Changelog

v0.1.2 — 2026-09-26​

Fixed​

• Deleting an entity whose cascade runs hooks that write into it (a child whose OnRemove hook sets a component of its parent while the parent is deleted) no longer leaves those ids on the freed slot: the deleted entity stayed in queries as a dead id, and the next entity that reused the slot inherited the ids.

Installation​

• Wally: mercs = "dubalda/mercs@0.1.2".
• Roblox model: mercs.rbxm, attached to this release.

v0.1.1 — 2026-09-26​

The Wally package gets a description, a homepage and a repository link; the code is the same as in v0.1.0.

Installation​

• Wally: mercs = "dubalda/mercs@0.1.1".
• Roblox model: mercs.rbxm, attached to this release.

v0.1.0 — 2026-09-26​

The first release of mErCS, an entity component system for Luau and Roblox built on inverted bitsets. Every component, tag and pair keeps a bitset of the entities that have it, and the values sit in pages indexed by the entity slot: adding or removing a component sets a bit and a value, never moves the entity and never creates anything per combination of components. The API follows jecs.

Highlights​

• O(1) structural changes. Tags for states, relationships, spawning and despawning cost the same whatever the entity has; memory does not grow with the number of component combinations, and there is nothing to clean up.
• The jecs feature set. Worlds, entities, components and tags, pairs and wildcards, Exclusive, cleanup policies (OnDelete / OnDeleteTarget with Delete / Remove), ChildOf hierarchies, hooks (OnAdd, OnChange, OnRemove), signals (world:added / changed / removed), Name, pre-registered ids.
• Beyond jecs.
  • query:each(fn), the fastest loop, and query:any(...) OR terms.
  • Batch operations on the matches of a query: count, add_all, set_all, remove_all, delete_all, plus world:remove_all(id).
  • Change tracking by ticks: world:track, world:tick, and the query filters added, changed, removed.
  • The Disabled tag, hierarchies of any depth, no limit on the number of components.
  • get / has with up to 8 ids (get_list / has_all for more), a fast path over the raw columns (query:spans, world:column), and debug worlds that raise errors for dead entities and ids.
• Typed API. Component<T> carries the data type into get, set and queries; the module is --!strict and --!native.
• jabby. The debugger connects through the adapter mErCS.jabby, a child module that the library itself never requires.

Performance​

Measured against jecs 0.11; time of mErCS relative to jecs, lower is better.

• A long session with changing relationship targets: the heap stays flat (+0.4 MB), while jecs grows by 15 MB in 5000 frames.
• An entity with 4 components takes 130 bytes (jecs: 320).
• Still slower than jecs: a for-in over sparse matches (about 1.2–1.5×; query:each is faster than the jecs loop), has with 4 ids in the interpreter, and many pairs of one relation with scattered targets.

All numbers, the benchmark sources and the Benchmarker screenshots are in the comparison with jecs.

Coming from jecs​

• 125 of the 125 applicable cases of the jecs test suite pass.
• A few jecs calls are not provided because they would do nothing here or only repeat native calls: World.new, w, bulk_insert / bulk_remove, new / new_w_id / new_low_id, world:cleanup(), query:fini(), ArchetypeCreate / ArchetypeDelete.
• query:iter(), query:archetypes(), world.entity_index, entity_index_try_get and is_tag exist only in the jabby adapter.
• The builtin ids after Name have other numbers: Exclusive is 268 (jecs: 270), Rest is 270 (jecs: 271).
• The migration guide lists what to write instead of each call.

Installation​

• Wally: mErCS = "dubalda/mercs@0.1.0".
• Roblox model: mercs.rbxm, attached to this release.
• Or copy src/init.luau into a ModuleScript (src/jabby.luau is the optional jabby adapter).

Documentation​

• Documentation site: the API reference and the guide.
• Guide in the repository.

Status​

Pre-release (0.x): the API may still change. Tested with the standalone luau 0.703 CLI (the interpreter and native code generation), in CI on Linux, and in Roblox Studio with the Benchmarker plugin and the jabby debugger. A full check in a running Roblox place (server and client) is still pending.