std/ecs/hierarchy

Whose space an entity's transform is written in. A hierarchy is a component, not a second structure: the column of Parent is the tree, and depthOrder computes the order a pass over it needs.

var parents: Column<Parent> = Column()
parents.put Entity(1), Parent(Entity(0))
assert(parents.get(Entity(1)) == Some(Parent(Entity(0))))

entities in an order where every parent comes before its children, each one listed once. Entities that do not depend on each other keep the order they have in entities; a parent that is not in entities is not listed, but its place in the chain still orders the entities below it.

A chain of any depth is walked with a loop, not a recursion. A cycle in parents is a bug in the program and cannot be prevented by the type system, so it panics with the entity the walk found it at and does not break it quietly.

When the parents form a cycle, with the parents of <entity> form a cycle: <ancestor> is its own ancestor.

var parents: Column<Parent> = Column()
const root = Entity 0
const child = Entity 1
const grandchild = Entity 2
parents.put grandchild, Parent(child)
parents.put child, Parent(root)
assert(depthOrder(parents, [grandchild, child, root]) == [root, child, grandchild])