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Docs/archive/CODEX-TASK-006-cave-morphology-radius-envelope.md

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+# Codex task 006 — the same `Min > Max` under-bound, in `BuildChunkCache` (both density paths)
+
+**Owner:** Codex (Model Luna, xHigh) · **Orchestrator:** Claude · **Branch:** `experimental`
+**Status:** specified, not started
+**Kind:** ⚠️ **correctness of a collection bound.** Third instance of this class; the worst of the three.
+**Credit:** found by the Sol-High read-only audit (`AUDIT-2026-08-CODEX.md`, VF-05) and **verified
+against the code** before being specified.
+
+---
+
+## Why this exists
+
+`CODEX-TASK-004` fixed two cell-sweep pads that assumed the field named `Max*` was numerically the
+larger one. **The same defect exists in `VoxelCaveMorphology.cpp`, and it matters more**, for three
+reasons:
+
+1. It is **`TunnelNetwork`** — the largest and most-used archetype.
+2. `BuildChunkCache` is called by **both** density paths: the original `switch` *and*
+   `FRoomGraphSource`, which deliberately calls it rather than transcribing it. **This is not an
+   operator-stack bug — it is in the shipped original code and always has been.**
+3. Its failure mode is a **window-invariance break** (`ARCHITECTURE §8.4`), not just a missing room:
+   whether a room exists depends on which chunk you queried from. In a multiplayer game that means
+   two peers generate different geometry from the same seed.
+
+### The mechanism
+
+Radii are interpolated:
+
+```cpp
+Room.RadiusXY = FMath::Lerp(Params.MinRoomRadius,   Params.MaxRoomRadius,   SizeFactor);  // ~262
+const float RadA = FMath::Lerp(Params.TunnelMinRadius, Params.TunnelMaxRadius, FactorA);  // ~467
+const float RadB = FMath::Lerp(Params.TunnelMinRadius, Params.TunnelMaxRadius, FactorB);  // ~468
+```
+
+`FMath::Lerp(A, B, t)` with `t ∈ [0,1]` yields anywhere in `[min(A,B), max(A,B)]` — it does **not**
+require `A ≤ B`. But every bound derived from those radii reads only the `Max*` field:
+
+| site | line | expression |
+|---|---|---|
+| `MaxInfluence` | ~127–130 | `FMath::Max(Params.MaxRoomRadius, Params.TunnelWarpStrength + Params.TunnelMaxRadius) + Params.SDFBlendRadius` |
+| `CollectMargin` | ~152 | `2.0f * MaxTunnelLen + MaxInfluence` (inherits it — **no separate edit needed**) |
+| `RoomZBuffer` | ~171 | `Params.MaxRoomRadius * Params.RoomHeightRatio` |
+| `EvaluateSDF`'s `Margin` | ~871–874 | the same expression as `MaxInfluence`, duplicated |
+
+With `MinRoomRadius > MaxRoomRadius`, rooms larger than `MaxInfluence` are generated, so a room that
+can reach a chunk may sit in a cell the collect region never visited. `RoomReachesSearchBox` uses the
+*actual* radius and is therefore correct — but it can only test rooms that were collected at all.
+
+## The fix
+
+Derive **bound-only envelopes** and use them at all four sites:
+
+```cpp
+const float RoomRadiusEnvelope   = FMath::Max(Params.MinRoomRadius,   Params.MaxRoomRadius);
+const float TunnelRadiusEnvelope = FMath::Max(Params.TunnelMinRadius, Params.TunnelMaxRadius);
+```
+
+- `MaxInfluence` → `FMath::Max(RoomRadiusEnvelope, Params.TunnelWarpStrength + TunnelRadiusEnvelope) + Params.SDFBlendRadius`
+- `RoomZBuffer` → `RoomRadiusEnvelope * Params.RoomHeightRatio`
+- `EvaluateSDF`'s `Margin` → the same corrected expression.
+
+⚠️ `MaxInfluence` and `EvaluateSDF`'s `Margin` are the **same formula written twice**. They must stay
+identical. If a shared helper is natural here, use one — two copies of a rule that must agree is a
+bug factory, and this file already has the duplication. If you introduce a helper, keep it local to
+this translation unit and do not change either call site's semantics.
+
+## ⛔ DO NOT reorder the `Lerp` endpoints
+
+Swapping to `Lerp(min, max, t)` maps the same hash `t` to a **different radius** for the same room,
+which changes generated geometry and breaks the eight bit-for-bit equivalence tests. **Only the
+bounds may become conservative. The three `Lerp` calls must not be touched at all.**
+This is the same rule as tasks 003 and 004, and it is the third time it applies.
+
+## ⚠️ Invariants
+
+1. **No `Lerp` line changes. No room/tunnel placement, hashing, or `bStore` logic changes.**
+   If your diff touches `Room.RadiusXY`, `RadA`, `RadB`, or `RoomReachesSearchBox`, stop and say so.
+2. The change is strictly conservative: a larger envelope collects **more** cells, never fewer.
+3. Exactly one file: `Source/VoxelForge/Private/VoxelCaveMorphology.cpp`.
+4. Comments are French + English; match the file. Say **why** the envelope is `max(Min, Max)` — the
+   next reader must not "simplify" it back to `MaxRoomRadius`.
+
+## Acceptance
+
+- One file changed, a handful of lines.
+- **With correctly ordered params, `max(Min, Max) == Max`, so every number is bit-identical and this
+  is a NO-OP. That is the acceptance signal.** The eight equivalence tests, every box-verdict line
+  and every `violations` count must be **unchanged** after the build. A moved number means the diff
+  did something it should not have.
+- It only changes behaviour for an asset whose range is inverted — which is precisely the case that
+  was silently producing window-dependent geometry.
+
+## Notes for the reviewer (Claude)
+
+- Confirm all four bound sites use the envelopes, and that `CollectMargin` inherits rather than being
+  edited separately.
+- Confirm `MaxInfluence` and `EvaluateSDF`'s `Margin` still compute the identical expression.
+- Confirm no `Lerp` argument order changed anywhere in the file.