Rayon uses a singleton global pool.
By default it's set to a regular spawn handler with number of logical CPU cores, but it can be overridden by Rust applications to customize number of threads, spawn handlers, exit handlers and other options.
Such customization should be usually done at the app level, because if a single library initialises the global pool, then Rayon will prevent any further overrides and they will error out. This can cause conflicts between libraries or library and user code and make them impossible to use together.
Hence, I've removed the `threads` option from the `Options` struct and instead moved initialisation to the CLI part of the codebase (main.rs).
Users of the library that didn't depend on custom `threads` number can keep using it as before - they'll still get same number of threads as number of logical CPU cores, while users who need fine-tuning, can do that by customizing rayon pool themselves at the top level of the app.
Note: another alternative to keep the option could've been to use `ThreadPoolBuilder::build` + `ThreadPool::install` to use a local pool just within OxiPNG, but that would ignore any customizations made by users in top-level pool and would prevent usage on targets that require custom spawn handlers like WebAssembly. As such, I've decided to avoid it.
Co-authored-by: Josh Holmer <jholmer.in@gmail.com>
In order to avoid problems with mutable references,
the approach is to use the eval sender for non-parallel.
Computations will still be done synchronously
with the fake synchronous rayon module.
* Skip baselines earlier in the evaluator
We know that we'll throw them away anyway, so there is no point in even sending them to Sender and comparing them with others once we used them for the best candidate size.
* Simplify comparison logic
* Improve performance consistency
Switch from HashMap / HashSet to IndexMap / IndexSet for consistent iteration order of various options and, as a result, more predictable performance.
libdeflater is a Rust wrapper around
[libdeflate](https://github.com/ebiggers/libdeflate) - an alternative
heavily optimised library for deflate/zlib/gzip compression and
decompression that is intended for situations where upper bounds of the
output are well-known.
In my benchmarks on test files in the repo it has shown to be usually
both slightly faster and providing better compressed output than
cloudflare-zlib, but in some cases showing the opposite, so rather
than swapping defaults, it's currently provided as another option,
similarly to zopfli.
Since it's not strictly better in all cases, I'm not providing median
numbers, but you can check distribution histograms for time and size
differences here (all using `oxipng -o 6 -t 6 -P`):
https://docs.google.com/spreadsheets/d/1WOKgeYZBhLkQvMGAC36snN4azilElzOFhx63RJu0EZY/edit?usp=sharing
* Avoid using time API when we don't need it
This avoids a syscall to the time API when the result is ignored later anyway.
This allows to use the library with default options on wasm32-unknown-unknown, where the unimplemented syscall would panic otherwise.
* eval_send doesn't need to be an Option
We can drop the value manually, thus avoiding unwrap on each access.
* Keep single `use rayon::prelude::*`
If either `rayon` is already imported, then `rayon::prelude::*` should always resolve.
* Extract comparator
* Fully enable non-parallel mode
