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fixed decoder behavior when nbSeqs==0 is encoded using 2 bytes

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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.
This commit is contained in:
Yann Collet
2023-06-05 16:03:00 -07:00
parent 3e815f5b3a
commit 3732a08f5b
6 changed files with 44 additions and 18 deletions
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12
doc/zstd_compression_format.md
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@ -16,7 +16,7 @@ Distribution of this document is unlimited.
### Version
0.3.9 (2023-03-08)
0.4.0 (2023-06-05)
Introduction
@ -650,15 +650,16 @@ __`Number_of_Sequences`__
This is a variable size field using between 1 and 3 bytes.
Let's call its first byte `byte0`.
- `if (byte0 == 0)` : there are no sequences.
The sequence section stops there.
Decompressed content is defined entirely as Literals Section content.
The FSE tables used in `Repeat_Mode` aren't updated.
- `if (byte0 < 128)` : `Number_of_Sequences = byte0` . Uses 1 byte.
- `if (byte0 < 255)` : `Number_of_Sequences = ((byte0 - 0x80) << 8) + byte1`. Uses 2 bytes.
Note that the 2 bytes format fully overlaps the 1 byte format.
- `if (byte0 == 255)`: `Number_of_Sequences = byte1 + (byte2<<8) + 0x7F00`. Uses 3 bytes.
`if (Number_of_Sequences == 0)` : there are no sequences.
The sequence section stops immediately,
FSE tables used in `Repeat_Mode` aren't updated.
Block's decompressed content is defined solely by the Literals Section content.
__Symbol compression modes__
This is a single byte, defining the compression mode of each symbol type.
@ -1698,6 +1699,7 @@ or at least provide a meaningful error code explaining for which reason it canno
Version changes
---------------
- 0.4.0 : fixed imprecise behavior for nbSeq==0, detected by Igor Pavlov
- 0.3.9 : clarifications for Huffman-compressed literal sizes.
- 0.3.8 : clarifications for Huffman Blocks and Huffman Tree descriptions.
- 0.3.7 : clarifications for Repeat_Offsets, matching RFC8878