As reported by @P-E-Meunier in https://github.com/facebook/zstd/issues/2662#issuecomment-1443836186,
seekable format ingestion speed can be particularly slow
when selected `FRAME_SIZE` is very small,
especially in combination with the recent row_hash compression mode.
The specific scenario mentioned was `pijul`,
using frame sizes of 256 bytes and level 10.
This is improved in this PR,
by providing approximate parameter adaptation to the compression process.
Tested locally on a M1 laptop,
ingestion of `enwik8` using `pijul` parameters
went from 35sec. (before this PR) to 2.5sec (with this PR).
For the specific corner case of a file full of zeroes,
this is even more pronounced, going from 45sec. to 0.5sec.
These benefits are unrelated to (and come on top of) other improvement efforts currently being made by @yoniko for the row_hash compression method specifically.
The `seekable_compress` test program has been updated to allows setting compression level,
in order to produce these performance results.
Memory constrained use cases that manage multiple archives benefit from
retaining multiple archive seek tables without retaining a ZSTD_seekable
instance for each.
* New opaque type for seek table: ZSTD_seekTable.
* ZSTD_seekable_copySeekTable() supports copying seek table out of a
ZSTD_seekable.
* ZSTD_seekTable_[eachSeekTableOp]() defines seek table API that mirrors
existing seek table operations.
* Existing ZSTD_seekable_[eachSeekTableOp]() retained; they delegate to
ZSTD_seekTable the variant.
These changes allow the above-mentioned use cases to initialize a
ZSTD_seekable, extract its ZSTD_seekTable, then throw the ZSTD_seekable
away to save memory. Standard ZSTD operations can then be used to
decompress frames based on seek table offsets.
The copy and delegate patterns are intended to minimize impact on
existing code and clients. Using copy instead of move for the infrequent
operation extracting a seek table ensures that the extraction does not
render the ZSTD_seekable useless. Delegating to *new* seek
table-oriented APIs ensures that this is not a breaking change for
existing clients while supporting all meaningful operations that depend
only on seek table data.
