credit to oss-fuzz
In rare circumstances, the block-splitter might cut a block at the exact beginning of a repcode.
In which case, since litlength=0, if the repcode expected 1+ literals in front, its signification changes.
This scenario is controlled in ZSTD_seqStore_resolveOffCodes(),
and the repcode is transformed into a raw offset when its new meaning is incorrect.
In more complex scenarios, the previous block might be emitted as uncompressed after all,
thus modifying the expected repcode history.
In the case discovered by oss-fuzz, the first block is emitted as uncompressed,
so the repcode history remains at default values: 1,4,8.
But since the starting repcode is repcode3, and the literal length is == 0,
its meaning is : = repcode1 - 1.
Since repcode1==1, it results in an offset value of 0, which is invalid.
So that's what the `assert()` was verifying : the result of the repcode translation should be a valid offset.
But actually, it doesn't matter, because this result will then be compared to reality,
and since it's an invalid offset, it will necessarily be discarded if incorrect,
then the repcode will be replaced by a raw offset.
So the `assert()` is not useful.
Furthermore, it's incorrect, because it assumes this situation cannot happen, but it does, as described in above scenario.
which properly represents the maximum bit size of compressed literals (11) as defined in the specification.
To be preferred from HUF_TABLELOG_DEFAULT which represents the same value but by accident.
Name selected to keep the same convention as existing width definitions,
MLFSELog, LLFSELog and OffFSELog.
In rare cases, the default huffman depth selector is a bit too harsh,
requiring brutal adaptations to the tree,
resulting is some loss of compression ratio.
This new heuristic avoids the worse cases, favoring compression ratio.
As an example, compression of a specific distribution of 771 literals
is now improved to 441 bytes, from 601 bytes before.
oss-fuzz uncovered a scenario where we're evaluating the cost of litLength = 131072,
which can't be represented in the zstd format, so we accessed 1 beyond LL_bits.
Fix the issue by making it cost 1 bit more than litLength = 131071.
There are still follow ups:
1. This happened because literals_cost[0] = 0, so the optimal parser chose 36 literals
over a match. Should we bound literals_cost[literal] > 0, unless the block truly only
has one literal value?
2. When no matches are found, the cost model isn't updated. In this case no matches were
found for an entire block. So the literals cost model wasn't updated at all. That made
the optimal parser think literals_cost[0] = 0, where it is actually quite high, since
the block was entirely random noise.
Credit to OSS-Fuzz.
When re-using a compression state, across multiple successive compressions,
the state should minimize the amount of allocation and initialization required.
This mostly matters in situations where initialization is an overwhelming task
compared to compression itself.
This can happen when the amount to compress is small,
while the compression state was given the impression that it would be much larger,
aka, streaming mode without providing a srcSize hint.
This lean-initialization optimization was broken in 980f3bbf83 .
This commit fixes it, making this scenario once again on par with v1.4.9.
Note that this does not completely fix#2966,
since another heavy initialization, specific to row mode,
is also happening (and was not present in v1.4.9).
This will be fixed in a separate commit.
directly at ZSTD_storeSeq() interface.
In the process, remove ZSTD_REP_MOVE.
This makes it possible, in future commits,
to update and effectively simplify the naming scheme
to properly label the updated processing pipeline :
offset | repcode => offBase => offCode + offBits
the new contracts seems to make more sense :
updateRep() updates an array of repeat offsets _in place_,
while newRep() generates a new structure with the updated repeat-offset array.
Most callers are actually expecting the in-place variant,
and a limited sub-section, in `zstd_opt.c` mainly, prefer `newRep()`.
to act on values stored / expressed in the sumtype numeric representation required by `storedSeq()`.
This makes it possible to abstract away this representation by using the macros to extract these values.
First user : ZSTD_updateRep() .
this meant to abstract the sumtype representation required
to transfert `offcode` to `ZSTD_storeSeq()`.
Unfortunately, the sumtype numeric representation is currently a leaky abstraction
that has permeated many other parts of the code,
especially within `zstd_lazy.c` and also within `zstd_opt.c` and `zstd_compress.c`.
While this PR makes a good job a transfering a large nb of call sites
to using the new macros, there are still a few sites where this transformation is more complex,
or where the numeric representation itself it used "as is".
One of the problematics area is the decision to use the numeric format of the sumtype
within the match finders of `zstd_lazy`.
This commit doesn't change the behavior, it only introduces and employes the macros,
but eventually the resulting code remains identical.
At target, if the numeric representation of the sumtype can be completely abstracted
and no other part of the code depends on it,
it will be possible to move it towards something slightly more efficient.
since this is effectively what is stored in this field (== matchLength - MINMATCH).
This makes it clearer what needs to be done when reading from / writing to this field.
