Deflate64 decoder · onearchive docs

Normative notes for Deflate64Decoder. The format facts (symbols, extra bits, window, header limits) are in deflate64-derivation.md, with the experiment behind each. This file records what the decoder does with them.

Input: one raw stream (zip method 9 stores nothing around it). No wrapper, no checksum; the zip layer checks the CRC-32.
Window: 65536 bytes of history, kept in a 128 KiB ring. A match whose distance exceeds the output produced so far is a format error naming the offset. Distance 65536 is accepted (fact F1).
Symbols: RFC 1951 for everything not listed in the derivation. Literal/length 285 = 3 plus 16 extra bits; distance symbols 30 and 31 = bases 32769 and 49153 with 14 extra bits. Literal/length 286 and 287 fail only when decoded.
Dynamic headers: HLIT up to 288 and HDIST up to 32 are accepted (fact F9); the code lengths must still form a complete or single-symbol code per RFC 1951, otherwise a format error.
Block type 3 is a format error. A stored block checks LEN against its complement.
Output limit: MaxOutputSize (default 1 GiB) is checked before bytes are written; reaching it throws CompressionLimitException and writes nothing past the limit. The decoder allocates only its fixed window and tables.
End: Done is returned after the final block; bytes after it are left unconsumed. The one-call and stream readers refuse them unless AllowTrailingData is set. A truncated stream is a CompressionFormatException with IsTruncated set, raised by ThrowIfIncomplete or by the readers.
Speed: a fast loop (Deflate64FastLoop, built like Deflate's) runs while 24 input and 280 output bytes remain. A second bit refill follows a length whose code and extra bits leave fewer than 29 bits (symbol 285: 15 + 16 bits, then a distance code and 14 extra bits do not fit one 56-bit refill). Matches longer than 258 bytes, or reaching into the window, leave the loop and are copied by the per-symbol path, which also handles the ends of the buffers.