3732a08f5b
The sequence section starts with a number, which tells how sequences are present in the section. If this number if 0, the section automatically ends. The number 0 can be represented using the 1 byte or the 2 bytes formats. That's because the 2-bytes formats fully overlaps the 1 byte format. However, when 0 is represented using the 2-bytes format, the decoder was expecting the sequence section to continue, and was looking for FSE tables, which is incorrect. Fixed this behavior, in both the reference decoder and the educational behavior. In practice, this behavior never happens, because the encoder will always select the 1-byte format to represent 0, since this is more efficient. Completed the fix with a new golden sample for tests, a clarification of the specification, and a decoder errata paragraph.
Educational Decoder
zstd_decompress.c is a self-contained implementation in C99 of a decoder,
according to the Zstandard format specification.
While it does not implement as many features as the reference decoder,
such as the streaming API or content checksums, it is written to be easy to
follow and understand, to help understand how the Zstandard format works.
It's laid out to match the format specification,
so it can be used to understand how complex segments could be implemented.
It also contains implementations of Huffman and FSE table decoding.
While the library's primary objective is code clarity,
it also happens to compile into a small object file.
The object file can be made even smaller by removing error messages,
using the macro directive ZDEC_NO_MESSAGE at compilation time.
This can be reduced even further by foregoing dictionary support,
by defining ZDEC_NO_DICTIONARY.
harness.c provides a simple test harness around the decoder:
harness <input-file> <output-file> [dictionary]
As an additional resource to be used with this decoder,
see the decodecorpus tool in the tests directory.
It generates valid Zstandard frames that can be used to verify
a Zstandard decoder implementation.
Note that to use the tool to verify this decoder implementation,
the --content-size flag should be set,
as this decoder does not handle streaming decoding,
and so it must know the decompressed size in advance.