nx_zstd_frame.nx

nx_zstd_frame.nx -- Zstandard frame and block layer, writer and reader. Sits above nx_zstd_fse.nx (the entropy tables). This layer is what makes a .zst file walkable: magic, a variable-length frame header, then a chain of blocks each with its own 3-byte header. Raw and RLE blocks decode fully here -- a frame built only from those is a completely valid zstd file that any zstd tool will read -- so this is a working decoder for that subset, not a stub. COMPRESSED BLOCKS ARE REFUSED, NOT GUESSED. The literals+sequences layer is not built yet, so a Compressed block returns a distinct refusal code rather than emitting whatever the raw bytes happen to look like. A decoder that silently passes through undecoded data is worse than one that stops. THE HEADER LENGTH IS COMPUTED, NOT ASSUMED. Frame_Content_Size is 0, 1, 2, 4 or 8 bytes, Dictionary_ID is 0, 1, 2 or 4, and the Window_Descriptor is ABSENT when Single_Segment_flag is set. Five independent choices, so the header is anywhere from 2 to 14 bytes. Assuming a length works on files from one encoder and fails on the next. THE 2-BYTE FCS IS BIASED BY 256. A two-byte Frame_Content_Size stores (size - 256), so a naive read reports every such frame 256 bytes short. This only affects frames between 256 and 65791 bytes, which is precisely the range small test files land in. genealogy_id: zstandard_rfc8878_frame lineage_id: nx_zstd_frame_v1 license_tier: ORIGINAL