Entity-Component System (ECS)

Object-oriented games typically use class hierarchies:

Entity
├── Player(health, speed)
├── Enemy(health, attack)
└── Bullet(velocity, damage)

Problems:

• Class explosion (PoisonedFlyingPlayer?).
• Diamond inheritance.
• Cache-unfriendly (each entity is allocation).

ECS: composition over inheritance.

• Entity: just an ID (integer).
• Component: pure data, attached to entities.
• System: function operating on entities with specific components.

Systems:

Pros:

• Composition: any combination of components.
• Cache-friendly: store components in arrays (SoA).
• Parallelizable: systems independent.
• Simple to add/remove behavior at runtime.

Cons:

• Indirection (entity ID → components).
• Less natural for traditional OOP folks.

Modern ECS frameworks:

• Bevy (Rust): default.
• EnTT (C++): widely-used.
• flecs (C): standalone.
• Unity DOTS: Unity's ECS (in transition).

Archetype-based ECS:

• Group entities by exact set of components.
• Each archetype = an array of entities + arrays of components.
• Iteration is contiguous memory access.
• Faster than dict-based.