Add claude docs

2026-06-16 10:15:59 -05:00
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+# Physics System
+
+Source: `src/dusk/physics/`, entity component at
+`src/dusk/entity/component/physics/entityphysics.h/.c`
+
+## Overview
+
+Dusk uses a lightweight, custom 3D physics simulation with no external
+library dependency. It is integrated with the ECS: only entities that
+have both a `COMPONENT_TYPE_PHYSICS` and a `COMPONENT_TYPE_POSITION`
+component participate in the simulation.
+
+## Shapes
+
+```c
+typedef enum {
+  PHYSICS_SHAPE_CUBE,     // Axis-aligned bounding box (AABB)
+  PHYSICS_SHAPE_SPHERE,
+  PHYSICS_SHAPE_CAPSULE,  // Y-axis aligned; radius + halfHeight
+  PHYSICS_SHAPE_PLANE,    // Infinite plane; normal + distance
+} physicshapetype_t;
+```
+
+All shape pairs are supported by the collision dispatch
+(`physicsTestShapeVsShape`). See `physicstest.h` for the individual
+test functions.
+
+## Body types
+
+```c
+typedef enum {
+  PHYSICS_BODY_STATIC,     // Never moves; immovable collision surface
+  PHYSICS_BODY_DYNAMIC,    // Driven by gravity, velocity, collisions
+  PHYSICS_BODY_KINEMATIC,  // Moved programmatically; collides but not
+                            // driven by the simulation (e.g. player)
+} physicsbodytype_t;
+```
+
+## World and gravity
+
+```c
+extern physicsworld_t PHYSICS_WORLD;
+// PHYSICS_WORLD.gravity -- default {0, -9.81, 0}
+```
+
+The simulation step is driven by `physicsManagerUpdate()`, which is
+called each fixed-timestep game loop tick. It skips dynamic-timestep
+sub-steps (`DUSK_TIME_DYNAMIC`).
+
+## Simulation phases (each step)
+
+1. **Integrate dynamics** -- apply gravity scaled by `gravityScale`,
+   advance velocity, update position.
+2. **Dynamic vs static/kinematic** -- resolve penetration and cancel
+   the normal velocity component.
+3. **Dynamic vs dynamic** -- split penetration 50/50; exchange
+   relative normal velocity.
+4. **Rebuild transforms** -- call `entityPositionRebuild()` for all
+   affected dynamic bodies.
+
+`PHYSICS_GROUND_THRESHOLD = 0.707f` -- a collision normal with a Y
+component above this value sets `onGround = true` on the dynamic body.
+
+## Entity component (`entityphysics_t`)
+
+```c
+typedef struct {
+  physicsbodytype_t type;
+  physicsshape_t shape;
+  vec3 velocity;
+  float_t gravityScale;  // default 1.0
+  bool_t onGround;       // set by the solver each step
+} entityphysics_t;
+```
+
+Default on init: DYNAMIC body, 0.5m half-extents AABB cube,
+`gravityScale = 1.0f`.
+
+### Component API
+
+```c
+entityphysics_t *entityPhysicsGet(entityid_t, componentid_t);
+
+void  entityPhysicsSetShape(entityid_t, componentid_t, physicsshape_t);
+physicsshape_t entityPhysicsGetShape(entityid_t, componentid_t);
+
+void  entityPhysicsSetVelocity(entityid_t, componentid_t, vec3);
+void  entityPhysicsGetVelocity(entityid_t, componentid_t, vec3 dest);
+void  entityPhysicsApplyImpulse(entityid_t, componentid_t, vec3);
+// No-op on STATIC bodies.
+
+bool_t entityPhysicsIsOnGround(entityid_t, componentid_t);
+void   entityPhysicsSetBodyType(entityid_t, componentid_t, physicsbodytype_t);
+physicsbodytype_t entityPhysicsGetBodyType(entityid_t, componentid_t);
+```
+
+## Collision detection primitives (`physicstest.h`)
+
+Each function returns `true` if overlapping and writes the push-out
+normal (pointing from B toward A) and penetration depth.
+
+| Function | Shapes |
+|----------|--------|
+| `physicsTestAabbVsAabb` | CUBE vs CUBE |
+| `physicsTestSphereVsSphere` | SPHERE vs SPHERE |
+| `physicsTestSphereVsAabb` | SPHERE vs CUBE |
+| `physicsTestSphereVsPlane` | SPHERE vs PLANE |
+| `physicsTestAabbVsPlane` | CUBE vs PLANE |
+| `physicsTestCapsuleVsSphere` | CAPSULE vs SPHERE |
+| `physicsTestCapsuleVsAabb` | CAPSULE vs CUBE |
+| `physicsTestCapsuleVsPlane` | CAPSULE vs PLANE |
+| `physicsTestCapsuleVsCapsule` | CAPSULE vs CAPSULE |
+| `physicsTestShapeVsShape` | Any pair via dispatch |
+
+Capsules are always Y-axis aligned. Planes are infinite (not half-spaces).
+
+## Limitations and known gaps
+
+- No rotation simulation -- bodies do not rotate from collisions.
+- No friction or damping model yet.
+- No sleeping / deactivation for resting bodies.
+- No broad-phase culling: the solver is O(n^2) per phase.
+  This is acceptable up to the ECS entity limit (64 entities) but must
+  be revisited if the entity count grows.
+- Capsule vs plane uses the bottom/top hemisphere centers as a
+  degenerate approximation -- accurate for large planes but
+  not for thin surfaces.