lz4_header.nx

lz4_header.nx -- parse LZ4 frame format headers (RFC-less spec: https://github.com/lz4/lz4/blob/dev/doc/lz4_Frame_format.md). Parses and validates the fixed magic + FLG + BD bytes that precede every LZ4-framed stream. Block decompression (the actual LZ4 algorithm) is a much larger port and ships in a future lz4.nx; this module is enough to: - detect whether a blob is an LZ4 frame - read the block-size code + content-checksum flag - know how many header bytes to skip before the first block Magic: 0x184D2294 little-endian (4 bytes). FLG byte: bit 7-6: version (must be 01 for v1) bit 5: block independence (1 = independent, 0 = linked) bit 4: block checksum present bit 3: content size present (if set, 8 bytes follow BD) bit 2: content checksum present (xxHash32 at end of stream) bit 1: reserved (must be 0) bit 0: dictionary ID present (if set, 4 bytes follow) BD byte: bit 7: reserved bit 6-4: block max size (4=64KB, 5=256KB, 6=1MB, 7=4MB) bit 3-0: reserved Then 1 byte HC (header checksum, low-byte of xxHash32 of (FLG, BD, [content_size], [dict_id])). Invariants: L1 Magic match required; otherwise LZ4_ERR_FORMAT. L2 FLG version must be 01. L3 Header length varies (7, 11, 15, or 19 bytes).