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fix a minor inefficiency in compress_superblock

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and in `decodecorpus`:
the specific case `nbSeq=127` can be represented using the 1-byte format.
Note that both the 1-byte and the 2-bytes formats are valid to represent this case,
so there was no "error", produced data remains valid,
it's just that the 1-byte format is more efficient.

fix #3667

Credit to @ip7z for finding this issue.
This commit is contained in:
Yann Collet
2023-06-05 09:51:52 -07:00
parent 5059618295
commit 1f83b7cfc4
3 changed files with 5 additions and 4 deletions
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doc/zstd_compression_format.md
+3 -2
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@@ -655,8 +655,9 @@ Let's call its first byte `byte0`.
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 = ((byte0-128) << 8) + byte1` . Uses 2 bytes.
- `if (byte0 == 255)`: `Number_of_Sequences = byte1 + (byte2<<8) + 0x7F00` . Uses 3 bytes.
- `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.
__Symbol compression modes__