zstd rejects blocks which do not compress by at least a certain amount.
In which case, such block is simply emitted uncompressed (even if a little bit of compression could be achieved).
This is better for decompression speed, hence for energy.
The logic is controlled by ZSTD_minGain().
The rule is applied uniformly, at all compression levels.
This change makes btultra accepts blocks with poor compression ratios.
We presume that users of btultra mode prefers compression ratio over some decompress speed gains.
The threshold for minimum gain is lowered for btultra
from s>>6 (~1.5% minimum gain)
to s>>7 (~0.8% minimum gain).
This is a prudent change.
Not sure if it's large enough.
ensure that, when frequency[symbol]==0,
result is (tableLog + 1) bits
with both upper-bit and fractional-bit estimates.
Also : enable BIT_DEBUG in /tests
Work around bug in zstd decoder
Pull request #1144 exercised a new path in the zstd decoder that proved to
be buggy. Avoid the extremely rare bug by emitting an uncompressed block.
This edge case is only possible with the new optimal encoding selector,
since before zstd would always choose `set_basic` for small numbers of
sequences.
Fix `FSE_readNCount()` to support buffers < 4 bytes.
Credit to OSS-Fuzz
Estimate the cost for using FSE modes `set_basic`, `set_compressed`, and
`set_repeat`, and select the one with the lowest cost.
* The cost of `set_basic` is computed using the cross-entropy cost
function `ZSTD_crossEntropyCost()`, using the normalized default count
and the count.
* The cost of `set_repeat` is computed using `FSE_bitCost()`. We check the
previous table to see if it is able to represent the distribution.
* The cost of `set_compressed` is computed with the entropy cost function
`ZSTD_entropyCost()`, together with the cost of writing the normalized
count `ZSTD_NCountCost()`.
Yann Collet