gzip_header.nx

gzip_header.nx -- parse RFC 1952 gzip stream headers. gzip = DEFLATE (RFC 1951) + gzip framing (RFC 1952). This module parses the framing, not the compression; a future deflate.nx will handle block decompression. Use cases: inspect a .gz file's original filename + mtime, detect corruption before paying the decompress cost, split a multi-member concatenated archive (gzip supports that), read NAS backup archives. Header layout (fixed-prefix then variable optionals): ID1 1 byte = 0x1F ID2 1 byte = 0x8B CM 1 byte compression method; 8 = DEFLATE FLG 1 byte flag bits MTIME 4 bytes modification time, seconds since epoch LE XFL 1 byte extra flags OS 1 byte originating filesystem FLG bits: bit 0 FTEXT probably ASCII text bit 1 FHCRC header CRC16 follows the header bit 2 FEXTRA extra fields follow (2-byte length + data) bit 3 FNAME original filename follows (NUL-terminated) bit 4 FCOMMENT comment follows (NUL-terminated) bits 5-7 reserved Invariants: G1 Magic (0x1F 0x8B) must match. G2 CM must be 8 (no other method is defined). G3 Unterminated FNAME / FCOMMENT returns GZIP_ERR_SHORT.