# `.voldoc` wire format (provisional)
> **Status: PROVISIONAL, not frozen v1.** The layout below is implemented and
> tested, but it is not a stability commitment. Version identity is carried both
> in the header and in the universe declaration string, and the universe string
> changes whenever any opcode, coder, limit semantic, adapter meaning, or hash
> semantic changes. See `PROJECT_STATE.md` and `docs/adr/` for the freeze policy.
All multi-byte integers are little-endian. Offsets are byte offsets from the
start of the file.
## File
```text
file := header record*
header := 64 bytes (fixed, see below)
record := tag:u8 flags:u8 reserved:u16=0 length:u32 payload:[u8;length] crc32c:u32
```
`crc32c` is CRC-32C (Castagnoli) over the 8-byte record header *and* the payload.
`reserved` must be zero. The final record is always `TRAILER`.
## Header (64 bytes)
| 0 | 8 | `magic` | `56 4F 4C 44 4F 43 1A 00` = ASCII `VOLDOC` + `0x1A` + `0x00` (source of truth: `MAGIC`) |
| 8 | 2 | `major` | format major version (this build: `0`) |
| 10 | 2 | `minor` | format minor version (this build: `1`) |
| 12 | 4 | `mandatory_features` | unknown bits fail closed |
| 16 | 4 | `optional_features` | may be ignored |
| 20 | 1 | `exactness_profile` | `0 = EXACT_BYTES` (only normative value) |
| 21 | 1 | `source_format` | `0 = OPAQUE` |
| 22 | 2 | `reserved_a` | must be `0` |
| 24 | 16 | `universe_id` | first 16 bytes of `SHA-256(universe_string)` |
| 40 | 8 | `declared_source_len` | exact reconstructed length |
| 48 | 12 | `reserved_b` | must be `0` |
| 60 | 4 | `header_crc32c` | CRC-32C over bytes `[0,60)` |
The magic constant is defined once in `src/container/header.rs` (`MAGIC`); this
document defers to that source of truth.
## Records
| `0x01` | `UNIVERSE` | UTF-8 universe declaration string |
| `0x02` | `FORMAT` | `source_format:u8`, `basis_len:u32`, `basis:[u8]` |
| `0x10` | `OBJECT` | raw object bytes |
| `0x20` | `GRAPH` | encoded reconstruction program |
| `0x30` | `MODEL` | canonical dense entropy model (see [Entropy records](#entropy-records-phase-2)) |
| `0x40` | `ENTROPY_CHANNEL` | typed channel capsule: 33-byte header + renorm payload |
| `0x50` | `RESIDUAL` | reserved (later) |
| `0x60` | `CHECKPOINT` | reserved (later) |
| `0x70` | `INDEX` | reserved (later) |
| `0x80` | `EXTERNAL_REF` | reserved (Phase 9+) |
| `0xF0` | `INTEGRITY` | `sha256:[u8;32]`, `source_len:u64` |
| `0xFF` | `TRAILER` | `record_count:u32`, `payload_bytes:u64`, `magic:[u8;8]` |
Rule: **unknown record whose `flags` lacks `FLAG_OPTIONAL` (`0x01`) fails closed**
with `UnsupportedFeature`. Unknown explicitly-optional records are skipped.
Requirements enforced by `Descriptor::parse`:
- exactly one `UNIVERSE`, `FORMAT`, `GRAPH`, `INTEGRITY`, `TRAILER`;
- `SHA-256(universe)[..16] == header.universe_id`;
- `FORMAT.source_format == header.source_format`;
- `INTEGRITY.source_len == header.declared_source_len`;
- `TRAILER.record_count` equals the number of records actually read;
- no record after `TRAILER`.
## Graph (reconstruction program)
```text
graph := version:u8=2 op_count:u32 op*
op := EMIT_OBJECT(0x01) u32_object_id
| INLINE(0x02) u32_len [u8;len]
| REPEAT_LAST(0x03) u32_count
| DECODE_CHANNEL(0x04) u32_channel_id
```
Semantics:
- `EMIT_OBJECT` appends the referenced object's bytes (authority: **Literal**).
- `INLINE` appends inline bytes (authority: **Literal**).
- `REPEAT_LAST` repeats the bytes produced by the *immediately preceding literal
instruction* `count` more times (authority: **Generated**). Consecutive
`REPEAT_LAST` and a leading `REPEAT_LAST` are invalid.
- `DECODE_CHANNEL` appends the exact decoded bytes of the referenced entropy
channel (authority: **EntropyChannel**). The channel's decoded length is known
statically from its descriptor, so the op's output length is still bounded at
parse time.
## Coverage certificate (checked invariant, not stored bytes)
The coverage map is **derived** deterministically from the program and object
lengths during `parse`, and validated before any byte materialization:
```text
union(spans) == [0, declared_source_len)
spans are contiguous (no gaps, no overlapping authorities)
```
A descriptor whose predicted length disagrees with `declared_source_len`, or
whose spans are not contiguous, is rejected with `CoverageViolation` **before**
allocation. This catches the dangerous "the parser forgot a source distinction"
class of bugs at the representation boundary.
## Universe declaration
The Phase-2 universe string is:
```text
vole-document;universe;phase-2;exact-bytes;dra-2;opaque+entropy
```
The header's `universe_id` is the first 16 bytes of `SHA-256` over this string.
Any change to an opcode, coder, limit semantic, adapter meaning, or hash semantic
requires a new universe string.
## Entropy records (Phase 2)
Phase 2 introduces exactly two entropy records and one graph op. They are
implemented and measured but remain **PROVISIONAL** (the wire format is not
frozen v1).
### `MODEL` (`0x30`)
A canonical, self-describing frequency table over the fixed 256-symbol byte
alphabet. The payload is the dense canonical model, exactly **516 bytes**:
```text
model := version:u8=1 scale_bits:u8 count:u16=256 freq:[u16;256]
```
- `freq` entries are little-endian and **must sum to exactly `1 << scale_bits`**.
- `scale_bits` is in `1..=15` (frequencies are stored as `u16`).
- Normalization from observed counts is integer-only, deterministic, and
tie-broken by lower symbol index; symbols seen zero times get frequency zero.
- A channel's `scale_bits` must equal the `scale_bits` of the model it names, and
a channel may not name a missing model; both are checked during `parse`.
### `ENTROPY_CHANNEL` (`0x40`)
One typed channel capsule. The payload is a fixed **33-byte header** followed by
the renormalization payload; the total record payload length must equal
`33 + payload_len` exactly (no trailing bytes, no truncation).
| 0 | 1 | `coder` | `1 = CODER_ORDER0_BYTE_RANS` |
| 1 | 2 | `coder_version` | `1` |
| 3 | 1 | `scale_bits` | must match the referenced model |
| 4 | 1 | `lane_count` | `1` (single-lane only; else `UnsupportedFeature`) |
| 5 | 4 | `model_id` | index into the descriptor's `MODEL` table |
| 9 | 8 | `symbol_count` | number of symbols encoded |
| 17 | 8 | `decoded_length` | exact decoded bytes |
| 25 | 4 | `initial_state` | scalar decoder entry state |
| 29 | 4 | `payload_len` | length of the following payload |
| 33 | `payload_len` | `payload` | renormalization bytes in forward decoder-consumption order |
A channel is never a bare seed: the model, decoder state, payload, and counts
are all required to reconstruct bytes (see [`docs/adr/0006-rans-substrate.md`](docs/adr/0006-rans-substrate.md)).
## Feature policy
- `default = ["rans"]`: the native scalar entropy decoder (`ryg-rans-rs`
`=0.5.1`, **safe manual** API only) is present by default.
- Built with `--no-default-features`, the exact RAW/RLE floor still compiles and
materializes channel-free descriptors byte-for-byte.
- A descriptor that declares `MODEL`/`ENTROPY_CHANNEL` records but is decoded
without the `rans` feature returns an explicit `UnsupportedFeature`
(`ErrorClass` exit code 6). It is never silently reinterpreted and never
partially materialized.
## Canonical descriptor encoding
The *source document* is never canonicalized. The *descriptor* is canonical: a
given `Descriptor` always serializes to the same bytes, and
`serialize(parse(x)) == x` holds for every descriptor this implementation
produces (verified by the conformance court). This makes receipts and hashes
stable across runs and implementations.
## Integrity levels
| framing | per-record CRC-32C; header CRC-32C |
| whole source | `INTEGRITY` SHA-256 of the reconstructed bytes |
A deep verify (`vole-document verify`) materializes the source and checks the
whole-source digest. During development, `byte_compare` is the court authority.