Finish basic scene graph implementation

Sources/FirebladeECS/Family.swift

@@ -134,25 +134,52 @@ extension Family.EntityComponentIterator: LazySequenceProtocol { }
 // MARK: - relatives iterator
 
 extension Family {
-    @inlinable public var descendRelatives: RelativesIterator {
-        return RelativesIterator(family: self)
+    @inlinable
+    public func descendRelatives(from root: Entity) -> RelativesIterator {
+        return RelativesIterator(family: self, root: root)
     }
 
     public struct RelativesIterator: IteratorProtocol {
-        @usableFromInline var memberIdsIterator: UnorderedSparseSetIterator<EntityIdentifier>
         @usableFromInline unowned let nexus: Nexus
+        @usableFromInline let familyTraits: FamilyTraitSet
 
-        public init(family: Family<R>) {
+        @usableFromInline var relatives: [(EntityIdentifier, EntityIdentifier)]
+
+        public init(family: Family<R>, root: Entity) {
             self.nexus = family.nexus
-            memberIdsIterator = family.memberIds.makeIterator()
+            self.familyTraits = family.traits
+
+            // FIXME: this is not the most efficient way to aggregate all parent child tuples
+            // Problems:
+            // - allocates new memory
+            // - needs to be build on every iteration
+            // - relies on isMember check
+            self.relatives = []
+            self.relatives.reserveCapacity(family.memberIds.count)
+            aggregateRelativesBreathFirst(root.identifier)
+            relatives.reverse()
+        }
+
+        mutating func aggregateRelativesBreathFirst(_ parent: EntityIdentifier) {
+            guard let children = nexus.parentChildrenMap[parent] else {
+                return
+            }
+            children
+                .compactMap { child in
+                    guard nexus.isMember(child, in: familyTraits) else {
+                        return nil
+                    }
+                    relatives.append((parent, child))
+                    return child
+                }
+            .forEach { aggregateRelativesBreathFirst($0) }
         }
 
         public mutating func next() -> R.RelativesDescending? {
-            guard let parentId = memberIdsIterator.next() else {
+            guard let (parentId, childId) = relatives.popLast() else {
                 return nil
             }
-
-            return nil
+            return R.relativesDescending(nexus: nexus, parentId: parentId, childId: childId)
         }
     }
 }