使用它限制大小字节后如何忽略编解码器

Question

I would create a model for the KMIP protocols that works with TTLV encoding ( Tag, Type, Length, Value )

The ttlv function is "high level" and accepts the tag, the type and the codec of the value.

 def ttlv[A<:HList](tag:ByteVector, itype:ByteVector, value: Codec[A]) =
   constant(tag) :: constant(itype) :: 
   (uint32 >>:~ {
     case length => limitedSizeBytes(length, value)
   })

The result is a Codec[Unit :: Unit :: Long :: A] . However, I would have a Codec[Unit :: Unit :: Unit :: A] (or Codec[A] ) to cast the codec to a case class with only the value A . How to ignore the uint32 after used by limitedSizeBytes ? Otherwise I am interested for comments for better approaches.

---

Here are case class examples:

case class RequestHeader(protocol:Int)
case class RequestPayload(payload:CompromiseDate, name:CertificateName)
case class RequestMessage(header:RequestHeader, payload: RequestPayload)
case class CompromiseDate(value:Int)
case class CertificateName(value:String)

More high level functions like ttlvStructure

def ttlvStructure[A<:HList](tag:ByteVector, struct:Codec[A]) =
  ttlv(tag, hex"01", struct)
def ttlvTextString(tag:ByteVector) =
  ttlv(tag, hex"07", utf8.hlist)
def ttlvInt(tag:ByteVector) =
  ttlv(tag, hex"02", int32.hlist)

And the final codec:

implicit val certificateNameCodec =
  ttlvTextString(hex"420020").as[CertificateName]

implicit val compromiseDateCodec =
  ttlvInt(hex"420021").as[CompromiseDate]

implicit val requestPayloadCodec =
  ttlvStructure(hex"420003", Codec[CompromiseDate] :: Codec[CertificateName]).as[RequestPayload]

implicit val requestHeaderCodec =
  ttlvInt(hex"420002").as[RequestHeader]

implicit val requestMessageCodec =
  ttlvStructure(hex"420001", Codec[RequestHeader] :: Codec[RequestPayload]).as[RequestMessage]

Example of object to encode:

val m = RequestMessage(
      RequestHeader(14),
      RequestPayload(
        CompromiseDate(8),
        CertificateName("certificate name")
      )
    )

---

Solution :

variableSizeBytesLong is here to do what I want:

def ttlv[A<:HList](tag:ByteVector, itype:ByteVector, value: Codec[A]): Codec[A] =
    (constant(tag) :~>: constant(itype) :~>: variableSizeBytesLong(uint32, value))

Answer 1

Try

val defaultLength: Long = ???

def ttlv[A<:HList](tag:ByteVector, itype:ByteVector, value: Codec[A]): Codec[A] =
  constant(tag) :~>: constant(itype) :~>:
    (uint32 >>:~ (length => limitedSizeBytes(length, value))).xmap[A](_.tail, defaultLength :: _)