acdream/memory/project_collision_port.md

# Collision System Port — Status and Plan

## Current State (2026-04-14)

The collision system has been patched multiple times but does NOT match
retail. The user has explicitly requested a **full faithful port** of
the retail collision system — no shortcuts, no simplifications.

## What Went Wrong

Instead of porting the decompiled code line-by-line (as CLAUDE.md
mandates), I wrote simplified approximations:
- Static overlap instead of swept-sphere FindTimeOfCollision
- Custom FindObjCollisions instead of porting Sphere.IntersectsSphere
- Custom BSP query instead of porting BSPTree.find_collisions dispatcher
- Ad-hoc push-out instead of proper SlideSphere crease-projection
- Incremental patches that don't address root architectural issues

Each patch fixed one symptom but introduced new edge cases. The result
is a patchwork that handles ~60-70% of cases but fails on the rest.

## What Must Happen Next

**Delete the existing collision code and start fresh.** Port from ACE's
complete C# implementation, cross-referencing the decompiled code for
ground truth. ACE has the ENTIRE system already in C#:

### Files to port from ACE (in order):

1. **Sphere.cs** — `IntersectsSphere` (FUN_005387c0), `SlideSphere` (both variants), `StepSphereUp`, `StepSphereDown`, `LandOnSphere`, `CollideWithPoint`, `CollidesWithSphere`

2. **BSPTree.cs** — `find_collisions` (6-path dispatcher)

3. **BSPNode.cs** — `sphere_intersects_poly` (tree traversal with movement), `find_walkable`, `hits_walkable`, `sphere_intersects_solid`

4. **BSPLeaf.cs** — leaf-level polygon tests

5. **Polygon.cs** — `pos_hits_sphere`, `adjust_sphere_to_plane`, `check_walkable`

6. **Transition.cs** — `FindTransitionalPosition`, `TransitionalInsert`, `StepUp`, `StepDown`, `ValidateTransition`, `AdjustOffset`

7. **SpherePath.cs** — `SetCheckPos`, `AddOffsetToCheckPos`, `CacheLocalSpaceSphere`, `SetCollide`, `SetWalkable`, `SetNegPolyHit`

8. **CollisionInfo.cs** — `SetContactPlane`, `SetSlidingNormal`, `SetCollisionNormal`

9. **ObjectInfo.cs** — `ValidateWalkable`

10. **LandCell.cs** — `FindEnvCollisions` (outdoor terrain)

11. **EnvCell.cs** — `FindEnvCollisions` (indoor BSP)

12. **ObjCell.cs** — `FindObjCollisions`, `find_cell_list`

### ACE source locations:
- `references/ACE/Source/ACE.Server/Physics/Sphere.cs`
- `references/ACE/Source/ACE.Server/Physics/BSP/BSPTree.cs`
- `references/ACE/Source/ACE.Server/Physics/BSP/BSPNode.cs`
- `references/ACE/Source/ACE.Server/Physics/BSP/BSPLeaf.cs`
- `references/ACE/Source/ACE.Server/Physics/Polygon.cs`
- `references/ACE/Source/ACE.Server/Physics/Transition.cs`
- `references/ACE/Source/ACE.Server/Physics/SpherePath.cs`
- `references/ACE/Source/ACE.Server/Physics/Collision/CollisionInfo.cs`
- `references/ACE/Source/ACE.Server/Physics/Collision/ObjectInfo.cs`

### Decompiled ground truth:
- `docs/research/decompiled/chunk_00530000.c` — BSP, Polygon, Sphere collision
- `docs/research/decompiled/chunk_00500000.c` — PhysicsObj, transition callers
- `docs/research/acclient_function_map.md` — mapped functions

### Pseudocode (already written):
- `docs/research/transition_pseudocode.md` — full system documented

## What to Keep

- `CollisionPrimitives.cs` — 9 low-level functions already faithfully ported from decompiled code. These are CORRECT and match retail.
- `PhysicsDataCache.cs` — GfxObj/Setup/CellStruct physics data loading from dats. Correct.
- `ShadowObjectRegistry.cs` — cell-based spatial index. Correct concept, may need refinement.
- `TransitionTypes.cs` data structures — SpherePath, CollisionInfo, ObjectInfo, PhysicsGlobals. Mostly correct, may need field additions.
- `PhysicsBody.cs` — Euler integration. Correct.
- `MotionInterpreter.cs` — Motion state machine. Correct.
- `PlayerWeenie.cs` — Run/Jump formulas. Correct.

## What to Replace

- `BSPQuery.cs` — replace with faithful port of BSPTree/BSPNode/BSPLeaf
- `TransitionTypes.cs` Transition methods — replace FindTransitionalPosition, TransitionalInsert, FindEnvCollisions, FindObjCollisions, SlideSphere, AdjustOffset with faithful ports
- `PhysicsEngine.ResolveWithTransition` — may need restructuring

## Approach (MANDATORY — per CLAUDE.md)

For EVERY function:

1. **DECOMPILE FIRST.** Find the matching function in the decompiled
   client (`docs/research/decompiled/`). Use the function map at
   `docs/research/acclient_function_map.md`. If not mapped, search
   by characteristic constants or struct offsets.

2. **CROSS-REFERENCE ACE.** Read ACE's C# port of the same function.
   ACE provides naming and structure. Note any differences.

3. **WRITE PSEUDOCODE.** Translate the decompiled C into readable
   pseudocode BEFORE porting to C#. Add to
   `docs/research/collision_port_pseudocode.md`.

4. **PORT FAITHFULLY.** Translate pseudocode to C# line-by-line.
   Same variable names, same control flow, same boundary conditions.
   Do NOT "improve" or "simplify" the algorithm.

5. **VERIFY.** When ACE and the decompiled code disagree, the
   decompiled code wins. Document the difference.

### Execution order:

1. Sphere collision (Sphere.cs) — FUN_005387c0 and sub-functions
2. BSP tree (BSPTree/Node/Leaf) — find_collisions dispatcher
3. Polygon tests (Polygon.cs) — pos_hits_sphere, adjust_sphere_to_plane
4. Transition orchestrator (Transition.cs) — FindTransitionalPosition
5. Cell collision (LandCell/EnvCell/ObjCell) — FindEnvCollisions, FindObjCollisions
6. Wire into PhysicsEngine.ResolveWithTransition
7. Test: terrain → indoor walls → objects → step-up → every object type