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# Objective - Implements change described in https://github.com/bevyengine/bevy/issues/3022 - Goal is to allow Entity to benefit from niche optimization, especially in the case of Option<Entity> to reduce memory overhead with structures with empty slots ## Discussion - First PR attempt: https://github.com/bevyengine/bevy/pull/3029 - Discord: https://discord.com/channels/691052431525675048/1154573759752183808/1154573764240093224 ## Solution - Change `Entity::generation` from u32 to NonZeroU32 to allow for niche optimization. - The reason for changing generation rather than index is so that the costs are only encountered on Entity free, instead of on Entity alloc - There was some concern with generations being used, due to there being some desire to introduce flags. This was more to do with the original retirement approach, however, in reality even if generations were reduced to 24-bits, we would still have 16 million generations available before wrapping and current ideas indicate that we would be using closer to 4-bits for flags. - Additionally, another concern was the representation of relationships where NonZeroU32 prevents us using the full address space, talking with Joy it seems unlikely to be an issue. The majority of the time these entity references will be low-index entries (ie. `ChildOf`, `Owes`), these will be able to be fast lookups, and the remainder of the range can use slower lookups to map to the address space. - It has the additional benefit of being less visible to most users, since generation is only ever really set through `from_bits` type methods. - `EntityMeta` was changed to match - On free, generation now explicitly wraps: - Originally, generation would panic in debug mode and wrap in release mode due to using regular ops. - The first attempt at this PR changed the behavior to "retire" slots and remove them from use when generations overflowed. This change was controversial, and likely needs a proper RFC/discussion. - Wrapping matches current release behaviour, and should therefore be less controversial. - Wrapping also more easily migrates to the retirement approach, as users likely to exhaust the exorbitant supply of generations will code defensively against aliasing and that defensive code is less likely to break than code assuming that generations don't wrap. - We use some unsafe code here when wrapping generations, to avoid branch on NonZeroU32 construction. It's guaranteed safe due to how we perform wrapping and it results in significantly smaller ASM code. - https://godbolt.org/z/6b6hj8PrM ## Migration - Previous `bevy_scene` serializations have a high likelihood of being broken, as they contain 0th generation entities. ## Current Issues - `Entities::reserve_generations` and `EntityMapper` wrap now, even in debug - although they technically did in release mode already so this probably isn't a huge issue. It just depends if we need to change anything here? --------- Co-authored-by: Natalie Baker <natalie.baker@advancednavigation.com> |
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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.
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.
Thanks
Bevy is the result of the hard work of many people. A huge thanks to all Bevy contributors, the many open source projects that have come before us, the Rust gamedev ecosystem, and the many libraries we build on.
A huge thanks to Bevy's generous sponsors. Bevy will always be free and open source, but it isn't free to make. Please consider sponsoring our work if you like what we're building.
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:
- MIT License (LICENSE-MIT or http://opensource.org/licenses/MIT)
- Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
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.