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Http2Hpack

HPACK header compression (RFC 7541). Integer encoding/decoding, static table lookup, dynamic table with FIFO eviction, and full header block encode/decode.

Limitations

No Huffman encoding or decoding. Literal strings are sent and received in their raw form. This is valid per RFC 7541 (Huffman is optional).

Procedures

Integer Codec

PROCEDURE EncodeInt(VAR b: Buf; value: CARDINAL;
                    prefixBits: CARDINAL; mask: CARDINAL);
PROCEDURE DecodeInt(firstByte: CARDINAL; prefixBits: CARDINAL;
                    v: BytesView; VAR pos: CARDINAL;
                    VAR ok: BOOLEAN): CARDINAL;

RFC 7541 Section 5.1 variable-length integer encoding. mask is OR'd into the first byte's upper bits (e.g. 128 for indexed header field representation). firstByte is already masked to the prefix bits by the caller.

Static Table

PROCEDURE StaticLookup(index: CARDINAL; VAR entry: HeaderEntry; VAR ok: BOOLEAN);
PROCEDURE StaticFind(name: ARRAY OF CHAR; nameLen: CARDINAL;
                     value: ARRAY OF CHAR; valLen: CARDINAL;
                     nameOnly: BOOLEAN): CARDINAL;

61 entries per RFC 7541 Appendix A. StaticFind returns the 1-based index of a matching entry (exact match preferred over name-only), or 0 if not found.

Dynamic Table

TYPE DynTable = RECORD
  entries:  ARRAY [0..127] OF HeaderEntry;
  head:     CARDINAL;
  count:    CARDINAL;
  byteSize: CARDINAL;
  maxSize:  CARDINAL;
END;

Ring buffer with FIFO eviction. Entry size = nameLen + valLen + 32 (RFC 7541 Section 4.1).

PROCEDURE DynInit(VAR dt: DynTable; maxSize: CARDINAL);
PROCEDURE DynInsert(VAR dt: DynTable; name: ARRAY OF CHAR; nameLen: CARDINAL;
                    value: ARRAY OF CHAR; valLen: CARDINAL);
PROCEDURE DynLookup(VAR dt: DynTable; index: CARDINAL;
                    VAR entry: HeaderEntry; VAR ok: BOOLEAN);
PROCEDURE DynResize(VAR dt: DynTable; newMaxSize: CARDINAL);
PROCEDURE DynCount(VAR dt: DynTable): CARDINAL;

Header Block

PROCEDURE DecodeHeaderBlock(v: BytesView; VAR dt: DynTable;
                            VAR headers: ARRAY OF HeaderEntry;
                            maxOut: CARDINAL;
                            VAR numHeaders: CARDINAL; VAR ok: BOOLEAN);
PROCEDURE EncodeHeaderBlock(VAR b: Buf; VAR dt: DynTable;
                            VAR headers: ARRAY OF HeaderEntry;
                            numHeaders: CARDINAL);

Full header block decode/encode. Supports indexed, literal with incremental indexing, literal without indexing, and dynamic table size updates.

Usage

VAR dt: DynTable;
    hdrs: ARRAY [0..15] OF HeaderEntry;
    buf: Buf;
DynInit(dt, 4096);
(* ... fill hdrs ... *)
Init(buf, 1024);
EncodeHeaderBlock(buf, dt, hdrs, 3);