gc_formats::chunks parses the container that fills the region after the last
shrub class body and before the constant trailer:
+0x00 16 bytes, last byte 0xff (opaque, differs per chunk)
+0x10 10 zero bytes
+0x1a N tagged offsets, top byte 0xf0
Each tagged offset is 0xF0000000 | low24, and a record is read at
chunk_start + low. Resolving that way gives records sharing a field layout,
which is what distinguishes the container from a coincidental byte pattern.
Fields are not named. The 16-byte header is opaque, the 0xf0 tag is
unexplained, and it is not established that a chunk is moby_anim -- a
49 x 368-byte table inside this region is byte-identical on all 27 levels,
which is evidence against that identification. What the parser provides is
chunk and record extraction, which is what the analysis established.
The gated test measures, over all 27 levels: 1123 chunks, 19546 records, 50
distinct strides, 12.4% of byte positions constant across records (per-chunk
range 2.8% to 75%). Chance gives a constant position probability of 1/256^n.
Three things the test got wrong first, recorded because they are the traps:
- A per-chunk threshold of 20% constant positions. Individual chunks run from
2.8% to 75%; the aggregate is the invariant, not the chunk. Asserting 20%
per chunk was asserting a property the format does not have.
- Requiring every offset to resolve. 27 chunks truncate at the window edge,
because the 900000-byte window does not align to chunk boundaries. Counted
now rather than asserted away.
- read_u32 in the chunk signature check was big-endian while Buf::u32 is
little-endian. The tag lives in the high byte of a little-endian word, so
the mismatch made every level report zero chunks while parse_chunk -- which
goes through Buf -- was correct. Worth remembering that the two paths can
disagree on a format this young.
Tests: 190 -> 191.
ReaperOfVeriod
681333535a