`zerodds-hpack`

HPACK (RFC 7541) header-compression codec for HTTP/2: variable- length integer, string literals (with/without Huffman, Appendix B), static table (61 entries, Appendix A), dynamic table with SETTINGS_HEADER_TABLE_SIZE lifecycle and all four header-field representations from §6. no_std + alloc, forbid(unsafe_code). Safety classification: STANDARD.

Spec mapping

Spec	Section
RFC 7541 (HPACK)	§2.3 (indexing tables), §4 (dynamic-table management), §5.1 (integer), §5.2 (string literals), §6.1 (indexed header), §6.2.1 (literal with indexing), §6.2.2 (literal without indexing), §6.2.3 (literal never indexed), §6.3 (dynamic-table size update), Appendix A (static table), Appendix B (Huffman)

What's inside

Encoder / Decoder — high-level codec with its own dynamic table and indexing strategy (full match → indexed; name-only → literal-with-indexing indexed name; otherwise → literal-with-indexing new name).
Table / HeaderField — combined lookup over static + dynamic (index 1..=61 = static, 62..N = dynamic), eviction per spec §4.4 (a single entry too large clears the table).
STATIC_TABLE / StaticTableEntry — the 61 Appendix A entries as a &'static constant.
encode_integer / decode_integer — variable-length integer with configurable prefix-bit position (§5.1).
encode_string / decode_string — string literal with optional Huffman compression (§5.2).
huffman::encode / huffman::decode — static Huffman code from Appendix B with EOS-padding detection.

Layer position

Layer 5 — Bridges. Substrate for:

zerodds-http2 — RFC 9113 framing + stream state machine.
zerodds-grpc-bridge — gRPC-over-HTTP/2 + gRPC-Web length-prefixed-message codec.

Quickstart

use zerodds_hpack::{Decoder, Encoder, HeaderField};

let mut encoder = Encoder::new();
let mut decoder = Decoder::new();

let headers = vec![
    HeaderField { name: ":method".into(), value: "GET".into() },
    HeaderField { name: ":scheme".into(), value: "https".into() },
    HeaderField { name: "custom-key".into(), value: "custom-value".into() },
];

let wire = encoder.encode(&headers);
let decoded = decoder.decode(&wire).expect("roundtrip");
assert_eq!(decoded, headers);

Enable Huffman compression:

use zerodds_hpack::Encoder;

let mut encoder = Encoder::new();
encoder.use_huffman = true;

Configure the dynamic-table size (e.g. when an HTTP/2 peer sends SETTINGS_HEADER_TABLE_SIZE):

use zerodds_hpack::Decoder;

let mut decoder = Decoder::with_max_size(8192);
decoder.table_mut().set_max_size(4096);

Feature flags

Feature	Default	Purpose
`std`	✅	`std::error::Error` for all error types.
`alloc`	✅ (via std)	`Vec` / `String` / `VecDeque`.

The crate is no_std-capable: default-features = false, features = ["alloc"].

Stability

1.0.0-rc.1 is the initial release materialization. The public API, wire format (RFC 7541) and error discriminants are RC1-stable; breaking changes require a major bump.

Tests

cargo test -p zerodds-hpack

49 unit tests: integer coding (8, of which 3 are RFC 7541 Appendix C.1 vectors), string coding (7, incl. Huffman roundtrip), Huffman (7), table management (14), encoder (7), decoder (8, incl. RFC 7541 Appendix C.2.1 vector + dynamic-table size update + invalid-index rejection).

License

Apache-2.0. See LICENSE.

zerodds-hpack 1.0.0-rc.4