Ymir

Ymir is the GameCult Box3D wrapper and physics-daemon boundary for Aetheria and future world-bearing projects. The name means Not Invented Here. We are taking the hint.

Box3D owns physics algorithms. Ymir owns the service boundary that makes those algorithms usable as durable, typed GameCult world truth. Unity, renderers, and game products consume Ymir state and contact facts; they do not become physics authorities.

Ownership

• Box3D owns rigid-body integration, collision geometry, broadphase, solver behavior, continuous collision, overlap and cast semantics, contact lifecycle, force and torque lifetime, tolerances, and deterministic step behavior for identical recorded inputs.
• Ymir currently owns the pinned Box3D version, stable GameCult entity ids, native ABI isolation, public in-process retained sessions, explicit mutation receipts, typed contact facts, snapshot lowering, deterministic result ordering, versioned CultCache snapshots, and a diagnostic Eve publication.
• The Ymir daemon will own named session routing, durable command receipts, checkpoint reconstruction, CultMesh command/fact publication, and process supervision. Those are the daemon cutover contract, not a claim that the current CLI already exposes them.
• Aetheria owns gameplay policy and supplies typed physical intent: bodies, fields, tractor targets, forces, projectiles, filters, and the gameplay consequences of Ymir contact facts.
• CultMath remains available for GameCult-side numeric projections and force preparation. It is not a second collision or integration engine.
• Unity and Eve lowerers render world state and submit commands. They never repair, override, or independently simulate authoritative physics.

Box3D opaque handles are process-local derived state. They are never durable entity ids and never cross the public contract boundary.

Current Cutover State

Box3D v0.1.0 is pinned at commit 8441b4a06d6d09dcfb0b0f704df4d847d1437b92. A Ymir-owned native shim and parity suite already witness released Box3D behavior for:

• sphere overlap and shape casts
• tractor capsule membership and rounded caps
• numeric overlap and cast slop
• planar sphere-world stepping
• contact-begin facts
• restitution mixing
• transient torque lifetime
• category/mask/group collision filtering
• bullet CCD against kinematic bodies, including Box3D's next-step contact-event timing

The production Ymir.Box3D boundary owns the native session ABI. The legacy snapshot YmirSimulator.Step contract now creates an isolated Box3D session, projects the result, and disposes it. It contains no managed integrator, collision loop, or impulse solver.

That snapshot facade deliberately does not pretend to own long-lived world identity. The public in-process YmirSession now exposes explicit body mutations, force and torque commands, revisioned receipts, retained stepping, and typed Begin/Hit/End facts. It never treats omission as removal. The daemon still lacks a named session registry and CultMesh command lowering. Aetheria will name one session per run and zone through that daemon contract; its old process-wide simulator objects are not session identifiers. Retained-world queries, checkpoint reconstruction, and non-Windows RID artifacts remain cutover work.

See the architecture map and the executable parity contract. The retained-session contract records the current ownership cut and the public daemon boundary being built.

Build And Test

Initialize the pinned dependency and run the solution:

git submodule update --init --recursive
dotnet test Ymir.slnx

The parity project builds the native shim from source and requires CMake plus a C17 compiler:

dotnet test tests\Ymir.Box3D.Parity\Ymir.Box3D.Parity.csproj

Box3D is MIT licensed. Its license is retained in extern/box3d.

GameCult.Ymir.Box3D packages the managed boundary, Box3D license, and native facade under runtimes/<rid>/native. A local Windows x64 package has loaded in an isolated empty consumer without CMake or the submodule. A committed release smoke is still required before that is a repeatable packaging guarantee. Linux and macOS packages must be produced and smoked on their own release workers before those RIDs are advertised.

The current package version is 0.3.0. ABI v4 exposes Box3D body type, bullet, field-participation, and collision-filter configuration without exposing Box3D handles. These are Ymir contract fields, but their behavior is Box3D behavior.

Cutover Rule

A Ymir cutover is complete only when all runtime stepping, collision queries, and contact facts come from Box3D through an explicitly owned Ymir session. Manual Unity callbacks and Aetheria geometry tests must be structurally unable to decide the result. Snapshot compatibility calls may use isolated sessions; they are not allowed to infer or share retained-world identity.