Scodec: How to create a codec for an optional byte - scodec

Question

I must create a codec for a message that has the following specification
The message length is indicated by a byte of which the least significant bit is an extension flag that, when set indicates that the following (optional) byte must be used as the most significant byte. (Hope it make sense) It can be depicted as follows:
+----------------------------------------------------------------------------------+
| LENGTH |
| |
+----------------------------------+-----+-+---------------------------------------+
| | | | |
| Len1 (LSB) | Ext | | Len2 (MSB) - Optional |
+----+----+----+----+----+----+----+-----+ +----+----+----+----+----+----+----+----+
| | | | | | | | | | | | | | | | | |
| | | | | | | | + | | | | | | | | | |
+----+----+----+----+----+----+----+--|--+ +----+----+----+----+----+----+----+----+
|
|
v
Boolean: if true then Len2 is used
else only Len1
The length of the data that will follow is determined by this field(s). I would like to use the codec along with predefined codecs and combinators.
I guess it will involve using flatZip but I am not clear on how to incorporate flatZip into an HList combinator.
Any pointers to examples or documentation will be much appreciated.

Answer 1

One way to do this is using the scodec.codecs.optional combinator, which returns a Codec[Option[A]] given a Codec[Boolean] and a Codec[A].
val structure: Codec[(Int, Option[Int])] = uint(7) ~ optional(bool, uint8)
This gives us a codec of (Int, Option[Int]) - we need to convert this to a codec of Int. To do so, we'll need to provide a conversion from Int to (Int, Option[Int]) and another conversion in the reverse direction. We know the size field is at most 2^15 - 1 (7 LSB bits and 8 MSB bits), so converting from (Int, Option[Int]) to Int is total, whereas converting in the reverse direction could possibly fail -- for example, 2^16 cannot be represented in this structure. Hence, we can use widen to do the conversion:
val size: Codec[Int] = structure.widen[Int](
{ case (lsb, msb) => lsb + msb.map(_ << 7).getOrElse(0) },
{ sz =>
val msb = sz >>> 7
if (msb > 255 || msb < 0) Attempt.failure(Err(s"invalid size $sz"))
else Attempt.successful((sz & 0x7F, if (msb > 0) Some(msb) else None))
})
Finally, we can use this size codec to encode a variable length structure via variableSizeBytes:
val str: Codec[String] = variableSizeBytes(size, ascii)
This gives us a Codec[String] which prefixes the encoded string bytes with the size in bytes, encoded according to the scheme defined above.