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[doc] Clarify requirements around Block_Maximum_Size in spec

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Clarify that it is legal to send compressed blocks that are larger than the
uncompressed size so long as their `Block_Content` is no larger than
`Block_Maximum_Size`.

See https://github.com/facebook/zstd/issues/4667
This commit is contained in:
Nick Terrell
2026-05-14 18:45:42 -04:00
committed by Nick Terrell
parent c367e0c149
commit 5233c58e6c
1 changed files with 11 additions and 6 deletions
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doc/zstd_compression_format.md
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@@ -16,7 +16,7 @@ Distribution of this document is unlimited.
### Version ### Version
0.4.4 (2025-03-22) 0.4.5 (2026-05-14)
Introduction Introduction
@@ -387,20 +387,24 @@ When `Block_Type` is `RLE_Block`, since `Block_Content`’s size is always 1,
__`Block_Content`__ and __`Block_Maximum_Size`__ __`Block_Content`__ and __`Block_Maximum_Size`__
The size of `Block_Content` is limited by `Block_Maximum_Size`, The size of `Block_Content` is limited by `Block_Maximum_Size`, which is
which is the smallest of: determined once for a given frame and is the smallest of:
- `Window_Size` - `Window_Size`
- 128 KiB (131.072 bytes) - 128 KiB (131.072 bytes)
`Block_Maximum_Size` is constant for a given frame. Both the `Block_Content` and the decompressed size of any block in the frame must
This maximum is applicable to both the decompressed size be no larger than `Block_Maximum_Size`.
and the compressed size of any block in the frame.
The reasoning for this limit is that a decoder can read this information 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. at the beginning of a frame and use it to allocate buffers.
The guarantees on the size of blocks ensure that The guarantees on the size of blocks ensure that
the buffers will be large enough for any following block of the valid frame. the buffers will be large enough for any following block of the valid frame.
If a compressed block is larger than its uncompressed content, it is recommended
to send it uncompressed (i.e., a `Raw_Block`). However, as long as
`Block_Content` is no larger than `Block_Maximum_Size`, it is legal to send such
a compressed block, even if it's larger than its uncompressed content.
Compressed Blocks Compressed Blocks
----------------- -----------------
@@ -1742,6 +1746,7 @@ or at least provide a meaningful error code explaining for which reason it canno
Version changes Version changes
--------------- ---------------
- 0.4.5 : minor clarification regarding Block_Maximum_Size
- 0.4.4 : minor clarification for block size - 0.4.4 : minor clarification for block size
- 0.4.3 : clarifications for Huffman prefix code assignment example - 0.4.3 : clarifications for Huffman prefix code assignment example
- 0.4.2 : refactor FSE table construction process, inspired by Donald Pian - 0.4.2 : refactor FSE table construction process, inspired by Donald Pian