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updated frame specification with recommended max window size

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Yann Collet
2016-06-30 17:05:42 +02:00
parent c35cd5b1db
commit 406dcca98a
1 changed files with 12 additions and 6 deletions
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zstd_frame_format.md
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@@ -194,11 +194,11 @@ __WD byte__ (Window Descriptor)
Provides guarantees on maximum back-reference distance
that will be used within compressed data.
This information can then be used by decoder to allocate enough memory.
This information is useful for decoders to allocate enough memory.
| BitNb | 7-3 | 0-2 |
| ------- | -------- | -------- |
|FieldName| Exponent | Mantissa |
| BitNb | 7-3 | 0-2 |
| --------- | -------- | -------- |
| FieldName | Exponent | Mantissa |
Maximum distance is given by the following formulae :
```
@@ -207,12 +207,18 @@ windowBase = 1 << windowLog;
windowAdd = (windowBase / 8) * Mantissa;
windowSize = windowBase + windowAdd;
```
`WD` byte is optional. It's not present in `single segment` mode.
The minimum window size is 1 KB.
The maximum value is (15*(2^38))-1 bytes, which is almost 1.875 TB.
In order to preserve decoder from unreasonable memory requirement,
`WD` byte is optional. It's not present in `single segment` mode.
In which case, the maximum back-reference distance is the content size itself, which can be any value from 1 to 2^64-1 bytes (16 EB).
In order to preserve decoder from unreasonable memory requirements,
a decoder can refuse a compressed frame
which requests a memory size beyond decoder's authorized range.
For better interoperability, decoders are recommended to be compatible with window sizes up to 8 MB. Encoders are recommended to not request more than 8 MB. It's just a recommendation, decoders are free to accept or refuse larger or lower values.
__Frame Content Size__
This is the original (uncompressed) size.