Module data_stream 

Draft-16 data stream header encoding and decoding.

Draft-16 subgroup type-byte flag layout:

• & 0x01: extensions present on objects
• & 0x06: SUBGROUP_ID_MODE. Section 10.4.2: “The SUBGROUP_ID_MODE field (bits 1-2, mask 0x06) is a two-bit field that determines the encoding of the Subgroup ID. To extract this value, perform a bitwise AND with mask 0x06 and right-shift by 1 bit”. Mode 0 puts no Subgroup ID on the wire and it is zero, mode 1 takes it from the first object, mode 2 reads it as a field after the Group ID, and mode 3 is reserved
• & 0x08: end-of-group marker
• & 0x20: no publisher_priority (0x30+ types)

Because the mode is one field and not two independent bits, the three carriers are mutually exclusive: a Type setting both 0x02 and 0x04 names the reserved mode, not two carriers at once. This module described those bits separately and read them separately, which let a single Type answer to two carriers.

Draft-15 spells the same three carriers out of the same two bits, as a pair of table columns rather than a named field, and the two drafts end up admitting the same twenty-four Types — draft-16 by excluding the reserved mode from the ranges 0x10-0x1F and 0x30-0x3F, draft-15 by listing them.

Draft-16 datagram type-byte flag layout:

• 0x01: extensions present (byte-length-prefixed blob)
• 0x02: end-of-group
• 0x04: no object_id (object_id = 0 implied)
• 0x08: default priority (priority omitted, inherited)
• 0x20: status datagram (carries object_status instead of payload)

Draft-16 fetch objects are framed differently again: a per-object Serialization Flags varint decides which of Group ID, Subgroup ID, Object ID, Priority and Extensions are on the wire, and a field left off is taken from the object before it. See FetchObjectHeader for the flag layout and FetchObjectReader for the inheritance. A fetch object carries no Object Status at all — draft-16 Section 10.2.1.1 puts that field on subscription deliveries only.

Extension headers in draft-16 are byte-length-prefixed opaque blobs (not count-prefixed as in draft-14).

Structs

DatagramHeader

FetchHeader

FetchObjectHeader

One Object as it is framed on a draft-16 fetch stream, before any field is resolved against the Object before it.

FetchObjectLocation

One fetch object’s Location and priority, with every field the flags left off the wire filled in from the Object before it.

FetchObjectReader

Stateful resolver for the Objects on one draft-16 fetch stream.

FetchObjectWriter

Re-encodes resolved fetch Objects onto one FETCH stream.

SubgroupHeader

SubgroupObject

One object within a draft-16 subgroup stream with its Object ID already resolved from the delta encoding. See SubgroupObjectReader for stateful encode/decode.

SubgroupObjectMeta

The framing of one draft-16 subgroup object, without its payload.

SubgroupObjectReader

Stateful reader/writer for draft-16 subgroup objects. Mirrors the draft-15 semantics (delta-encoded object IDs and header-typed extension presence).

Enums

FetchEndOfRange

The two Serialization Flags values draft-16 gives a meaning of their own instead of reading as a bit field, from Section 10.4.4 Table 4.

FetchSubgroupMode

How an Object on a draft-16 fetch stream states its Subgroup ID.

PayloadPermission

Whether an Object carrying a given status is allowed a non-empty payload.