Bevy procedural generation

When to use

Load this when a level, loot table, or map should be reproducible from a seed, and you need the ECS shape. Spawning the finished layout uses Commands on Startup or a later schedule you already use. Saving the seed belongs on bevy-save-load.

ECS shape

• Components: cell facts you define, spawned with Commands.
• Resources: one RunSeed for the whole run. The generator reads it; it does not invent a Bevy RNG type. rand 0.9 supplies StdRng.
• Systems: insert the seed on Startup, then spawn from that data. .chain() so the spawn system sees the seed.
• Schedule: Startup for a one-shot map. A streaming map is a later Update system that still reads the same seed.

[dependencies]
bevy = "0.20"
rand = "0.9"

The seed is a resource. The walk is plain data. Entities are spawned only after the seed exists.

use rand::rngs::StdRng;
use rand::{Rng, SeedableRng};

#[derive(Resource)]
struct RunSeed(u64);

#[derive(Component, Clone, Copy)]
struct FloorCell {
    x: i32,
    y: i32,
}

fn insert_seed(mut commands: Commands) {
    commands.insert_resource(RunSeed(42));
}

fn spawn_walk(mut commands: Commands, seed: Res<RunSeed>) {
    let mut rng = StdRng::seed_from_u64(seed.0);
    let mut x = 0;
    let mut y = 0;
    for _ in 0..24 {
        commands.spawn((
            FloorCell { x, y },
            Transform::from_xyz(x as f32, y as f32, 0.0),
        ));
        match rng.random_range(0..4) {
            0 => x += 1,
            1 => x -= 1,
            2 => y += 1,
            _ => y -= 1,
        }
    }
}

pub struct DungeonPlugin;

impl Plugin for DungeonPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(Startup, (insert_seed, spawn_walk).chain());
    }
}

Height samples stay in a resource. The same seed always rebuilds the same grid. Sampling happens once, not every frame.

#[derive(Resource)]
struct HeightGrid {
    width: i32,
    samples: Vec<f32>,
}

#[derive(Resource)]
struct RunSeed(u64);

fn hash_unit(x: i32, y: i32, seed: u64) -> f32 {
    let mut n = seed
        .wrapping_mul(0x9E37_79B9_7F4A_7C15)
        ^ (x as u64).wrapping_mul(0xBF58_476D_1CE4_E5B9)
        ^ (y as u64).wrapping_mul(0x94D0_49BB_1331_11EB);
    n = (n ^ (n >> 30)).wrapping_mul(0xBF58_476D_1CE4_E5B9);
    n = (n ^ (n >> 27)).wrapping_mul(0x94D0_49BB_1331_11EB);
    n ^= n >> 31;
    (n >> 40) as f32 / (1u32 << 24) as f32
}

fn build_heights(mut commands: Commands, seed: Res<RunSeed>) {
    let width = 16;
    let mut samples = Vec::with_capacity((width * width) as usize);
    for y in 0..width {
        for x in 0..width {
            samples.push(hash_unit(x, y, seed.0));
        }
    }
    commands.insert_resource(HeightGrid { width, samples });
}

Rooms are rectangles carved into a grid you own. Spawn markers only for floor cells.

struct Room {
    x: i32,
    y: i32,
    w: i32,
    h: i32,
}

fn carve(width: i32, rooms: &[Room]) -> Vec<bool> {
    let mut floor = vec![false; (width * width) as usize];
    for room in rooms {
        for y in room.y..room.y + room.h {
            for x in room.x..room.x + room.w {
                if (0..width).contains(&x) && (0..width).contains(&y) {
                    floor[(y * width + x) as usize] = true;
                }
            }
        }
    }
    floor
}

#[derive(Component, Clone, Copy)]
struct FloorCell {
    x: i32,
    y: i32,
}

fn spawn_rooms(mut commands: Commands) {
    let rooms = [
        Room { x: 1, y: 1, w: 4, h: 3 },
        Room { x: 8, y: 6, w: 5, h: 4 },
    ];
    let width = 16;
    for (i, open) in carve(width, &rooms).into_iter().enumerate() {
        if open {
            let x = (i as i32) % width;
            let y = (i as i32) / width;
            commands.spawn(FloorCell { x, y });
        }
    }
}

Choose the owner

Need	Owner
Seeded run	A Resource holding a seed you define
Chunk or dungeon grid	Your components, spawned with Commands
Hash, rooms, or walk	Your functions over a RunSeed. rand::rngs::StdRng is the seeded generator.
Floor cells	Entities spawned from the grid. Tilemaps belong on the tile skill.
Nav bake after generation	Nav bake after generation belongs on bevy-navigation.

Scope

• Seeded walks and room carves above use rand 0.9 and your own grid. Bevy does not ship a dungeon solver.
• Build the grid on the main schedule, then spawn. Commands stay on the system that runs there.
• Navigation rebake after a generated map belongs on bevy-navigation.

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

Piece	Owner	Who may mutate	Schedule
Data on one entity	Component	One system via Query	Update or FixedMain
One value for the whole app	Resource	One system	Startup to insert, then the schedule that reads it
Spawn	Commands	The spawning system	Startup for the initial world; later schedules for runtime spawn
Order that must not race	.chain()	The chained systems only	Same schedule
Plugin boundary	Plugin	build registers systems and resources	Before the app runs

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

If you need	Do this	Do not
Per-entity data	Component + Query	A value that is not a component
App-wide config or clock	Resource	Copy the value onto every entity
Stable simulation step	FixedMain and the Fixed clock	Tie simulation to raw frame Update
Startup spawn	Commands in Startup	Spawn from a render system
Cross-system notice that is not shared state	Message or observer slot in bevy-events-messaging	A new event type invented in this file
An API outside the documented set	Check the official docs first	A made-up crate or API

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.

• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.

• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• No Godot noise, tile, or thread-pool types. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No invented noise, wave-function, or chunk APIs. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No named third-party generation crate as the chosen tool. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.0.

Official Documentation

• Assets - Official asset catalog.
• Examples - Example tree on the latest branch.
• ECS - Component, Commands, Startup, and .chain().
• Resources - App-wide seed state. API limits are on bevy-data-resources.

Related Skills

Prerequisites

• bevy-master - ECS ownership.
• bevy-data-resources - Resource versus Component.
• bevy-ecs-composition - Spawn tuples and .chain().

Complements

• bevy-asset-scenes - Spawn a scene asset. Not a generator.
• bevy-save-load - Seed persistence.
• bevy-navigation - No bake after a generated map.
• godot-procedural-generation - Equivalent Godot skill.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.