# Phase L.2 - Movement & Collision Conformance

**Status:** ACTIVE planning document, created 2026-04-29.
**Roadmap owner:** Phase L.2 in `docs/plans/2026-04-11-roadmap.md`.
**Scope:** player movement prediction, retail collision/transition behavior,
building boundaries, edge and wall sliding, cell ownership, outbound movement
packets, and server-correction diagnostics.

## Purpose

Phase B.3 shipped the first usable physics foundation: terrain contact,
basic resolver behavior, streaming-populated collision inputs, and enough
movement wire support to walk on ACE. That was not the complete retail
collision system.

Phase L.2 is the conformance program that turns that foundation into a
retail-faithful movement stack. It is the single organizing bucket for work
that otherwise looks scattered across B.3 physics, L.1 animation/motion, and
G.3 dungeon/portal space.

The active movement spine is:

```text
input + motion command
  -> local body prediction / root-motion source
  -> PhysicsEngine.ResolveWithTransition
  -> TransitionTypes + BSPQuery + ShadowObjectRegistry contact/cell result
  -> MoveToState / AutonomousPosition outbound packets
  -> server echo or correction diagnostics
```

Live ACE accepting a position, or the absence of visible rubber-banding, is
not proof of retail collision parity. ACE can tolerate coarse or locally
invalid fine-grained movement. L.2 therefore requires retail-decomp evidence,
synthetic conformance tests, real-DAT fixtures, and live retail-observer checks.

## Current Foundation

Already active in acdream:

- `PhysicsEngine.ResolveWithTransition` is the local player collision path.
- `BSPQuery` contains a partial retail-style BSP dispatcher and step/contact
  logic.
- `TransitionTypes` carries `SpherePath`, `CollisionInfo`, `ObjectInfo`,
  transition validation, step-up/down, and partial slide behavior.
- `PhysicsDataCache` loads GfxObj, Setup, and CellStruct physics data from DATs.
- `ShadowObjectRegistry` gives the resolver a broadphase over nearby world
  objects.
- `TerrainSurface` uses triangle-aware terrain sampling rather than the older
  bilinear placeholder.

Known incomplete areas:

- Full `CELLARRAY` ownership and `CObjCell::find_cell_list` / adjacent-cell
  checks are not ported.
- `cell_bsp` / `CellBSP` is not fully represented as a first-class runtime
  owner.
- Building entry/exit and indoor/outdoor portal transit are not solved by the
  normal walking path.
- Retail `edge_slide`, `cliff_slide`, and `precipice_slide` behavior is
  incomplete; failed edge/step-down cases often hard-block instead of sliding.
- `NegPolyHit` handling is a stub relative to the retail transition dispatch.
- Live entities collapse to a simplified cylinder shape; exact retail
  sphere/cylsphere and object-shape behavior is not yet matched.
- Outbound contact/cell fields can be too optimistic, so server agreement does
  not necessarily mean local conformance.

## Lane Model

L.2 uses five working lanes. The roadmap breaks them into six sub-lanes because
real-DAT and live verification spans every lane.

| Lane | Owns | Roadmap slice |
|---|---|---|
| Diagnostics | Truth probes, dump flags, server-correction logging, retail observer harness | L.2a, L.2f |
| Transition parity | `FindTransitionalPosition`, step-up/down, edge-slide, cliff-slide, precipice-slide, `NegPolyHit` dispatch | L.2c |
| Geometry fidelity | `CSphere`, `CCylSphere`, object shape extraction, building object collision, walkable polygon context | L.2d |
| Cell/building ownership | outdoor cell seams, low-cell id updates, `CELLARRAY`, `cell_bsp`, building entry/exit | L.2e |
| Movement/network authority | contact byte, full cell id, MoveToState / AutonomousPosition cadence, root motion vs velocity prediction, correction response | L.2b, L.2f |

## Roadmap Slices

### L.2a - Truth & Diagnostics

Goal: make every bad movement outcome explainable.

- Add targeted diagnostics for local placement, contact plane, object hit,
  water, cell id, outbound packet fields, server echo, and correction delta.
- Keep diagnostics opt-in via env vars and devtools panels.
- Record enough data for side-by-side retail-observer runs without drowning
  normal logs.
- Build real-DAT fixture capture for known walls, building ledges, rooftops,
  slopes, landblock seams, and dungeon entrances.

Current shipped slices:

- 2026-04-30: cdb + TTD retail-observer toolchain (`tools/pdb-extract/`,
  `tools/ttd-record.ps1`, `tools/ttd-query.ps1`) with PDB pairing checker
  and ring-buffer trace replay. The "retail observer harness" line item.
- 2026-04 (pre-L.2 rename): `ACDREAM_DUMP_MOVE_TRUTH` paired
  outbound/server-echo dumper in `GameWindow` covers outbound packet
  fields + server echo + correction delta with cell-id mismatch.
- Pre-L.2: scenario-specific dumps `ACDREAM_DUMP_MOTION`,
  `ACDREAM_DUMP_STEEP_ROOF`, `ACDREAM_DUMP_STEPUP`,
  `ACDREAM_DUMP_EDGE_SLIDE` for the codepaths hit during prior bug chases.
- 2026-05-12 (slice 1): general-purpose probes via new
  `AcDream.Core.Physics.PhysicsDiagnostics` static class.
  `ACDREAM_PROBE_RESOLVE` emits one `[resolve]` line per
  `PhysicsEngine.ResolveWithTransition` call (input/output pos+cell,
  ok-vs-partial, grounded-in, contact-plane status, wall normal if hit,
  walkable polygon valid, moving entity id).
  `ACDREAM_PROBE_CELL` emits one `[cell-transit]` line per
  `PlayerMovementController.CellId` change with old→new + position +
  reason tag (`resolver`/`teleport`). Both flippable live via the
  DebugPanel "Diagnostics" section — checkbox toggles take effect on
  the next resolve, no relaunch required.

Remaining L.2a work: contact-plane probe (general, not just steep-roof),
ShadowObjectRegistry hit log ("you collided with entity X"), water probe,
real-DAT fixture-capture pipeline, and folding the older sticky-at-startup
`ACDREAM_DUMP_*` flags into `PhysicsDiagnostics` for unified runtime
toggling.

### L.2b - Movement Wire / Contact Authority

Goal: stop sending movement packets that claim more certainty than the local
resolver has earned.

- Fix outbound contact state so `AutonomousPosition` and `MoveToState` do not
  always claim grounded contact.
- Track local result cell id and outbound full cell id separately from the last
  server placement until correction proves they agree.
- Reconcile packet cadence with retail/holtburger references.
- Wire routine server correction handling and diagnostics, not only portal
  reseating.

### L.2c - Transition Parity: Edge / Slide / Neg-Poly

Goal: match retail movement at walls, roof edges, step boundaries, and
precipices.

- Port and test `edge_slide`, `cliff_slide`, `precipice_slide`, and
  `step_up_slide` behavior from named retail.
- Preserve walkable polygon context needed for precipice/edge decisions.
- Replace `NegPolyHit` stub behavior with the retail dispatch path.
- Confirm the user-visible rule: walk-only motion is blocked by step,
  edge, walkable, and collision rules; jumping clears `OnWalkable` and only
  succeeds when the airborne path actually clears geometry.

Current shipped slice (2026-04-30): wall-adjacent `step_up_slide` feels
acceptable in live testing; player/remote movers pass `EdgeSlide`; terrain and
BSP step-down/find-walkable now preserve walkable polygon vertices; failed
step-down edge cases perform the retail back-probe before
`SPHEREPATH::precipice_slide`; precipice slide results now re-enter the
`TransitionalInsert` retry loop so tangent edge motion is preserved instead of
being reverted by outer validation. Remaining L.2c work is live visual
confirmation at real building/roof edges, real-DAT building-edge fixtures,
fuller `cliff_slide` coverage, and `NegPolyHit` dispatch.

### L.2d - Shape Fidelity: Sphere / CylSphere / Building Objects

Goal: object collisions use retail shape semantics, not one simplified
fallback.

- Finish `CSphere` / `CCylSphere` parity for static and live objects.
- Stop treating all live entities as one root-centered cylinder.
- Preserve enough building identity to model `CBuildingObj` collision and
  `bldg_check` behavior.
- Audit `Setup.Radius` and cylinder fallback behavior against retail before
  relying on them for conformance.

Current sub-direction (revised 2026-05-13 evening after slice 1 + 1.5
shipped and Holtburg-doorway capture analyzed — third reframe):
L.2d as scoped ("shape fidelity: Sphere / CylSphere / Building Objects")
is **essentially closed at the Holtburg site that motivated this phase**.
Building BSP collision works correctly — the slice-1.5 probe captured
real triangles in plausible world positions for `gfxObj=0x01000A2B` with
`bspR=13.99m`. The 121 wall hits the L.2a probe attributed to
`obj=0xA9B47900` were **side effects of the player already being pushed
back by a separate Door cylinder entity** at the same doorway threshold.

The actual blocker is a server-spawned **Door** entity — Setup
`0x020019FF` named `"Door"` — that ACE places at each Holtburg-town
building threshold (five doors total observed across `0xA9B40029`,
`0xA9B40154`, `0xA9B40155`). It registers as a Cylinder shadow entry
via the server-spawn path; its Cylinder collision blocks the player
walking into the doorway. That's **door-state handling**, a different
class of problem from L.2d's shape-fidelity scope — it touches network
(`CreateObject` PhysicsState bits), interaction (Use action on door
entity), animation (door open/close), and collision-state-toggle.

Recommend: **leave L.2d in "watch-and-wait" mode** with slice 1's probe
infrastructure in place. No more L.2d slices until a NEW shape-fidelity
bug is observed at a different site (dungeon walls, stairs, roofs) with
the probe-armed client. The door-state work becomes its own sub-phase
(probably nested under B.4 interaction or filed as a new L.2 sub-phase
like L.2g) scoped separately.

Full slice 1 + 1.5 handoff:
[docs/research/2026-05-13-l2d-slice1-shipped-handoff.md](../research/2026-05-13-l2d-slice1-shipped-handoff.md).
Design spec (now mostly historical, framing was wrong but probe
infrastructure shipped from it):
[docs/superpowers/specs/2026-05-13-l2d-cbuildingobj-collision-design.md](../superpowers/specs/2026-05-13-l2d-cbuildingobj-collision-design.md).
Predecessor L.2a handoff:
[docs/research/2026-05-12-l2a-shipped-l2d-handoff.md](../research/2026-05-12-l2a-shipped-l2d-handoff.md).

### L.2e - Cell Ownership: Outdoor Seams, CELLARRAY, cell_bsp

Goal: the resolver knows which cell owns the movement and which adjacent cells
must be checked.

- Update low outdoor cell id across 24m cell boundaries and landblock seams.
- Port the retail adjacent-cell search: `find_cell_list`, `check_other_cells`,
  and `adjust_check_pos`.
- Promote `cell_bsp` / `CellBSP` from partial data to active runtime owner.
- Hand G.3 a trustworthy building/portal boundary so dungeon streaming is not
  asked to solve collision ownership after the fact.

### L.2f - Real-DAT and Live Retail-Observer Conformance

Goal: prove the stack against real terrain/building/cell data and what a retail
client sees when observing acdream.

- Add real-DAT fixtures for representative movement cases.
- Use retail client observer runs to verify motion packets, animation/movement
  coupling, and server-visible placement.
- Treat ACE acceptance as a coarse compatibility check only.
- Require conformance notes in tests or research docs for every AC-specific
  algorithm ported under L.2.

### L.2g - Dynamic PhysicsState Toggling

Goal: server-driven post-spawn state changes (chiefly `ETHEREAL` flips) are
honored by the local collision stack.

Triggered 2026-05-12 evening by the L.2d slice 1.5 trace: the Holtburg
doorway blocker is a closed Door entity (Setup `0x020019FF`) whose
`PhysicsState.Ethereal` bit flips when the player Uses the door. The L.2d
shape-fidelity work doesn't cover this — the door's collision shape is
already correct; what's missing is honoring the *runtime* state change.

Scope is intentionally narrow:

- Parse inbound `GameMessageSetState (opcode 0xF74B)`.
- Plumb the new `PhysicsState` value into `ShadowObjectRegistry`'s cached
  per-entity state so the existing `CollisionExemption.IsExempt(...)` check
  sees the up-to-date bits.
- Verify the Holtburg inn-door scenario: walk into doorway → blocked, Use
  door → door swings open AND player can walk through, auto-close after
  30s → door closes AND player is blocked again.
- Confirm the existing `UpdateMotion` pipeline drives `(NonCombat, On/Off)`
  on non-creature entities (door swing animation). If not, one-line fix.

Excluded from L.2g scope (deferred):

- Door-specific UX polish: "door is locked" sound, creature-AI bump-open.
- Any Door-specific class hierarchy — generic state-flip infrastructure
  is enough; doors are the verification scenario, not a privileged case.

Lane: informal sixth lane "dynamic state." The existing five-lane table
treats per-entity state as static-after-spawn; L.2g makes it dynamic.

Full design spec:
[docs/superpowers/specs/2026-05-12-l2g-dynamic-physicsstate-design.md](../superpowers/specs/2026-05-12-l2g-dynamic-physicsstate-design.md).

M1 critical path: this slice unblocks the *"open the inn door"* demo
scenario.

Current shipped slice (2026-05-12):

| Commit | Subject |
|---|---|
| `2459f28` | `feat(phys L.2g slice 1): inbound SetState (0xF74B) parser` |
| `d538915` | `feat(phys L.2g slice 1): ShadowObjectRegistry.UpdatePhysicsState` |
| `536a608` | `feat(phys L.2g slice 1): WorldSession dispatches SetState (0xF74B) + hex probe` |
| `108e386` | `feat(phys L.2g slice 1): GameWindow routes SetState + extends [entity-source] log` |

Slice 1 is CODE-COMPLETE: parser + registry mutator + WorldSession
dispatcher + GameWindow subscriber. 6 new tests pass (3 parser + 3
registry). Build clean. Per-commit + final integration code reviews
all approved. **Visual verification deferred to Phase B.4b** — the
inbound SetState chain can't fire at runtime until B.4b finishes the
outbound Use handler. See
[docs/research/2026-05-12-l2g-slice1-shipped-handoff.md](../research/2026-05-12-l2g-slice1-shipped-handoff.md)
for full evidence + the 4 minor + 1 Important review notes.

## Named Retail Anchors

Primary source: `docs/research/named-retail/acclient_2013_pseudo_c.txt`.
Struct source: `docs/research/named-retail/acclient.h`.
Address lookup: `docs/research/named-retail/symbols.json`.

Use these names before falling back to older `docs/research/decompiled/`
chunks:

- `CTransition::find_transitional_position` - `0x0050BDF0`
- `CTransition::transitional_insert` - `0x0050B6F0`
- `CTransition::step_up` - `0x0050B610`
- `CTransition::step_down` - `0x0050B2A0`
- `CTransition::edge_slide` - `0x0050B3D0`
- `CTransition::cliff_slide` - `0x0050A6D0`
- `SPHEREPATH::step_up_slide` - `0x0050C3B0`
- `SPHEREPATH::precipice_slide` - `0x0050CC80`
- `SPHEREPATH::adjust_check_pos` - `0x0050CC00`
- `CTransition::adjust_offset` - `0x0050A370`
- `CTransition::check_other_cells` - `0x0050AE50`
- `CPhysicsObj::is_valid_walkable` - `0x0050F530`
- `CObjCell::find_cell_list` - `0x0052B4E0`
- `CBuildingObj::find_building_collisions`
- `CCellStruct::point_in_cell`
- `CCellStruct::sphere_intersects_cell`
- `CCellStruct::box_intersects_cell`
- `CCylSphere::intersects_sphere`
- `CSphere::intersects_sphere`
- `CSphere::slide_sphere`

## Implementation Order

1. Land L.2a diagnostics first. Do not make another physics change blind.
2. Fix L.2b packet/contact truth so logs and server echoes describe reality.
3. Port L.2c transition parity in narrow slices with named-retail citations and
   conformance tests.
4. Improve L.2d shape fidelity where transition parity depends on object
   contact semantics.
5. Land L.2e cell/building ownership before G.3 dungeon/portal work relies on
   indoor/outdoor walking.
6. Promote each synthetic case to L.2f real-DAT and live observer coverage.

## Acceptance

- A developer can name the active movement path and the current incomplete
  pieces without reading old chat logs.
- `dotnet build` and `dotnet test` stay green for each implementation slice.
- Every AC-specific port cites named retail decomp or a documented fallback.
- Real-DAT fixtures cover buildings, walls, roof edges, outdoor seams, and at
  least one dungeon/building entrance path before L.2 is marked shipped.
- Retail observer view and acdream local view both agree on contact, position,
  and movement state for the representative cases.