Codec to extract value w/o consuming source BitVector

PeekCodec.markdown

The scodec discriminated.by(tag).typecase(n,message) does not suffice to match all message structures:

1. tag is at start of structure
2. value of tag is not required by the message codec.

The PeekCodec uses the target codec to extract a value, but returns the original BitVector so the tag value can be re-decoded by a subsequent codec.

PeekCodec.scala

private class PeekCodec[T](target: Codec[T]) extends Codec[T] {

    def sizeBound = target.sizeBound

    def encode(value: T) = target.encode(value)

    def decode(buffer: BitVector) =
      target.decode(buffer) match {
        case Attempt.Successful(DecodeResult(value, rest)) => Attempt.successful(DecodeResult(value, buffer))
        case f: Attempt.Failure                            => f
      }
  }

def peek[T](target: Codec[T]) = new PeekCodec(target)