# Codex task 002 — is the op-stack column memo thrashing? Count, don't guess.

**Owner:** Codex (Model Luna, xHigh) · **Orchestrator:** Claude · **Branch:** `experimental`
**Status:** specified, not started
**Depends on:** `CODEX-TASK-001` — this adds counters to the **same** `stat VoxelForge` group.
Do 001 first; this task assumes `VoxelStats.h` already exists.

---

## Why this exists

The operator-stack density path is measurably slower than the `switch` it replaces, and the cause
has never been attributed. There are three standing suspects. **This task measures the first one,
and does not fix anything.** That order is deliberate: `AUDIT §C10` cost six builds and five refuted
hypotheses by reasoning first, and the last session re-learned it.

### The hypothesis, derived from the code

`FSurfaceColumnSource::GetColumn` (`VoxelDensityOpStack.cpp` ~615) memoises a computed column in a
**direct-mapped, 4096-entry `thread_local` table**, indexed by a hash of the two XY floats:

```cpp
struct FSlot { uint64 Key; float X, Y; FColumn C; };   // 40 bytes
thread_local FSlot Slots[4096] = {};                   // 160 KB per worker
const uint32 Idx = ((HX * 0x9E3779B9u) ^ (HY * 0x85EBCA6Bu)) >> 20;   // [0,4095]
```

The original path, `GSurfColCache` in `GetDensityAt`, is instead a **direct-indexed box**:
`CI = (IY - Box.BaseY) * Dim + (IX - Box.BaseX)`, with a `Computed[CI]` flag. **No hash, therefore
no collisions, therefore every column is computed exactly once.**

Now the sampling order, which is the premise that makes this bite. `FVoxelMarchingCubesMesher`
pre-samples with **Z as the OUTERMOST loop** (`VoxelMarchingCubesMesher.cpp` ~226):

```cpp
for (int32 gz = GzLo; gz <= GzHi; gz++)
    for (int32 gy = -1; gy <= GridDim; gy++)
        for (int32 gx = -1; gx <= GridDim; gx++)
            Generator->GetDensityAt(...);
```

So the mesher sweeps a **whole XY plane at every Z level**. Every column in the tile is revisited
once per Z plane — roughly 34 times.

The table's own comment sized it for this: *"A chunk is CHUNK_SIZE² columns (1024), so the first
draft's 256 entries could not even hold one chunk and thrashed inside a single tile. 4096 covers
four chunks."* **That reasoning has a gap.** A direct-mapped table does not need to be full to
evict — it needs two live keys to collide. At ~1156 columns per plane in 4096 slots (load factor
0.28), the expected number of columns sharing a slot with another is **~285, about 25 %**. Those
columns evict each other, miss again on the next Z plane, and recompute the **entire height stack** —
structural source, cliff (four structural resamples), terrace, layer-line, beach, and the ceiling
stack — every single plane.

Order of magnitude if that is right: ~1156 column computations on the original path versus
~1156 + 285 × 34 ≈ **10 000** on the op path. Roughly **9×** the column work, on the plugin's most
expensive archetype.

**That is a derivation, not a measurement, and it is exactly the kind of confident chain this
project has watched reverse six times.** Hence: count first.

## What to build — two counters, no behaviour change

Add to the existing `stat VoxelForge` group from task 001:

| counter | incremented when |
|---|---|
| `ColumnMemoHit` | `GetColumn` found a live entry (the `if` body did **not** run) |
| `ColumnMemoMiss` | `GetColumn` recomputed (the `if` body ran) |

That is the entire change. Two `INC_DWORD_STAT` calls inside `FSurfaceColumnSource::GetColumn`,
around the existing `if (S.Key != ColumnKey || S.X != WorldX || S.Y != WorldY)`.

## ⚠️ Invariants

1. **Change nothing else in `GetColumn`.** Not the table size, not the hash, not the key comparison.
   The point of this task is to produce a number that decides whether the fix is worth writing; a
   change bundled in makes the number unattributable. **Do not "obviously improve" the table while
   you are in there** — if the fix ships in the same build as the instrument, we learn nothing, and
   this project has a written lesson about exactly that.
2. **`GetColumn` is `const` and runs on mesher workers.** `INC_DWORD_STAT`, never a `static int32++`
   — same rule as task 001, same reason.
3. **The key comparison is load-bearing and must stay complete.** `S.Key != ColumnKey || S.X != X ||
   S.Y != Y` — the full key is compared on every touch precisely so a hash collision can only cost a
   recompute and never return **someone else's column**. Do not shorten it to feed a counter.
4. **Zero cost when `STATS == 0`.** No value computed outside the macros.
5. `FCaveCliffMod` and the overhang read this same memo through
   `Column->GetColumn(...)` (~line 864). They are legitimate traffic and must be counted, not
   excluded — they are part of why a miss is expensive.

## Acceptance

Fly a route through a **SurfaceWorld** strate that has `bUseOperatorStack` ticked, `stat VoxelForge`
on screen, and read the ratio.

- **`ColumnMemoMiss` ≈ one per distinct column per tile** (hit rate climbing toward ~97 % as Z
  planes accumulate) ⇒ **the hypothesis is WRONG**, the table is fine, and the perf cost is
  elsewhere. Record that and move to suspect 2 (per-voxel virtual dispatch). *A negative result here
  is a real result and must be written down, not quietly dropped.*
- **`ColumnMemoMiss` in the neighbourhood of 20–30 % of all lookups, and staying there** ⇒
  **confirmed**: the table is evicting on collision every Z plane. The fix is then obvious and cheap
  (direct-indexed box keyed like `GSurfColCache`, or a set-associative table), and gets its own
  task with the before/after this run supplies.

Report **both numbers**, not the ratio alone — a ratio cannot distinguish "few lookups" from "many".

## Notes for the reviewer (Claude)

- Confirm the miss counter sits inside the `if`, and the hit counter in an `else` — not computed
  from a subtraction, which would silently agree with itself.
- Confirm `GetColumn`'s early-out path (if any is added later) cannot skip both counters.
- Confirm nothing else in `VoxelDensityOpStack.cpp` changed. `git diff --stat` should show one file
  and a handful of lines.