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Save an instruction in EntityHasher (#10648)
# Objective

Keep essentially the same structure of `EntityHasher` from #9903, but
rephrase the multiplication slightly to save an instruction.

cc @superdump 
Discord thread:
https://discord.com/channels/691052431525675048/1172033156845674507/1174969772522356756

## Solution

Today, the hash is
```rust
        self.hash = i | (i.wrapping_mul(FRAC_U64MAX_PI) << 32);
```
with `i` being `(generation << 32) | index`.

Expanding things out, we get
```rust
i | ( (i * CONST) << 32 )
= (generation << 32) | index | ((((generation << 32) | index) * CONST) << 32)
= (generation << 32) | index | ((index * CONST) << 32)  // because the generation overflowed
= (index * CONST | generation) << 32 | index
```

What if we do the same thing, but with `+` instead of `|`? That's almost
the same thing, except that it has carries, which are actually often
better in a hash function anyway, since it doesn't saturate. (`|` can be
dangerous, since once something becomes `-1` it'll stay that, and
there's no mixing available.)

```rust
(index * CONST + generation) << 32 + index
= (CONST << 32 + 1) * index + generation << 32
= (CONST << 32 + 1) * index + (WHATEVER << 32 + generation) << 32 // because the extra overflows and thus can be anything
= (CONST << 32 + 1) * index + ((CONST * generation) << 32 + generation) << 32 // pick "whatever" to be something convenient
= (CONST << 32 + 1) * index + ((CONST << 32 + 1) * generation) << 32
= (CONST << 32 + 1) * index +((CONST << 32 + 1) * (generation << 32)
= (CONST << 32 + 1) * (index + generation << 32)
= (CONST << 32 + 1) * (generation << 32 | index)
= (CONST << 32 + 1) * i
```

So we can do essentially the same thing using a single multiplication
instead of doing multiply-shift-or.

LLVM was already smart enough to merge the shifting into a
multiplication, but this saves the extra `or`:

![image](https://github.com/bevyengine/bevy/assets/18526288/d9396614-2326-4730-abbe-4908c01b5ace)
<https://rust.godbolt.org/z/MEvbz4eo4>

It's a very small change, and often will disappear in load latency
anyway, but it's a couple percent faster in lookups:

![image](https://github.com/bevyengine/bevy/assets/18526288/c365ec85-6adc-4f6d-8fa6-a65146f55a75)

(There was more of an improvement here before #10558, but with `to_bits`
being a single `qword` load now, keeping things mostly as it is turned
out to be better than the bigger changes I'd tried in #10605.)

---

## Changelog

(Probably skip it)

## Migration Guide

(none needed)
2023-11-28 12:37:30 +00:00
.cargo Change recommended linker: zld to lld for MacOS (#7496) 2023-02-06 18:24:12 +00:00
.github Standardize toml format with taplo (#10594) 2023-11-21 01:04:14 +00:00
assets Fix load scene example to use proper serialization format for rotation field (#10638) 2023-11-21 01:14:50 +00:00
benches Ran cargo fmt on benches crate (#10758) 2023-11-27 18:41:35 +00:00
crates Save an instruction in EntityHasher (#10648) 2023-11-28 12:37:30 +00:00
docs Standardize toml format with taplo (#10594) 2023-11-21 01:04:14 +00:00
docs-template Improve WebGPU unstable flags docs (#10163) 2023-10-18 17:30:44 +00:00
errors Add some more docs for bevy_text. (#9873) 2023-10-27 18:53:57 +00:00
examples Add clippy::manual_let_else at warn level to lints (#10684) 2023-11-28 04:15:27 +00:00
src Schedule-First: the new and improved add_systems (#8079) 2023-03-18 01:45:34 +00:00
tests Deferred Renderer (#9258) 2023-10-12 22:10:38 +00:00
tools Move remaining clippy lint definitions to Cargo.toml (#10672) 2023-11-28 04:12:48 +00:00
.gitattributes Enforce linux-style line endings for .rs and .toml (#3197) 2021-11-26 21:05:35 +00:00
.gitignore Fix example showcase (#10366) 2023-11-04 01:33:51 +00:00
Cargo.toml Add clippy::manual_let_else at warn level to lints (#10684) 2023-11-28 04:15:27 +00:00
CHANGELOG.md 0.12 Changelog (#10361) 2023-11-04 01:57:29 +00:00
clippy.toml Use clippy::doc_markdown more. (#10286) 2023-10-27 22:49:02 +00:00
CODE_OF_CONDUCT.md Update CODE_OF_CONDUCT.md 2020-08-19 20:25:58 +01:00
CONTRIBUTING.md Standardize toml format with taplo (#10594) 2023-11-21 01:04:14 +00:00
CREDITS.md Add morph targets (#8158) 2023-06-22 20:00:01 +00:00
deny.toml Standardize toml format with taplo (#10594) 2023-11-21 01:04:14 +00:00
LICENSE-APACHE Let the project page support GitHub's new ability to display open source licenses (#4966) 2022-06-08 17:55:57 +00:00
LICENSE-MIT Let the project page support GitHub's new ability to display open source licenses (#4966) 2022-06-08 17:55:57 +00:00
README.md Fix orphaned contributing paragraph (#10174) 2023-10-18 15:52:04 +00:00
rustfmt.toml Cargo fmt with unstable features (#1903) 2021-04-21 23:19:34 +00:00

Bevy

License Crates.io Downloads Docs CI Discord

What is Bevy?

Bevy is a refreshingly simple data-driven game engine built in Rust. It is free and open-source forever!

WARNING

Bevy is still in the early stages of development. Important features are missing. Documentation is sparse. A new version of Bevy containing breaking changes to the API is released approximately once every 3 months. We provide migration guides, but we can't guarantee migrations will always be easy. Use only if you are willing to work in this environment.

MSRV: Bevy relies heavily on improvements in the Rust language and compiler. As a result, the Minimum Supported Rust Version (MSRV) is generally close to "the latest stable release" of Rust.

Design Goals

  • Capable: Offer a complete 2D and 3D feature set
  • Simple: Easy for newbies to pick up, but infinitely flexible for power users
  • Data Focused: Data-oriented architecture using the Entity Component System paradigm
  • Modular: Use only what you need. Replace what you don't like
  • Fast: App logic should run quickly, and when possible, in parallel
  • Productive: Changes should compile quickly ... waiting isn't fun

About

  • Features: A quick overview of Bevy's features.
  • News: A development blog that covers our progress, plans and shiny new features.

Docs

  • The Bevy Book: Bevy's official documentation. The best place to start learning Bevy.
  • Bevy Rust API Docs: Bevy's Rust API docs, which are automatically generated from the doc comments in this repo.
  • Official Examples: Bevy's dedicated, runnable examples, which are great for digging into specific concepts.
  • Community-Made Learning Resources: More tutorials, documentation, and examples made by the Bevy community.

Community

Before contributing or participating in discussions with the community, you should familiarize yourself with our Code of Conduct.

  • Discord: Bevy's official discord server.
  • Reddit: Bevy's official subreddit.
  • GitHub Discussions: The best place for questions about Bevy, answered right here!
  • Bevy Assets: A collection of awesome Bevy projects, tools, plugins and learning materials.

Contributing

If you'd like to help build Bevy, check out the Contributor's Guide. For simple problems, feel free to open an issue or PR and tackle it yourself!

For more complex architecture decisions and experimental mad science, please open an RFC (Request For Comments) so we can brainstorm together effectively!

Getting Started

We recommend checking out The Bevy Book for a full tutorial.

Follow the Setup guide to ensure your development environment is set up correctly. Once set up, you can quickly try out the examples by cloning this repo and running the following commands:

# Switch to the correct version (latest release, default is main development branch)
git checkout latest
# Runs the "breakout" example
cargo run --example breakout

To draw a window with standard functionality enabled, use:

use bevy::prelude::*;

fn main(){
  App::new()
    .add_plugins(DefaultPlugins)
    .run();
}

Fast Compiles

Bevy can be built just fine using default configuration on stable Rust. However for really fast iterative compiles, you should enable the "fast compiles" setup by following the instructions here.

Libraries Used

Bevy is only possible because of the hard work put into these foundational technologies:

  • wgpu: modern / low-level / cross-platform graphics library based on the WebGPU API.
  • glam-rs: a simple and fast 3D math library for games and graphics
  • winit: cross-platform window creation and management in Rust

Bevy Cargo Features

This list outlines the different cargo features supported by Bevy. These allow you to customize the Bevy feature set for your use-case.

Third Party Plugins

Plugins are very welcome to extend Bevy's features. Guidelines are available to help integration and usage.

Thanks and Alternatives

Additionally, we would like to thank the Amethyst, macroquad, coffee, ggez, Fyrox, and Piston projects for providing solid examples of game engine development in Rust. If you are looking for a Rust game engine, it is worth considering all of your options. Each engine has different design goals, and some will likely resonate with you more than others.

This project is tested with BrowserStack.

License

Bevy is free, open source and permissively licensed! Except where noted (below and/or in individual files), all code in this repository is dual-licensed under either:

at your option. This means you can select the license you prefer! This dual-licensing approach is the de-facto standard in the Rust ecosystem and there are very good reasons to include both.

Some of the engine's code carries additional copyright notices and license terms due to their external origins. These are generally BSD-like, but exact details vary by crate: If the README of a crate contains a 'License' header (or similar), the additional copyright notices and license terms applicable to that crate will be listed. The above licensing requirement still applies to contributions to those crates, and sections of those crates will carry those license terms. The license field of each crate will also reflect this. For example, bevy_mikktspace has code under the Zlib license (as well as a copyright notice when choosing the MIT license).

The assets included in this repository (for our examples) typically fall under different open licenses. These will not be included in your game (unless copied in by you), and they are not distributed in the published bevy crates. See CREDITS.md for the details of the licenses of those files.

Your contributions

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.