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Reuse seeded rng in tilemap_chunk for more determinism (#19812)

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# Objective

This example uses a seeded RNG for the initial setup, but new unseeded
RNGs during each timed update.

This is causing the example to produce different output each time in the
[example report](https://bevyengine.github.io/bevy-example-runner/).

<img width="838" alt="image"
src="https://github.com/user-attachments/assets/05d78083-97d0-4bcf-aebc-f645b570d144"
/>

## Solution

Store and reuse the RNG, following the pattern used in other examples.

## Testing

`cargo run --example tilemap_chunk`
This commit is contained in:
Rob Parrett 2025-06-25 09:10:04 -07:00 committed by GitHub
parent dd4479ed30
commit 35f1af522a
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GPG Key ID: B5690EEEBB952194
1 changed files with 14 additions and 3 deletions
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17
examples/2d/tilemap_chunk.rs
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@ -9,7 +9,7 @@ use rand_chacha::ChaCha8Rng;
fn main() {
App::new()
.add_plugins((DefaultPlugins.set(ImagePlugin::default_nearest()),))
.add_plugins(DefaultPlugins.set(ImagePlugin::default_nearest()))
.add_systems(Startup, setup)
.add_systems(Update, (update_tileset_image, update_tilemap))
.run();
@ -18,8 +18,14 @@ fn main() {
#[derive(Component, Deref, DerefMut)]
struct UpdateTimer(Timer);
#[derive(Resource, Deref, DerefMut)]
struct SeededRng(ChaCha8Rng);
fn setup(mut commands: Commands, assets: Res<AssetServer>) {
// We're seeding the PRNG here to make this example deterministic for testing purposes.
// This isn't strictly required in practical use unless you need your app to be deterministic.
let mut rng = ChaCha8Rng::seed_from_u64(42);
let chunk_size = UVec2::splat(64);
let tile_display_size = UVec2::splat(8);
let indices: Vec<Option<u16>> = (0..chunk_size.element_product())
@ -39,6 +45,8 @@ fn setup(mut commands: Commands, assets: Res<AssetServer>) {
));
commands.spawn(Camera2d);
commands.insert_resource(SeededRng(rng));
}
fn update_tileset_image(
@ -55,12 +63,15 @@ fn update_tileset_image(
}
}
fn update_tilemap(time: Res<Time>, mut query: Query<(&mut TilemapChunkIndices, &mut UpdateTimer)>) {
fn update_tilemap(
time: Res<Time>,
mut query: Query<(&mut TilemapChunkIndices, &mut UpdateTimer)>,
mut rng: ResMut<SeededRng>,
) {
for (mut indices, mut timer) in query.iter_mut() {
timer.tick(time.delta());
if timer.just_finished() {
let mut rng = ChaCha8Rng::from_entropy();
for _ in 0..50 {
let index = rng.gen_range(0..indices.len());
indices[index] = Some(rng.gen_range(0..5));