TeslaRel410/restoration/source410/BT/MECH4.NOTES.md

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. Ghidra never decompiled it (objdump territory). Owns: the turn-in-place dispatcher (arms state 4 on both channels in lockstep), the status→mode promotion, likely the runSpeedMax (+0x7a0) writer, the gyro rumble timer.
• @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.