`ZSTD_insertBt1()` has a speed optimization that skips the prefix of
very long matches.
40def70387/lib/compress/zstd_opt.c (L476)
This optimization is based off the length longest match found. However,
when indices are reset, we only ensure that we can reference the whole
window starting from `ip`. If the previous block ended with a long match
then `nextToUpdate` could be much less than `ip`. It might be far enough
back that `nextToUpdate < maxDist`, so it doesn't have a full window of
data to reference. This can cause non-determinism bugs, because we may
find a match that is beyond `ip - maxDist`, and may sometimes be
un-referencable, and that match triggers the speed optimization.
The fix is to base the `windowLow` off of the `target` of
`ZSTD_updateTree_internal()`, because anything below that value will be
obsolete by the time `ZSTD_updateTree_internal()` completes.
* Switch to yearless copyright per FB policy
* Fix up SPDX-License-Identifier lines in `contrib/linux-kernel` sources
* Add zstd copyright/license header to the `contrib/linux-kernel` sources
* Update the `tests/test-license.py` to check for yearless copyright
* Improvements to `tests/test-license.py`
* Check `contrib/linux-kernel` in `tests/test-license.py`
Fixes:
Enable RLE blocks for superblock mode
Fix the limitation that the literals block must shrink. Instead, when we're within 200 bytes of the next header byte size, we will just use the next one up. That way we should (almost?) always have space for the table.
Remove the limitation that the first sub-block MUST have compressed literals and be compressed. Now one sub-block MUST be compressed (otherwise we fall back to raw block which is okay, since that is streamable). If no block has compressed literals that is okay, we will fix up the next Huffman table.
Handle the case where the last sub-block is uncompressed (maybe it is very small). Before it would skip superblock in this case, now we allow the last sub-block to be uncompressed. To do this we need to regenerate the correct repcodes.
Respect disableLiteralsCompression in superblock mode
Fix superblock mode to handle a block consisting of only compressed literals
Fix a off by 1 error in superblock mode that disabled it whenever there were last literals
Fix superblock mode with long literals/matches (> 0xFFFF)
Allow superblock mode to repeat Huffman tables
Respect ZSTD_minGain().
Tests:
Simple check for the condition in #2096.
When the simple_round_trip fuzzer enables superblock mode, it checks that the compressed size isn't expanded too much.
Remaining limitations:
O(targetCBlockSize^2) because we recompute statistics every sequence
Unable to split literals of length > targetCBlockSize into multiple sequences
Refuses to generate sub-blocks that don't shrink the compressed data, so we could end up with large sub-blocks. We should emit those sections as uncompressed blocks instead.
...
Fixes#2096
