如何在使用Codec限制大小字节后忽略它

Question

我将为使用TTLV编码(标签，类型，长度，值)的KMIP协议创建一个模型。

ttlv函数是“高级”函数，接受该值的标记、类型和编解码器。

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

结果是一个Codec[Unit :: Unit :: Long :: A]。但是，我需要一个Codec[Unit :: Unit :: Unit :: A] (或Codec[A])将编解码器转换为只有值A的case class。如何在被uint32使用后忽略limitedSizeBytes？否则，我有兴趣就更好的办法发表意见。

下面是case class示例：

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)

更高级别的函数，如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)

最后的编解码器：

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]

要编码的对象示例：

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

溶液

variableSizeBytesLong来这里做我想做的事：

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

Answer 1

试一试

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 :: _)