# BT 4.10 — the damage model, end to end

How a shot becomes lost armor, dead equipment, and a mech kill. Synthesized
from the surviving 1995 engine source (`CODE/RP/MUNGA/`), the reconstructed
BT-side TUs (`restoration/source410/BT/`), and the BT411 binary reversal.
Per-TU depth lives in the sibling `*.NOTES.md` files; this doc is the
cross-cutting flow.

```
weapon fire ──► Damage record ──► TakeDamageMessage ──► victim handler
                                                          │ (unaimed? cylinder table)
                                                          ▼
                                              damageZones[zone]->TakeDamage
                                                          │
                        armor economy: level += amount × scale[type]
                                                          │
              ┌──────────────────┬────────────────────────┤
              ▼                  ▼                        ▼
        level ≥ 1.0        Energy special           level < 1.0
        zone destroyed     (generator short)        (leg ≥ 0.5 → limp)
              │
   ┌──────────┼──────────────┐
   ▼          ▼              ▼
 VITAL      LEG           other zone
 mech kill  mech down     SendSubsystemDamage (crit allotments → equipment)
                          + RecurseSegmentTable (SIBS / DESCEND)
```

## 1. The Damage record (engine: `MUNGA/DAMAGE.HPP`)

Every hit travels as one `Damage` struct:

| field | meaning |
|---|---|
| `damageType` | 0 Collision · 1 Ballistic · 2 Explosive · 3 Laser · 4 Energy |
| `damageAmount` | points (the only field in the armor formula) |
| `damageForce` | impulse vector (physics/feel, not armor) |
| `surfaceNormal`, `impactPoint` | world-space impact geometry |
| `burstCount` | "times to apply" — **NOT in the armor formula**; one message = one application. Feeds splash falloff and the gyro bounce only. |

## 2. Producers — where Damage records are born

**Beam weapons** (`EMITTER.CPP`, PPC/lasers): instant-hit at the owner's
current target when `rangeToTarget <= effectiveRange`. The discharge energy
splits by the authored ratio `damageFraction = dmg/(dmg+heat)`; the delivered
amount scales with charge: `damagePortion = authored × chargeRatio²` (an
undercharged PPC hits soft). The heat portion goes into the FIRER's own heat
sinks. Delivery = `MechWeapon::SendDamage`.

**Ballistic / missile weapons** (`PROJWEAP.CPP`, `MISLANCH.CPP`,
`MISSILE.CPP`): the weapon FSM's Loaded case pulls ammo (`FeedAmmo`), checks
jam, then `FireWeapon` spawns ONE cluster `Missile` entity per salvo
(salvo-split `damageAmount`, `burstCount` = missile count). The round flies
guided (seeker + thruster); the proximity fuse delivers the whole record
once, `impactPoint` = the round's world position, zone = −1 (unaimed).

**Explosions / splash** (`MUNGA/EXPLODE.CPP`): a boxed splash volume gathers
movers + cultural objects sorted by distance; each target gets
`burstCount = original / r^1.2` (min 1), force along the radius vector,
zone = −1.

**Collisions** (`MUNGA/MOVER.CPP` `ProcessCollisionList`): type Collision,
amount computed by the bounce resolver (`StaticBounce` from velocity,
elasticity, friction), impact point from the collision slice. Multiple
same-frame collisions are averaged, damage summed.

## 3. Delivery — `Entity::TakeDamageMessage` (`ENTITY3.HPP`)

Fields: `inflictingEntity`, `damageZone` (+`invalidDamageZone` = zone < 0),
`damageData`, `inflictingSubsystemID` (so the BT message manager can bundle
explosion resource IDs). Dispatched AT the victim entity.

The authored contract (ENTITY3.HPP warning): **only reticle-based (aimed)
weapons carry a valid zone**. Everything else — missiles, splash, rams —
arrives zone = −1 and must be resolved by the victim.

The attacker also posts a `ScoreInflicted` message to its own player per
delivered hit (the damage score).

## 4. Victim routing — `Mech::TakeDamageMessageHandler` (`MECH.CPP`)

Binary hub @004a0230, in order:

1. Feed the RAW record to the gyro (cockpit bounce — even an invalid-zone
   hit shakes the pilot). *(staged: feel wave)*
2. Latch `lastInflictingID` — keys the LOD damage-clustering below.
3. **Unaimed resolve**: if `invalidDamageZone`, map `impactPoint` through the
   cylinder hit-location table (`DMGTABLE.CPP`, type-29 resource, cached
   mech+0x444): world → mech-local; local height picks a ROW (feet rows are
   chassis-fixed, upper rows add the LIVE torso twist to the impact angle);
   `atan2(z,x)` picks the angular CELL; a uniform roll walks the cell's
   cumulative distribution (the BattleTech dice scatter) → hull zone.
4. Chain to `Entity::TakeDamageMessageHandler`: zone −1 is DROPPED (base
   contract), otherwise `damageZones[zone]->TakeDamage(damageData)`.

## 5. The hull zone — `Mech__DamageZone::TakeDamage` (`MECHDMG.CPP`)

**Artifact (LOD) zones** — a zone with a non-empty redirect table is a
low-LOD hull shell: it never takes damage itself, it routes to a real child
zone. Same-attacker clustering: within 0.25 s the SAME child is hit again;
past 10 s re-roll; in between, 33 % reuse. The artifact's displayed level =
mean of its children.

**Real zones — the armor economy**:

    damageLevel += damageAmount × damageScale[damageType]      clamp [0,1]

Authoring streams armor POINTS per type; the ctor normalizes
`scale[type] = 1/(points[type] × armorPoints)`, so 1.0 = the zone's full
point budget spent. **Leg zones halve every scale** (legs effectively carry
double points). 1.0 → BurningState + DestroyedGraphicState.

**Per-hit specials**: an Energy hit on a zone with critical subsystems rolls
ONE of them; if the pick is a Generator it is force-shorted (screens
flicker, weapons drop dead until recovery; novice cockpits exempt).

**State outcomes** by the new level:

| condition | result |
|---|---|
| VITAL zone hits 1.0 | mech kill (`statusAlarm` 9) |
| leg zone hits 1.0 | fall → mech kill |
| leg zone ≥ 0.5 | limp gait graphic (left 3 / right 4) |
| non-leg, non-vital hits 1.0 | destruction descent (below) |

## 6. Criticals and equipment — `MECHDMG.CPP` + `MECHSUB.CPP`

Each hull zone streams a critical table: `{weight, damagePercentage
allotment, roster subsystem index}` per entry (plug binding is
master-authoritative — replicants never bind).

Every subsystem owns a PRIVATE `DamageZone` (index 0, not in the hull
array) with its own points + per-type scales (`structureReference` +
`armorByFacing[5]`, same normalization rule). That's what makes criticals
measurable.

- **`CriticalHit`** (aimed/critical fire; not yet on the weapon path —
  reticle wave): HALF the damage (cap 1.0) is carved off as the critical
  bite, applied to ONE weight-rolled crit subsystem via
  `ApplyDamageAndMeasure`, charged against that entry's allotment; the
  remainder runs the normal zone armor model.
- **Zone death → `SendSubsystemDamage`**: pins the zone at 1.0 and pushes
  each entry's UNUSED allotment into its subsystem's private zone. A
  subsystem at 1.0 → `ForceCriticalFailure`: alarm level 1 (Destroyed),
  `SetSimulationState(DestroyedState)` — the hard gate every weapon FSM
  polls, so destroyed weapons fall silent; a VITAL subsystem kills the mech.
  Repeat hits on a dead zone re-run the push (binary-authentic) — contained
  equipment keeps degrading under continued fire.

## 7. The destruction cascade — `RecurseSegmentTable`

A destroyed zone walks the skeleton by its streamed flags:

- **SIBS** (`destroySiblingsOnDestruction`): destroy the other zones on the
  same segment.
- **DESCEND** (`descendOnDestruction`): destroy every zone on the child
  segments, recursing.

Fight-verified chain: arm zone dies → SIBS kills the searchlight zone (its
Searchlight/ThermalSight crits destroyed) → DESCEND kills the gun zone →
PPC_2 + ERMLaser_2 + Condenser6 destroyed and STOP FIRING, while the
untouched PPC_1 keeps shooting.

## 8. Data authoring (the resource chain)

- **Type-20 DamageZoneStream** (per mech): zone count, then per zone the
  engine record (name, 5 effect-site segment lists, `defaultArmorPoints`,
  `damageScale[5]`, material lists) + the BT tail (descend/sibs flags,
  `segmentIndex`, leftLeg/rightLeg/vital, crit table, LOD redirect table).
  Authored in `.dmg` notation files: `WeaponDamagePoints` default with
  `Collision/Ballistic/Explosive/Laser/EnergyDamagePoints` overrides
  (stored as 1/points).
- **Type-29 DamageLookupTableStream**: the cylinder table (rows ×
  angular cells × cumulative zone distributions; 18 tables shipped).
- **Weapon subsystem resources**: `damageAmount`, `damageType`,
  `heatCostToFire`, range, recharge.
- Reference magnitudes (bhk1): legs 70 pts, upper torso 124, searchlights
  25; PPC ~12 (Energy), ER-M laser 3.43 (Laser), SRM 5.83 × 6 (Explosive).

## 9. Replication and respawn

- Criticals resolve on the MASTER instance only; `DamageZone` state
  replicates via update records (`damageLevel`, zone state, graphic state,
  changed flags). `SubsystemMessageManager` consolidates per-frame damage
  and bundles explosion resources (not yet reconstructed).
- `Mech::Reset` (respawn) heals every hull zone and DeathResets the roster,
  but crit `damagePercentageUsed` PERSISTS across lives — spent crit
  budgets stay spent (binary-authentic).

## 10. Known deltas from the 1995 binary (staged)

- Aimed fire: beam `SendDamage` currently rolls a uniform random hull zone;
  authentic = the reticle hit. `CriticalHit` is unwired for the same reason.
- Cylinder height = 10.0 constant (binary reads the collision cylinder).
- Leg-branch gates use "not already destroyed" instead of the live
  MovementMode/IsDisabled checks (gait FSM pending).
- Gyro hit-feed and destroyed-skin graphics are log stubs (feel/render waves).