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Diffstat (limited to 'doc/zstd_compression_format.md')
| -rw-r--r-- | doc/zstd_compression_format.md | 34 |
1 files changed, 25 insertions, 9 deletions
diff --git a/doc/zstd_compression_format.md b/doc/zstd_compression_format.md index 90ac0fe9bcd5..fc61726fc98c 100644 --- a/doc/zstd_compression_format.md +++ b/doc/zstd_compression_format.md @@ -16,7 +16,7 @@ Distribution of this document is unlimited. ### Version -0.3.4 (16/08/19) +0.3.5 (13/11/19) Introduction @@ -341,6 +341,8 @@ The structure of a block is as follows: |:--------------:|:---------------:| | 3 bytes | n bytes | +__`Block_Header`__ + `Block_Header` uses 3 bytes, written using __little-endian__ convention. It contains 3 fields : @@ -385,17 +387,30 @@ There are 4 block types : __`Block_Size`__ The upper 21 bits of `Block_Header` represent the `Block_Size`. + When `Block_Type` is `Compressed_Block` or `Raw_Block`, -`Block_Size` is the size of `Block_Content`, hence excluding `Block_Header`. -When `Block_Type` is `RLE_Block`, `Block_Content`’s size is always 1, -and `Block_Size` represents the number of times this byte must be repeated. -A block can contain and decompress into any number of bytes (even zero), -up to `Block_Maximum_Decompressed_Size`, which is the smallest of: -- Window_Size +`Block_Size` is the size of `Block_Content` (hence excluding `Block_Header`). + +When `Block_Type` is `RLE_Block`, since `Block_Content`’s size is always 1, +`Block_Size` represents the number of times this byte must be repeated. + +`Block_Size` is limited by `Block_Maximum_Size` (see below). + +__`Block_Content`__ and __`Block_Maximum_Size`__ + +The size of `Block_Content` is limited by `Block_Maximum_Size`, +which is the smallest of: +- `Window_Size` - 128 KB -If this condition cannot be respected when generating a `Compressed_Block`, -the block must be sent uncompressed instead (`Raw_Block`). +`Block_Maximum_Size` is constant for a given frame. +This maximum is applicable to both the decompressed size +and the compressed size of any block in the frame. + +The reasoning for this limit is that a decoder can read this information +at the beginning of a frame and use it to allocate buffers. +The guarantees on the size of blocks ensure that +the buffers will be large enough for any following block of the valid frame. Compressed Blocks @@ -1658,6 +1673,7 @@ or at least provide a meaningful error code explaining for which reason it canno Version changes --------------- +- 0.3.5 : clarifications for Block_Maximum_Size - 0.3.4 : clarifications for FSE decoding table - 0.3.3 : clarifications for field Block_Size - 0.3.2 : remove additional block size restriction on compressed blocks |