64) where def = 0
-- | Used in generated records to represent @sfixed32@
instance HasDefault (Fixed Int32) where def = 0
-- | Used in generated records to represent @sfixed64@
instance HasDefault (Fixed Int64) where def = 0
instance HasDefault (Fixed Word32) where def = 0
instance HasDefault (Fixed Word64) where def = 0
instance HasDefault (Signed (Fixed Int32)) where def = 0
instance HasDefault (Signed (Fixed Int64)) where def = 0
instance HasDefault Float where def = 0
instance HasDefault Double where def = 0

instance HasDefault Bool where
  def = False

instance HasDefault T.Text where
  def = mempty

instance HasDefault TL.Text where
  def = mempty

instance HasDefault B.ByteString where
  def = mempty

instance HasDefault BL.ByteString where
  def = mempty

instance ProtoEnum e => HasDefault (Enumerated e) where
  def = Enumerated $ maybe (Left 0) Right (toProtoEnumMay 0)
  isDefault = (== 0) . either id fromProtoEnum . enumerated

instance HasDefault (UnpackedVec a) where
  def = mempty
  isDefault = null . unpackedvec

instance HasDefault (PackedVec a) where
  def = mempty
  isDefault = null . packedvec

instance HasDefault (NestedVec a) where
  def = mempty
  isDefault = null . nestedvec

instance HasDefault (Nested a) where
  def = Nested Nothing
  isDefault = isNothing . nested

instance (HasDefault a) => HasDefault (ForceEmit a) where
  def       = ForceEmit def
  isDefault = isDefault . forceEmit

-- | Used in fields of generated records to represent an unwrapped
-- 'PackedVec'/'UnpackedVec'
instance HasDefault (Vector a) where
  def       = mempty
  isDefault = null

-- | Used in generated records to represent an unwrapped 'Nested'
instance HasDefault (Maybe a) where
  def       = Nothing
  isDefault = isNothing

instance HasDefault (M.Map k v) where
  def = M.empty
  isDefault = M.null

class GenericHasDefault (f :: * -> *) where
  genericDef :: f x
instance HasDefault f => GenericHasDefault (K1 i f) where
  genericDef = K1 (def @f)
instance (GenericHasDefault f, GenericHasDefault g) => GenericHasDefault (f :*: g) where
  genericDef = genericDef @f :*: genericDef @g
instance (GenericHasDefault f, GenericHasDefault g) => GenericHasDefault (f :+: g) where
  genericDef = L1 (genericDef @f)
instance GenericHasDefault U1 where
  genericDef = U1 -- unit constructor
instance (Constructor i, GenericHasDefault f) => GenericHasDefault (C1 i f) where
  genericDef = M1 (genericDef @f)
instance (Datatype i, GenericHasDefault f) => GenericHasDefault (D1 i f) where
  genericDef = M1 (genericDef @f)
instance (Selector i, GenericHasDefault f) => GenericHasDefault (S1 i f) where
  genericDef = M1 (genericDef @f)

-- | This class captures those types whose names need to appear in .proto files.
--
-- It has a default implementation for any data type which is an instance of the
-- 'Generic' class, which will extract the name of the type constructor.
class Named a where
  -- | Get the name of a type constructor
  nameOf :: IsString string => Proxy# a -> string

  default nameOf :: (IsString string, GenericNamed (Rep a)) => Proxy# a -> string
  nameOf _ = genericNameOf (proxy# :: Proxy# (Rep a))

class GenericNamed (f :: * -> *) where
  genericNameOf :: IsString string => Proxy# f -> string

instance Datatype d => GenericNamed (M1 D d f) where
  genericNameOf _ = fromString (datatypeName (undefined :: M1 D d f ()))

-- | Enumerable types with finitely many values.
--
-- This class can be derived whenever a sum type is an instance of 'Generic',
-- and only consists of zero-argument constructors. The derived instance should
-- be compatible with `ProtoEnum` instances, in the sense that
--
-- > map (fromJust . toProtoEnumMay . snd) enumerate
--
-- should enumerate all values of the type without runtime errors.
class ProtoEnum a => Finite a where
  -- | Enumerate values of a finite type, along with names of constructors.
  enumerate :: IsString string => Proxy# a -> [(string, Int32)]

  default enumerate ::
    (IsString string, Generic a, GenericFinite (Rep a)) =>
    Proxy# a -> [(string, Int32)]
  enumerate _ =
    fmap (fromProtoEnum . (to :: Rep a p -> a)) <$> genericEnumerate

-- | Generate metadata for an enum type.
enum :: (Finite e, Named e) => Proxy# e -> DotProtoDefinition
enum pr = DotProtoEnum "" (Single $ nameOf pr) (map enumField $ enumerate pr)
  where
    enumField (name, value) = DotProtoEnumField (Single name) value []

class GenericFinite (f :: * -> *) where
  genericEnumerate :: IsString string => [(string, f p)]

instance ( GenericFinite f
         , GenericFinite g
         ) => GenericFinite (f :+: g) where
  genericEnumerate =
    (fmap L1 <$> genericEnumerate) <>
    (fmap R1 <$> genericEnumerate)

instance Constructor c => GenericFinite (M1 C c U1) where
  genericEnumerate = [ (fromString name, M1 U1) ]
    where
      name = conName (undefined :: M1 C c f ())

instance GenericFinite f => GenericFinite (M1 D t f) where
  genericEnumerate = fmap M1 <$> genericEnumerate

-- | This class captures those types which correspond to primitives in
-- the protocol buffers specification.
--
-- It should be possible to fully reconstruct values of these types from
-- a single 'RawPrimitive'. Notably, then, `Nested` is not `Primitive` even
-- though it can be 'embedded', since a nested message may by split up over
-- multiple 'embedded' fields.
class Primitive a where
  -- | Encode a primitive value
  encodePrimitive :: FieldNumber -> a -> Encode.MessageBuilder
  -- | Decode a primitive value
  decodePrimitive :: Parser RawPrimitive a
  -- | Get the type which represents this type inside another message.
  primType :: Proxy# a -> DotProtoPrimType

  default primType :: Named a => Proxy# a -> DotProtoPrimType
  primType pr = Named (Single (nameOf pr))

-- | Serialize a message as a lazy 'BL.ByteString'.
toLazyByteString :: Message a => a -> BL.ByteString
toLazyByteString = Encode.toLazyByteString . encodeMessage (fieldNumber 1)

-- | Parse any message that can be decoded.
fromByteString :: Message a => B.ByteString -> Either ParseError a
fromByteString = Decode.parse (decodeMessage (fieldNumber 1))

-- | As 'fromByteString', except the input bytestring is base64-encoded.
fromB64 :: Message a => B.ByteString -> Either ParseError a
fromB64 = fromByteString . B64.decodeLenient

instance Primitive Int32 where
  encodePrimitive = Encode.int32
  decodePrimitive = Decode.int32
  primType _ = Int32

instance Primitive Int64 where
  encodePrimitive = Encode.int64
  decodePrimitive = Decode.int64
  primType _ = Int64

instance Primitive Word32 where
  encodePrimitive = Encode.uint32
  decodePrimitive = Decode.uint32
  primType _ = UInt32

instance Primitive Word64 where
  encodePrimitive = Encode.uint64
  decodePrimitive = Decode.uint64
  primType _ = UInt64

instance Primitive (Signed Int32) where
  encodePrimitive num = Encode.sint32 num . coerce
  decodePrimitive = coerce Decode.sint32
  primType _ = SInt32

instance Primitive (Signed Int64) where
  encodePrimitive num = Encode.sint64 num . coerce
  decodePrimitive = coerce Decode.sint64
  primType _ = SInt64

instance Primitive (Fixed Word32) where
  encodePrimitive num = Encode.fixed32 num . coerce
  decodePrimitive = coerce Decode.fixed32
  primType _ = DotProto.Fixed32

instance Primitive (Fixed Word64) where
  encodePrimitive num = Encode.fixed64 num . coerce
  decodePrimitive = coerce Decode.fixed64
  primType _ = DotProto.Fixed64

instance Primitive (Signed (Fixed Int32)) where
  encodePrimitive num = Encode.sfixed32 num . coerce
  decodePrimitive = coerce Decode.sfixed32
  primType _ = SFixed32

instance Primitive (Signed (Fixed Int64)) where
  encodePrimitive num = Encode.sfixed64 num . coerce
  decodePrimitive = coerce Decode.sfixed64
  primType _ = SFixed64

instance Primitive Bool where
  encodePrimitive = Encode.bool
  decodePrimitive = Decode.bool
  primType _ = Bool

instance Primitive Float where
  encodePrimitive = Encode.float
  decodePrimitive = Decode.float
  primType _ = Float

instance Primitive Double where
  encodePrimitive = Encode.double
  decodePrimitive = Decode.double
  primType _ = Double

instance Primitive T.Text where
  encodePrimitive fn = Encode.text fn . TL.fromStrict
  decodePrimitive = fmap TL.toStrict Decode.text
  primType _ = String

instance Primitive TL.Text where
  encodePrimitive = Encode.text
  decodePrimitive = Decode.text
  primType _ = String

instance Primitive B.ByteString where
  encodePrimitive = Encode.byteString
  decodePrimitive = Decode.byteString
  primType _ = Bytes

instance Primitive BL.ByteString where
  encodePrimitive = Encode.lazyByteString
  decodePrimitive = Decode.lazyByteString
  primType _ = Bytes

instance forall e. (Named e, ProtoEnum e) => Primitive (Enumerated e) where
  encodePrimitive num = either (Encode.int32 num) (Encode.enum num) . enumerated
  decodePrimitive = coerce
    @(Parser RawPrimitive (Either Int32 e))
    @(Parser RawPrimitive (Enumerated e))
    Decode.enum
  primType _ = Named (Single (nameOf (proxy# :: Proxy# e)))

instance (Primitive a) => Primitive (ForceEmit a) where
  encodePrimitive num = encodePrimitive num . forceEmit
  decodePrimitive     = coerce @(Parser RawPrimitive a) @(Parser RawPrimitive (ForceEmit a)) decodePrimitive
  primType _          = primType (proxy# :: Proxy# a)

-- | This class captures those types which can appear as message fields in
-- the protocol buffers specification, i.e. 'Primitive' types, or lists of
-- 'Primitive' types
class MessageField a where
  -- | Encode a message field
  encodeMessageField :: FieldNumber -> a -> Encode.MessageBuilder
  -- | Decode a message field
  decodeMessageField :: Parser RawField a

  default encodeMessageField :: (HasDefault a, Primitive a)
                             => FieldNumber -> a -> Encode.MessageBuilder
  encodeMessageField num x
    | isDefault x = mempty
    | otherwise = encodePrimitive num x

  default decodeMessageField :: (HasDefault a, Primitive a) => Parser RawField a
  decodeMessageField = one decodePrimitive def

  -- | Get the type which represents this type inside another message.
  protoType :: Proxy# a -> DotProtoField
  default protoType :: Primitive a => Proxy# a -> DotProtoField
  protoType p = messageField (Prim $ primType p) Nothing

messageField :: DotProtoType -> Maybe DotProto.Packing -> DotProtoField
messageField ty packing = DotProtoField
    { dotProtoFieldNumber = fieldNumber 1
    , dotProtoFieldType = ty
    , dotProtoFieldName = Anonymous
    , dotProtoFieldOptions = packingOption
    , dotProtoFieldComment = ""
    }
  where
    packingOption = maybe [] (toDotProtoOption . isPacked) packing

    toDotProtoOption b = [DotProtoOption (Single "packed") (BoolLit b)]

    isPacked DotProto.PackedField   = True
    isPacked DotProto.UnpackedField = False

instance MessageField Int32
instance MessageField Int64
instance MessageField Word32
instance MessageField Word64
instance MessageField (Signed Int32)
instance MessageField (Signed Int64)
instance MessageField (Fixed Word32)
instance MessageField (Fixed Word64)
instance MessageField (Signed (Fixed Int32))
instance MessageField (Signed (Fixed Int64))
instance MessageField Bool
instance MessageField Float
instance MessageField Double
instance MessageField T.Text
instance MessageField TL.Text
instance MessageField B.ByteString
instance MessageField BL.ByteString
instance (Named e, ProtoEnum e) => MessageField (Enumerated e)

instance (Ord k, Primitive k, MessageField k, Primitive v, MessageField v) => MessageField (M.Map k v) where
  encodeMessageField num = foldMap (Encode.embedded num . encodeMessage (fieldNumber 1)) . M.toList

  -- Data.Map.fromList will retain the last key/value mapping. From the spec:
  --
  -- > When parsing from the wire or when merging, if there are duplicate map
  -- > keys the last key seen is used.
  decodeMessageField = M.fromList . Foldable.toList
                       <$> repeated (Decode.embedded' (decodeMessage (fieldNumber 1)))
  protoType _ = messageField (Map (primType (proxy# :: Proxy# k)) (primType (proxy# :: Proxy# v))) Nothing

instance {-# OVERLAPS #-} (Ord k, Primitive k, Named v, Message v, MessageField k) => MessageField (M.Map k (Nested v)) where
  encodeMessageField num = foldMap (Encode.embedded num . encodeMessage (fieldNumber 1)) . M.toList

  -- Data.Map.fromList will retain the last key/value mapping. From the spec:
  --
  -- > When parsing from the wire or when merging, if there are duplicate map
  -- > keys the last key seen is used.
  decodeMessageField = M.fromList . Foldable.toList
                       <$> repeated (Decode.embedded' (decodeMessage (fieldNumber 1)))
  protoType _ = messageField (Map (primType (proxy# :: Proxy# k)) (Named . Single $ nameOf (proxy# :: Proxy# v))) Nothing

instance (HasDefault a, Primitive a) => MessageField (ForceEmit a) where
  encodeMessageField = encodePrimitive

instance (Named a, Message a) => MessageField (Nested a) where
  encodeMessageField num = foldMap (Encode.embedded num . encodeMessage (fieldNumber 1))
                           . coerce @(Nested a) @(Maybe a)
  decodeMessageField = coerce @(Parser RawField (Maybe a)) @(Parser RawField (Nested a))
                       (Decode.embedded (decodeMessage (fieldNumber 1)))
  protoType _ = messageField (Prim . Named . Single $ nameOf (proxy# :: Proxy# a)) Nothing

instance Primitive a => MessageField (UnpackedVec a) where
  encodeMessageField fn = Encode.vectorMessageBuilder (encodePrimitive fn) . unpackedvec
  decodeMessageField =
    UnpackedVec . fromList . Foldable.toList <$> repeated decodePrimitive
  protoType _ = messageField (Repeated $ primType (proxy# :: Proxy# a)) (Just DotProto.UnpackedField)

instance forall a. (Named a, Message a) => MessageField (NestedVec a) where
  encodeMessageField fn = Encode.vectorMessageBuilder (Encode.embedded fn . encodeMessage (fieldNumber 1)) . nestedvec
  decodeMessageField =
      fmap (coerce @(Vector a) @(NestedVec a) . fromList . Foldable.toList)
           (repeated (Decode.embedded' oneMsg))
    where
      oneMsg :: Parser RawMessage a
      oneMsg = decodeMessage (fieldNumber 1)
  protoType _ = messageField (NestedRepeated . Named . Single $ nameOf (proxy# :: Proxy# a)) Nothing

instance (Named e, ProtoEnum e) => MessageField (PackedVec (Enumerated e)) where
  encodeMessageField fn = omittingDefault (Encode.packedVarintsV fromIntegral fn) . Vector.mapMaybe omit . packedvec
    where
      -- omit values which are outside the enum range
      omit :: Enumerated e -> Maybe Int32
      omit (Enumerated (Right e)) = Just (fromProtoEnum e)
      omit _                      = Nothing
  decodeMessageField = decodePacked (foldMap retain <$> Decode.packedVarints @Word64)
    where
      -- retain only those values which are inside the enum range
      retain = foldMap (pure . Enumerated . Right) . toProtoEnumMay . fromIntegral
  protoType _ = messageField (Repeated . Named . Single $ nameOf (proxy# :: Proxy# e)) (Just DotProto.PackedField)

instance MessageField (PackedVec Bool) where
  encodeMessageField fn = omittingDefault (Encode.packedBoolsV id fn) . packedvec
  decodeMessageField = fmap (fmap toBool) (decodePacked Decode.packedVarints)
    where
      toBool :: Word64 -> Bool
      toBool 1 = True
      toBool _ = False
  protoType _ = messageField (Repeated Bool) (Just DotProto.PackedField)

instance MessageField (PackedVec Word32) where
  encodeMessageField fn = omittingDefault (Encode.packedVarintsV fromIntegral fn) . packedvec
  decodeMessageField = decodePacked Decode.packedVarints
  protoType _ = messageField (Repeated UInt32) (Just DotProto.PackedField)

instance MessageField (PackedVec Word64) where
  encodeMessageField fn = omittingDefault (Encode.packedVarintsV id fn) . packedvec
  decodeMessageField = decodePacked Decode.packedVarints
  protoType _ = messageField (Repeated UInt64) (Just DotProto.PackedField)

instance MessageField (PackedVec Int32) where
  encodeMessageField fn = omittingDefault (Encode.packedVarintsV fromIntegral fn) . packedvec
  decodeMessageField = decodePacked Decode.packedVarints
  protoType _ = messageField (Repeated Int32) (Just DotProto.PackedField)

instance MessageField (PackedVec Int64) where
  encodeMessageField fn = omittingDefault (Encode.packedVarintsV fromIntegral fn) . packedvec
  decodeMessageField = decodePacked Decode.packedVarints
  protoType _ = messageField (Repeated Int64) (Just DotProto.PackedField)

instance MessageField (PackedVec (Signed Int32)) where
  encodeMessageField fn = omittingDefault (Encode.packedVarintsV zigZag fn) . coerce @_ @(Vector Int32)
    where
      zigZag = fromIntegral . Encode.zigZagEncode

  decodeMessageField = decodePacked (fmap (fmap zagZig) Decode.packedVarints)
    where
      -- This type signature is important: `Decode.zigZagDecode` will not undo
      -- `Encode.zigZagEncode` if given a signed value with the high order bit
      -- set. So we don't allow GHC to infer a signed input type.
      zagZig :: Word32 -> Signed Int32
      zagZig = Signed . fromIntegral . Decode.zigZagDecode

  protoType _ = messageField (Repeated SInt32) (Just DotProto.PackedField)

instance MessageField (PackedVec (Signed Int64)) where
  encodeMessageField fn = omittingDefault (Encode.packedVarintsV zigZag fn) . coerce @_ @(Vector Int64)
    where
      zigZag = fromIntegral . Encode.zigZagEncode

  decodeMessageField = decodePacked (fmap (fmap zagZig) Decode.packedVarints)
    where
      -- This type signature is important: `Decode.zigZagDecode` will not undo
      -- `Encode.zigZagEncode` if given a signed value with the high order bit
      -- set. So we don't allow GHC to infer a signed input type.
      zagZig :: Word64 -> Signed Int64
      zagZig = Signed . fromIntegral . Decode.zigZagDecode

  protoType _ = messageField (Repeated SInt64) (Just DotProto.PackedField)


instance MessageField (PackedVec (Fixed Word32)) where
  encodeMessageField fn = omittingDefault (Encode.packedFixed32V id fn) . coerce @_ @(Vector Word32)
  decodeMessageField = coerce @(Parser RawField (PackedVec Word32))
                              @(Parser RawField (PackedVec (Fixed Word32)))
                              (decodePacked Decode.packedFixed32)
  protoType _ = messageField (Repeated DotProto.Fixed32) (Just DotProto.PackedField)

instance MessageField (PackedVec (Fixed Word64)) where
  encodeMessageField fn = omittingDefault (Encode.packedFixed64V id fn) . coerce @_ @(Vector Word64)
  decodeMessageField = coerce @(Parser RawField (PackedVec Word64))
                              @(Parser RawField (PackedVec (Fixed Word64)))
                              (decodePacked Decode.packedFixed64)
  protoType _ = messageField (Repeated DotProto.Fixed64) (Just DotProto.PackedField)

instance MessageField (PackedVec (Signed (Fixed Int32))) where
  encodeMessageField fn = omittingDefault (Encode.packedFixed32V fromIntegral fn) . coerce @_ @(Vector Int32)
  decodeMessageField = coerce @(Parser RawField (PackedVec Int32))
                              @(Parser RawField (PackedVec (Signed (Fixed Int32))))
                             (decodePacked Decode.packedFixed32)
  protoType _ = messageField (Repeated SFixed32) (Just DotProto.PackedField)

instance MessageField (PackedVec (Signed (Fixed Int64))) where
  encodeMessageField fn = omittingDefault (Encode.packedFixed64V fromIntegral fn) . coerce @_ @(Vector Int64)
  decodeMessageField = coerce @(Parser RawField (PackedVec Int64))
                              @(Parser RawField (PackedVec (Signed (Fixed Int64))))
                              (decodePacked Decode.packedFixed64)
  protoType _ = messageField (Repeated SFixed64) (Just DotProto.PackedField)

instance MessageField (PackedVec Float) where
  encodeMessageField fn = omittingDefault (Encode.packedFloatsV id fn) . packedvec
  decodeMessageField = decodePacked Decode.packedFloats
  protoType _ = messageField (Repeated Float) (Just DotProto.PackedField)

instance MessageField (PackedVec Double) where
  encodeMessageField fn = omittingDefault (Encode.packedDoublesV id fn) . packedvec
  decodeMessageField = decodePacked Decode.packedDoubles
  protoType _ = messageField (Repeated Double) (Just DotProto.PackedField)

instance (MessageField e, KnownSymbol comments) => MessageField (e // comments) where
  encodeMessageField fn = encodeMessageField fn . unCommented
  decodeMessageField = coerce @(Parser RawField e)
                              @(Parser RawField (Commented comments e))
                              decodeMessageField
  protoType p = (protoType (lowerProxy1 p))
                  { dotProtoFieldComment = symbolVal (lowerProxy2 p) }
    where
      lowerProxy1 :: forall k f (a :: k). Proxy# (f a) -> Proxy# a
      lowerProxy1 _ = proxy#

      lowerProxy2 :: forall k f (a :: k) b. Proxy# (f a b) -> Proxy a
      lowerProxy2 _ = Proxy

decodePacked
  :: Parser RawPrimitive [a]
  -> Parser RawField (PackedVec a)
decodePacked = Parser
             . fmap (fmap (pack . Foldable.toList))
             . TR.traverse
             . runParser
  where
    pack :: forall a. [[a]] -> PackedVec a
    pack = fromList . join . reverse


-- | This class captures those types which correspond to protocol buffer messages.
class Message a where
  -- | Encode a message
  encodeMessage :: FieldNumber -> a -> Encode.MessageBuilder
  -- | Decode a message
  decodeMessage :: FieldNumber -> Parser RawMessage a
  -- | Generate a .proto message from the type information.
  dotProto :: Proxy# a -> [DotProtoField]

  default encodeMessage :: (Generic a, GenericMessage (Rep a))
                        => FieldNumber -> a -> Encode.MessageBuilder
  encodeMessage num = genericEncodeMessage num . from

  default decodeMessage :: (Generic a, GenericMessage (Rep a))
                        => FieldNumber -> Parser RawMessage a
  decodeMessage = fmap to . genericDecodeMessage

  default dotProto :: GenericMessage (Rep a)
                   => Proxy# a -> [DotProtoField]
  dotProto _ = genericDotProto (proxy# :: Proxy# (Rep a))

instance (MessageField k, MessageField v) => Message (k, v)

-- | Generate metadata for a message type.
message :: (Message a, Named a) => Proxy# a -> DotProtoDefinition
message proxy = DotProtoMessage ""
                                (Single $ nameOf proxy)
                                (DotProtoMessageField <$> dotProto proxy)

-- * Generic Instances

class GenericMessage (f :: * -> *) where
  type GenericFieldCount f :: Nat

  genericEncodeMessage :: FieldNumber -> f a -> Encode.MessageBuilder
  genericDecodeMessage :: FieldNumber -> Parser RawMessage (f a)
  genericDotProto      :: Proxy# f -> [DotProtoField]

instance GenericMessage U1 where
  type GenericFieldCount U1 = 0
  genericEncodeMessage _ = mempty
  genericDecodeMessage _ = pure U1
  genericDotProto _      = mempty

instance (KnownNat (GenericFieldCount f), GenericMessage f, GenericMessage g)
           => GenericMessage (f :*: g)
  where
    type GenericFieldCount (f :*: g) = GenericFieldCount f + GenericFieldCount g
    genericEncodeMessage num (x :*: y) =
        genericEncodeMessage num x <>
        genericEncodeMessage (FieldNumber (getFieldNumber num + offset)) y
      where
        offset = fromIntegral $ natVal (Proxy @(GenericFieldCount f))

    genericDecodeMessage num =
        liftA2 (:*:) (genericDecodeMessage num)
                     (genericDecodeMessage num2)
      where
        num2 = FieldNumber $ getFieldNumber num + offset
        offset = fromIntegral $ natVal (Proxy @(GenericFieldCount f))

    genericDotProto _ =
        genericDotProto (proxy# :: Proxy# f) <>
        adjust (genericDotProto (proxy# :: Proxy# g))
      where
        offset = fromIntegral $ natVal (Proxy @(GenericFieldCount f))
        adjust = map adjustPart
        adjustPart part = part
          { dotProtoFieldNumber = FieldNumber . (offset +)
                                  . getFieldNumber . dotProtoFieldNumber
                                  $ part
          }

instance MessageField c => GenericMessage (K1 i c) where
  type GenericFieldCount (K1 i c) = 1
  genericEncodeMessage num (K1 x) = encodeMessageField num x
  genericDecodeMessage num        = K1 <$> at decodeMessageField num
  genericDotProto _               = [protoType (proxy# :: Proxy# c)]

instance (Selector s, GenericMessage f) => GenericMessage (M1 S s f) where
  type GenericFieldCount (M1 S s f) = GenericFieldCount f
  genericEncodeMessage num (M1 x)   = genericEncodeMessage num x
  genericDecodeMessage num          = M1 <$> genericDecodeMessage num
  genericDotProto _                 = map applyName $ genericDotProto (proxy# :: Proxy# f)
    where
      applyName :: DotProtoField -> DotProtoField
      applyName mp = mp { dotProtoFieldName = fromMaybe Anonymous newName}
      -- [issue] this probably doesn't match the intended name generating semantics

      newName :: Maybe DotProtoIdentifier
      newName = guard (not (null name)) $> Single name
        where
          name = selName (undefined :: S1 s f ())

instance GenericMessage f => GenericMessage (M1 C t f) where
  type GenericFieldCount (M1 C t f) = GenericFieldCount f
  genericEncodeMessage num (M1 x)   = genericEncodeMessage num x
  genericDecodeMessage num          = M1 <$> genericDecodeMessage num
  genericDotProto _                 = genericDotProto (proxy# :: Proxy# f)

instance GenericMessage f => GenericMessage (M1 D t f) where
  type GenericFieldCount (M1 D t f) = GenericFieldCount f
  genericEncodeMessage num (M1 x)   = genericEncodeMessage num x
  genericDecodeMessage num          = M1 <$> genericDecodeMessage num
  genericDotProto _                 = genericDotProto (proxy# :: Proxy# f)