# MECH4 — pre-TU reconstruction notes

**Status: no MECH4.CPP exists yet.** The ground snap, frame rejection,
knockdown stagger and collision economy that the binary keeps in mech4's
master performance are currently RE-HOSTED inside `Mech::Simulate`
([[MECH]], 5.3.98–100) at seams marked STAGED. This file carries the decode
work for the pieces not yet written, so it survives session boundaries.

## Mech::ProcessCollision (@004abb40) — decoded, not yet reconstructed

The override of the engine's protected virtual (`MOVER.CPP:1354`), called
per contact by `Mover::ProcessCollisionList`. The decomp
(part_012.c:15206–15416) reads as the BASE function plus four additions:

1. **The base head, verbatim**: `collisionVolume->ProcessCollision(collision,
   worldLinearVelocity, lastCollisionList, &damage->surfaceNormal,
   &penetration)`; on a miss NOTHING runs. Clamp penetration to the slice,
   ratio = pen/dt, `damage->damageAmount = StaticBounce(old, dt, r, normal,
   &elasticity, minimumBounceSpeed, &friction)` with the elasticity/friction
   locals seeded from the mover's coefficients.

2. **The Mover-family branch** (owner = the collided solid's owner entity,
   solid+0x1c; family test FUN_0041a1a4 against the "Mover" derivation GUID
   @0x4e4518):
   * SEPARATING-CONTACT GATE: `(ourWorldVel − other->WorldVel[vtbl+0x20])
     · normal < −1e-4` (`_DAT_004ac044`) → return, no damage, no dispatch.
   * If the other's classID == **0xbb9** (mech), build a TakeDamage message
     (raw wire shape `{id, len, inflictor = DAT_0050b9ac global, zone −1,
     the Damage verbatim}`) and Dispatch it AT THE OTHER — the mech-vs-mech
     ram. Use our `Entity::TakeDamageMessage` ctor as the weapon path does.

3. **The CulturalIcon branch** (GUID @0x4e7038):
   * `stopping = other->instanceFlags & 0x8000` (the StoppingCollisionVolume
     flag, entity+0x28).
   * Same separating gate against a ZERO other-velocity (icons are static).
   * Dispatch the crunch TakeDamage at the icon (crushable props have no
     zones → their base handler no-ops it, but the dispatch is real).
   * If NOT stopping: `damage->damageAmount = 0.00123f` (bits 0x3aa137f4) —
     the WALK-THROUGH SENTINEL. The caller (the response block in Simulate)
     must treat exactly 0.00123f as "the move stands": restore the post-snap
     saves, zero the amount, feed the gyro crunch (torque 0.4 along the
     normalized force, upward impulse 0.2 — binary @4aa81e/@4aa86c).

4. **The material tail**: the elasticity/friction OUT values from
   StaticBounce are compared against `_DAT_004ac048`; sets
   `collisionTemporaryState` (+0x44c) to 1, or 2 when both match and the
   state was 0. The state is pushed into the `collisionState` StateIndicator
   once per frame by the response block (0→1 fires the impact sound). Needs
   the `_DAT_004ac048` constant identified before writing (likely the
   default-material coefficient — check the literal pool at 0x4ac048).

## Two hazards the donor fought that DO NOT apply here

* **Compounded StaticBounce reflections**: our engine's
  `Mover::ProcessCollisionList` (MOVER.CPP:1203) restores
  `worldLinearVelocity = initial_velocity` and position PER CONTACT before
  each ProcessCollision call, and averages the results. The port's
  `frameEntryWorldVelocity` economy guard has no counterpart hazard in this
  engine — do not import it.
* **The AlarmIndicator ODR split**: a port-only problem; our tree reads
  `MovementMode()` / the alarms directly.

## Already re-hosted in Mech::Simulate (move here when mech4.cpp exists)

| piece | binary | landed |
|---|---|---|
| ground probe + snap | @4aa630–4aa6cc | 5.3.98 |
| frame rejection + crash self-damage + knockdown | @4aa6cf–4aab0b | 5.3.99 |
| collision economy divert + distributor | @0x4a0368 → @0049ffcc | 5.3.100 (in the damage hub, where the binary has it) |
| status→mode limp promotion | master perf (un-decompiled) | 5.3.97, STAGED |
| forwardCycleRate re-pick (ground/airborne) | IntegrateMotion head | partially — the ctor sets it once; the per-frame re-pick needs +0x3f4 |

## Still to decode

* `FUN_004a9b5c` — the master performance.  **ISOLATED AND MAPPED
  (2026-08-04):** one single 5,676-byte body @004a9b5c..@004ab168 (one
  `ret`), hidden inside the manifest's mis-sized @004a9770 entry (that fn
  is really the stream-builder's 6.6KB name→SharedData strcmp resolver,
  and Ghidra swallowed everything to @004ab188 behind it).  Registration
  proven from the ctor decomp: `(flags & 0xC) == 4` → replicant perf
  @004ab9d8 (also hidden, inside @004ab430's claimed range); else master
  @004a9b5c.  Recipe: extract CODE bytes (va 0x1000 → file 0xe00),
  `ndisasm -b 32 -o 0x4a9b5c`.

  **The ordered call skeleton (111 calls)** — blocks in execution order,
  matching our Simulate's structure and locating every staged gap:
  1. controls/demand intake (mech.cpp helpers 0x4a4eab/0x4a4ee9/0x4a4f9e)
  2. three cerr+SetLevel triplets — illegal-state alarm guards
  3. **THE DUCK DRIVER**: three arms of {mech2 SetAnim 0x4a7fc4,
     MarkUpdate ×2, AlarmIndicator::SetLevel} — duck down/hold/up
  4. verify-guarded state read (mech4 accessors + Fail 0x40385c)
  5. gyro feed a (vec pair → gyro_b 0x4b2de4, gyro_a 0x4b2d8c)
  6. death arm: InDeathTransition 0x49fb54 → the 1.9KB mech2 advancer
     0x4a71f4 under it (death-clip arming)
  7. **status→mode LIMP PROMOTION** (0x4a5028 + SetAnim/Mark/SetLevel)
  8. update-record block (0x421bac/0x40e5f0/0x4227e0/0x41db7c/0x422bac +
     Point3D copies) — the dead-reckon/update writer
  9. collision → gyro crunch (gyro feeds + alarms)
  10. knockdown/bump arms (EntityID pair + two SetAnim/Mark/SetLevel arms)
  11. IntegrateMotion @004ab1c8 + position dirty marks
  12. pose/joint tail: gyro_d/gyro_c → **Torso::UpdateJoints @004b66b4** →
      five StateIndicator::SetState publishes + four GetState reads
  13. frame close (position latch)

  **MarkUpdate = FUN_004a4c54 (×20!)** — the update-record dirty-mask OR
  (death strips motion bits); our `ForceUpdate(int)` is an EMPTY STUB, so
  none of these marks exist in our tree yet (MP replication feed).
  Next sitting: instruction-level read of blocks 3 and 7 (the two STAGED
  items), then the mark masks.
* `@004ab430` ReplicantPerformance (1432B) — the replicant-side interior.
* `@004ab1c8` IntegrateMotion — decoded (see 5.3.95 notes) but only
  partially re-hosted: the local-velocity/orientation integration and the
  dead-reckon latency fold (+0x778/+0x77c) remain. **`+0x598` is a Point3D**
  (its replicant branch writes it with a vector subtract), so
  `motionEventName` in MECH.HPP is mistyped as CString — fix when this
  lands.