The only fixture was one .hgtz container written by one producer at level 19.
An audit ran 16,490 real blocks through an instrumented decoder and found
thirteen branches of RFC 8878 that corpus never reaches at all: raw blocks,
RLE blocks, RLE literals, treeless literals at the three-byte size format,
frames with no content size, frames with a checksum, the one and eight byte
content sizes, the three-byte sequence count, a block with no sequences, and
RLE sequence tables for literal and match length.
make_corpus.py builds frames for them, either through libzstd or by hand and
then past pyzstd for a verdict, so legal and malformed are libzstd's opinion
rather than ours. The expected plain bytes of a legal frame are whatever
pyzstd decompressed it to, gzipped to keep the fixture small, and the
manifest records the exact command behind every frame. 46 KB in all.
A feasibility spike, standalone so it can move to its own repository. Not
wired into the app and not a dependency of it.
Decompression only, and only what our containers need: no dictionaries, no
multi-frame, no streaming, and the content checksum is parsed but never
verified. Native only, since the bit reader carries 56 bits in an int.
It is correct on real data. The committed fixture decodes to the samples
the generator wrote, and tool/oracle.py drives a byte-for-byte comparison
against pyzstd over 32 containers and 2,615 blocks from both tiers, every
thinned column count from 3601 to 361, and 88 S to 80 N. All identical.
The numbers, AOT on this box, 131,072 bytes a block: 0.66 ms median
against pyzstd's 0.175, so about 200 MB/s against 750. Across three tiles
the range is 0.66 to 0.94 ms, and a 113 block sweep is 67 to 105 ms. The
cost is the sequence loop and nothing else: a terrain block at level 19
carries around 1,700 literals and 14,400 sequences, so optimising the
literal path bought nothing.
