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updated spec on dictID==0

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Specified decoder behavior on receiving a frame with dictID=0.

Pushed paragraph on reserved DictID ranges into the Dictionary Format section.
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Yann Collet 2020-05-25 08:15:09 -07:00
parent 9eb2ccc9fb
commit bb3c9bf43a
1 changed files with 17 additions and 23 deletions
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doc/zstd_compression_format.md
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@ -3,7 +3,7 @@ Zstandard Compression Format
### Notices
Copyright (c) 2016-present Yann Collet, Facebook, Inc.
Copyright (c) 2016-2020 Yann Collet, Facebook, Inc.
Permission is granted to copy and distribute this document
for any purpose and without charge,
@ -16,7 +16,7 @@ Distribution of this document is unlimited.
### Version
0.3.5 (13/11/19)
0.3.6 (25/05/20)
Introduction
@ -291,21 +291,10 @@ Format is __little-endian__.
It's allowed to represent a small ID (for example `13`)
with a large 4-bytes dictionary ID, even if it is less efficient.
_Reserved ranges :_
Within private environments, any `Dictionary_ID` can be used.
However, for frames and dictionaries distributed in public space,
`Dictionary_ID` must be attributed carefully.
Rules for public environment are not yet decided,
but the following ranges are reserved for some future registrar :
- low range : `<= 32767`
- high range : `>= (1 << 31)`
Outside of these ranges, any value of `Dictionary_ID`
which is both `>= 32768` and `< (1<<31)` can be used freely,
even in public environment.
A value of `0` has same meaning as no `Dictionary_ID`,
in which case the frame may or may not need a dictionary to be decoded,
and the ID of such a dictionary is not specified.
The decoder must know this information by other means.
#### `Frame_Content_Size`
@ -1429,14 +1418,18 @@ __`Dictionary_ID`__ : 4 bytes, stored in __little-endian__ format.
It's used by decoders to check if they use the correct dictionary.
_Reserved ranges :_
If the frame is going to be distributed in a private environment,
any `Dictionary_ID` can be used.
However, for public distribution of compressed frames,
the following ranges are reserved and shall not be used :
If the dictionary is going to be distributed in a public environment,
the following ranges of `Dictionary_ID` are reserved for some future registrar
and shall not be used :
- low range : <= 32767
- high range : >= (2^31)
Outside of these ranges, any value of `Dictionary_ID`
which is both `>= 32768` and `< (1<<31)` can be used freely,
even in public environment.
__`Entropy_Tables`__ : follow the same format as tables in [compressed blocks].
See the relevant [FSE](#fse-table-description)
and [Huffman](#huffman-tree-description) sections for how to decode these tables.
@ -1673,6 +1666,7 @@ or at least provide a meaningful error code explaining for which reason it canno
Version changes
---------------
- 0.3.6 : clarifications for Dictionary_ID
- 0.3.5 : clarifications for Block_Maximum_Size
- 0.3.4 : clarifications for FSE decoding table
- 0.3.3 : clarifications for field Block_Size