---

name: sparze description: Expert guidance for Sparze ECS apps including systems, queries, components, resources, events, and performance trade-offs.

Sparze ECS Skill

Expert guidance for building high-performance ECS applications with Sparze.

Core Concepts

World: ECS coordinator with compile-time component, resource, event, and group types.

const World = sparze.World(
    .{ Position, Velocity, Health },     // Components
    .{ DeltaTime, Score },               // Resources
    .{ CollisionEvent },                 // Events
    .{ struct { Position, Velocity } },  // Groups
);

Entity: 32-bit handle [version:16 | index:16]. index selects dense slot; version invalidates stale handles. Use .index, .version, or getIndex() / getVersion(). Create via Entity.init(), serialize with toInt() / fromInt(u32).

Components: Data attached to entities. Tag components (empty structs) use 1 bit per entity.

Resources: Global singletons injected via Resource(T) / ResourceMut(T). Can implement init() / deinit(), or opt out with pub const auto_init = false (manual initialization required). POD resources are zero-initialized for compatibility.

Events: 1-frame delayed, double-buffered communication. Events written in frame N are read in frame N+1.

Docs: @docs/ARCHITECTURE.md, @docs/ENTITY_LIFECYCLE.md

System Functions

System functions use injected parameters and must use commands: anytype for world mutation. Return void or !void.

fn mySystem(
    allocator: std.mem.Allocator,
    movement: Group(struct { Position, Velocity }),
    health: SingleQuery(Health),
    delta: Resource(DeltaTime),
    score: ResourceMut(Score),
    reader: EventReader(CollisionEvent),
    writer: EventWriter(DamageEvent),
    commands: anytype,
) !void {
    // Implementation
}

Commands API

Immediate (returns now):

• commands.createEntity()
• try commands.createEntityWith(.{ Position{...}, Velocity{...} })
• commands.setResource(T, value) / commands.getResource(T) / commands.getResourcePtrMut(T)
• try commands.initResources(.{ ... })
• try commands.serializeToFile(path) / try commands.deserializeFromFile(path)

Deferred (applied at world.endFrame()):

• try commands.addComponent(e, T, value) / try commands.removeComponent(e, T)
• try commands.addTag(e, T) / try commands.removeTag(e, T)
• try commands.destroyEntity(e)

Why deferred: avoids invalidating iterators or shifting dense arrays mid-iteration.

Docs: @docs/SYSTEM_PATTERNS.md

Queries

Decision guide:

• SingleQuery: single component, any frequency
• Query: multi-component, occasional use
• Group: multi-component, hot path

SingleQuery(Component)

fn healthSystem(query: SingleQuery(Health)) !void {
    for (query.entities, query.components) |entity, *health| {
        health.hp = @max(0, health.hp);
    }
}

Query(struct { ... })

fn combatSystem(query: Query(struct { Position, Health, ?Shield })) !void {
    for (query.entities) |entity| {
        if (query.filter(entity)) {
            const pos = query.getComponent(entity, Position);
            const health = query.getComponentMut(entity, Health);
            const shield = query.getOptional(entity, Shield);
            _ = pos; _ = health; _ = shield;
        }
    }
}

Group(struct { ... })

const MovementGroup = struct { Position, Velocity };
const World = sparze.World(.{ Position, Velocity }, .{}, .{}, .{ MovementGroup });

fn movementSystem(group: Group(MovementGroup)) !void {
    const positions = group.getMutArrayOf(Position);
    const velocities = group.getArrayOf(Velocity);
    for (positions, velocities) |*pos, vel| {
        pos.x += vel.x;
        pos.y += vel.y;
    }
}

Free(Component): use in groups that do not own a component. Access via group.getComponent(entity, T) instead of array access.

Tag Queries

fn enemySystem(query: TagQuery(struct { Enemy, ?Boss, Exclude(Dead) })) !void {
    for (query.entities) |entity| {
        if (query.filter(entity)) {
            _ = entity;
        }
    }
}

Docs: @docs/QUERY_PATTERNS.md

Query Modifiers

• Optional (?T): entity may or may not have component
• Exclude(T): filter out entities with component
• Free(T): group does not own component; access by lookup

System Organization

Principles:

• One system, one responsibility
• Writers run before readers
• Use events for loose coupling (1-frame delay)
• Group systems by domain (physics, combat, rendering)

Frame lifecycle:

world.beginFrame();
try world.runSystem(inputSystem);
try world.runSystem(physicsSystem);
try world.runSystem(renderSystem);
try world.endFrame();

Critical: Always call endFrame() to flush deferred commands.

Resources

Initialization options:

• init() / deinit() for auto-lifecycle
• pub const auto_init = false for manual setup
• POD resources are zero-initialized

Safety helpers:

• isResourceInitialized(T)
• tryGetResource(T)
• initResources(.{ ... }) for bulk init

Performance

Fastest to slowest:

1. Group owned components (getMutArrayOf)
2. SingleQuery
3. Group free components (getComponent)
4. Query

Best practices:

• Use Group for hot-path systems
• Use Query for ad-hoc/dynamic access
• Validate group ownership with World.validateGroups()

Storage notes:

• Tag components use 1 bit per entity
• Reserve with getSparseSetPtrMut(T).reserve(capacity)
• Pagination: 4096 entities per page

Docs: @docs/PERFORMANCE.md, @docs/STORAGE_INTERNALS.md

Quick Reference

| Task | API | |------|-----| | Create entity | commands.createEntity() | | Add component | try commands.addComponent(e, T, value) | | Query single | SingleQuery(Health) | | Query multi | Query(struct { Position, Velocity }) | | Fastest iteration | Group(struct { Position, Velocity }) | | Optional component | Query(struct { Position, ?Sprite }) | | Exclude entities | Query(struct { Enemy, Exclude(Dead) }) | | Read resource | Resource(DeltaTime) | | Write resource | ResourceMut(Score) | | Read events | EventReader(CollisionEvent) | | Write events | EventWriter(DamageEvent) | | Tag iteration | SingleTag(Enemy) |

Documentation Index

• @docs/ARCHITECTURE.md
• @docs/QUERY_PATTERNS.md
• @docs/ENTITY_LIFECYCLE.md
• @docs/STORAGE_INTERNALS.md
• @docs/SYSTEM_PATTERNS.md
• @docs/PERFORMANCE.md