ADR-0006 · accepted

Identified pools give stable ids and reuse them

Removing an entity from a pool moves the last entity into its index, so an index can't name an entity across a removal. Push-puzzle must name a box across turns for undo and for its view, so core's createIdentifiedPool gives each entity a stable id, reused after removal, with O(1) idOf, indexOf and addWithId that allocate nothing (#17, finding 3).

Considered Options

• Generational handles (an index plus a generation), the usual answer in entity-component systems. They catch an id used after its entity is gone. Neither genre needs that, so they were left out (PR #22).

Consequences

• A removed entity's id becomes free, and a later add may hand it out again. An id names its entity only while that entity is in the pool.
• addWithId brings a removed entity back under its old id. Push-puzzle's undo uses it for a box that filled a hole. Push-puzzle adds no box after a level starts, so that id is still free.
• createPool is unchanged. Survivors' entities stay anonymous, and its replay hash didn't move.
• Push-puzzle's turn records and its view name boxes by id (ADR-0007, finding 12).
• Revisit if a genre must keep an id after its entity is removed while other entities are still added, such as a unit that targets an enemy that dies. A reused id would then name a different entity, and generational handles would catch it.