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fixed description of Frame_Content_Size_flag

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inikep 2016-07-25 12:26:39 +02:00
parent f9c3ccec49
commit 49ec6d11d9
1 changed files with 20 additions and 12 deletions
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zstd_compression_format.md
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@ -168,12 +168,29 @@ Decoding this byte is enough to tell the size of `Frame_Header`.
In this table, bit 7 is highest bit, while bit 0 is lowest.
__`Frame_Content_Size_flag`__
This is a 2-bits flag (`= Frame_Header_Descriptor >> 6`),
specifying if decompressed data size is provided within the header.
The `Value` can be converted to `Field_Size` that is number of bytes used by `Frame_Content_Size` according to the following table:
| `Value` | 0 | 1 | 2 | 3 |
| ---------- | --- | --- | --- | --- |
|`Field_Size`| 0-1 | 2 | 4 | 8 |
The meaning of `Value` equal 0 depends on `Single_Segment_flag` :
it either means `0` (size not provided) _if_ the `Window_Descriptor` byte is present,
or `1` (frame content size <= 255 bytes) otherwise.
__`Single_Segment_flag`__
If `Single_Segment_flag` is not set then `Window_Descriptor` is mandatory and `Frame_Content_Size_flag` will be ignored.
If this flag is set,
data shall be regenerated within a single continuous memory segment.
If `Single_Segment_flag` is set then `Window_Descriptor` should be absent and `Frame_Content_Size_flag` will be used along with a mandatory `Frame_Content_Size` field.
As a consequence, the decoder must allocate a single continuous memory segment of size equal or bigger than `Frame_Content_Size`.
In this case, `Window_Descriptor` byte __is not present__,
but `Frame_Content_Size_flag` field necessarily is.
As a consequence, the decoder must allocate a memory segment
of size equal or bigger than `Frame_Content_Size`.
In order to preserve the decoder from unreasonable memory requirement,
a decoder can reject a compressed frame
@ -185,15 +202,6 @@ This is just a recommendation,
each decoder is free to support higher or lower limits,
depending on local limitations.
__`Frame_Content_Size_flag`__
This is a 2-bits flag (`= FHD >> 6`) used only if `Single_Segment_flag` is set.
In this case Value can be converted to Field size that is number of bytes used by `Frame_Content_Size` according to the following table:
| Value | 0 | 1 | 2 | 3 |
|----------| --- | --- | --- | --- |
|Field size| 1 | 2 | 4 | 8 |
__`Unused_bit`__
The value of this bit should be set to zero.