Relations

A relation enables adding multiple "components" of the same type to an entity.

Info Relations are not components but both have similar interfaces too add, remove or access them. Relations are not stored within archetypes to avoid archetype fragmentation.

A typical limitation of an archetype-based ECS is that an entity can only contain one component of a certain type. When adding a component of a specific type to an entity already present the component is updated. This is the common behavior implemented by most ECS implementations like EnTT, flecs, BEVY, fennecs, ... .

Terminology A relation in mathematical context describes a connection between the elements of two sets: Set-1 & Set-2. In other words - a relation is a pair (element of Set-1, element of Set-2).

In friflo ECS a relation is a type implementing either IRelation<> or ILinkRelation. An entity containing a relation creates a relation between this entity and the relation key. So Set-1 are always all entities and Set-2 are all possible relation keys.

• A relation implements IRelation<> and is a pair (entity, relation key)

• A relationship implements ILinkRelation and is a pair (entity, linked entity)

Relation

Multiple relations can be added to a single entity and must implement IRelation<>. To distinguish between different relations, a relation type must implement GetRelationKey(). Any type can be used as relation key type. E.g. enum, string, int, long, Guid, DateTime, ... .

Relations are added, removed and queried with

    entity.AddRelation(new InventoryItem { type = ItemType.Coin });
    entity.AddRelation(new InventoryItem { type = ItemType.Axe  });
    entity.RemoveRelation <InventoryItem,ItemType>(ItemType.Coin);
    entity.GetRelations   <InventoryItem>();                        // O(1)
    entity.GetRelation    <InventoryItem,ItemType>(ItemType.Axe);   // O(N)
    // N: number of relations on a single entity

The following example illustrates the state changes of a specific entity regarding its relations. It uses an enum as relation key type.

enum ItemType {
    Coin    = 1,
    Axe     = 2,
}

struct InventoryItem : IRelation<ItemType> { // relation key type: ItemType
    public  ItemType    type;
    public  int         count;
    public  ItemType    GetRelationKey() => type;     // unique relation key
}

public static void Relations()
{
    var store   = new EntityStore();
    var entity  = store.CreateEntity();
    
    // add multiple relations of the same component type
    entity.AddRelation(new InventoryItem { type = ItemType.Coin, count = 42 });
    entity.AddRelation(new InventoryItem { type = ItemType.Axe,  count =  3 });
    
    // Get all relations added to an entity.   O(1)
    entity.GetRelations  <InventoryItem>();                       // { Coin, Axe }
    
    // Get a specific relation from an entity. O(N)
    entity.GetRelation   <InventoryItem,ItemType>(ItemType.Coin); // {type=Coin, count=42}
    
    // Remove a specific relation from an entity
    entity.RemoveRelation<InventoryItem,ItemType>(ItemType.Axe);
    entity.GetRelations  <InventoryItem>();                       // { Coin }
}

The relations of an Entity can be modified within a foreach loop by accessing the elements by ref.

    var relations = entity.GetRelations<InventoryItem>();
    foreach(ref var relation in relations) {
        relation.count += 1;
    }

Important Within loop iteration relations must not be added or removed. Doing this invalidates the iteration result.

Retrieving all entity relations is shown by the example below.

public static void AllRelations()
{
    var store   = new EntityStore();
    var entity1 = store.CreateEntity();
    var entity2 = store.CreateEntity();
    
    // add relations to multiple entities
    entity1.AddRelation(new InventoryItem { type = ItemType.Coin, count = 10 });
    entity2.AddRelation(new InventoryItem { type = ItemType.Coin, count = 20 });
    entity2.AddRelation(new InventoryItem { type = ItemType.Axe,  count =  1 });
    
    var allRelations = store.EntityRelations<InventoryItem>();
    
    // all entities with relations
    var uniqueEntities = allRelations.Entities;     // count: 2
    
    // all entity relation pairs
    var (entities, relations) = allRelations.Pairs; // count: 3
}

Note - Breaking changes since 3.0.0-preview.16

1. To avoid mixing up relations with components accidentally IRelation does not extends IComponent anymore.

2. Renamed public API's IRelationComponent<> -> IRelation<> RelationComponents<> -> Relations<>

Serialization

Serialization of relations as JSON is supported since 3.0.0-preview.17 or higher. Its encoding similar to the serialization of components. In contrast to components relations are serialized as an array of components as a single entity can have multiple relations.

{
    "id": 42,
    "components": {
        "relations": [{"value":42},{"value":43}],
        "component": {"value":42}
    }
}

Benchmark - Relations

The number of link relations / relations per entity should not exceed 100. The reason is that inserting and removing a relation is executed in O(N). N: number of relations per entity.

Benchmark see Tests Add 1.000.000 int relations. Type:

internal struct IntRelation : IRelation<int> {
    public          int     value;
    public          int     GetRelationKey()    => value;
}

relationsPerEntity	duration ms	entities
1	60.17	1000000
2	64.11	500000
4	62.68	250000
8	66.66	125000
16	67.71	62500
32	90.53	31250
64	150.14	15625
128	154.39	7813
256	254.26	3907
512	416.20	1954
1024	772.39	977
2048	1504.50	489
4096	2993.34	245
8192	5867.59	123