chore(repo): archive historical docs under Docs/archive, drop tracked pycache
Move closed audits, task briefs, design rounds, perf reports, the OPSTACK plan/progress logs and WORK-NEXT.md into Docs/archive/ (git mv, history kept). Whitelist Docs/ in the strict .gitignore, ignore Tools/**/__pycache__/, and remove the tracked .pyc. Repoint references in the kept docs and in non-test source comments to the new paths.
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# Codex task 002 — is the op-stack column memo thrashing? Count, don't guess.
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**Owner:** Codex (Model Luna, xHigh) · **Orchestrator:** Claude · **Branch:** `experimental`
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**Status:** specified, not started
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**Depends on:** `CODEX-TASK-001` — this adds counters to the **same** `stat VoxelForge` group.
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Do 001 first; this task assumes `VoxelStats.h` already exists.
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---
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## Why this exists
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The operator-stack density path is measurably slower than the `switch` it replaces, and the cause
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has never been attributed. There are three standing suspects. **This task measures the first one,
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and does not fix anything.** That order is deliberate: `AUDIT §C10` cost six builds and five refuted
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hypotheses by reasoning first, and the last session re-learned it.
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### The hypothesis, derived from the code
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`FSurfaceColumnSource::GetColumn` (`VoxelDensityOpStack.cpp` ~615) memoises a computed column in a
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**direct-mapped, 4096-entry `thread_local` table**, indexed by a hash of the two XY floats:
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```cpp
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struct FSlot { uint64 Key; float X, Y; FColumn C; }; // 40 bytes
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thread_local FSlot Slots[4096] = {}; // 160 KB per worker
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const uint32 Idx = ((HX * 0x9E3779B9u) ^ (HY * 0x85EBCA6Bu)) >> 20; // [0,4095]
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```
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The original path, `GSurfColCache` in `GetDensityAt`, is instead a **direct-indexed box**:
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`CI = (IY - Box.BaseY) * Dim + (IX - Box.BaseX)`, with a `Computed[CI]` flag. **No hash, therefore
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no collisions, therefore every column is computed exactly once.**
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Now the sampling order, which is the premise that makes this bite. `FVoxelMarchingCubesMesher`
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pre-samples with **Z as the OUTERMOST loop** (`VoxelMarchingCubesMesher.cpp` ~226):
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```cpp
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for (int32 gz = GzLo; gz <= GzHi; gz++)
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for (int32 gy = -1; gy <= GridDim; gy++)
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for (int32 gx = -1; gx <= GridDim; gx++)
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Generator->GetDensityAt(...);
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```
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So the mesher sweeps a **whole XY plane at every Z level**. Every column in the tile is revisited
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once per Z plane — roughly 34 times.
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The table's own comment sized it for this: *"A chunk is CHUNK_SIZE² columns (1024), so the first
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draft's 256 entries could not even hold one chunk and thrashed inside a single tile. 4096 covers
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four chunks."* **That reasoning has a gap.** A direct-mapped table does not need to be full to
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evict — it needs two live keys to collide. At ~1156 columns per plane in 4096 slots (load factor
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0.28), the expected number of columns sharing a slot with another is **~285, about 25 %**. Those
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columns evict each other, miss again on the next Z plane, and recompute the **entire height stack** —
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structural source, cliff (four structural resamples), terrace, layer-line, beach, and the ceiling
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stack — every single plane.
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Order of magnitude if that is right: ~1156 column computations on the original path versus
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~1156 + 285 × 34 ≈ **10 000** on the op path. Roughly **9×** the column work, on the plugin's most
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expensive archetype.
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**That is a derivation, not a measurement, and it is exactly the kind of confident chain this
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project has watched reverse six times.** Hence: count first.
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## What to build — two counters, no behaviour change
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Add to the existing `stat VoxelForge` group from task 001:
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| counter | incremented when |
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|---|---|
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| `ColumnMemoHit` | `GetColumn` found a live entry (the `if` body did **not** run) |
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| `ColumnMemoMiss` | `GetColumn` recomputed (the `if` body ran) |
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That is the entire change. Two `INC_DWORD_STAT` calls inside `FSurfaceColumnSource::GetColumn`,
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around the existing `if (S.Key != ColumnKey || S.X != WorldX || S.Y != WorldY)`.
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## ⚠️ Invariants
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1. **Change nothing else in `GetColumn`.** Not the table size, not the hash, not the key comparison.
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The point of this task is to produce a number that decides whether the fix is worth writing; a
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change bundled in makes the number unattributable. **Do not "obviously improve" the table while
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you are in there** — if the fix ships in the same build as the instrument, we learn nothing, and
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this project has a written lesson about exactly that.
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2. **`GetColumn` is `const` and runs on mesher workers.** `INC_DWORD_STAT`, never a `static int32++`
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— same rule as task 001, same reason.
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3. **The key comparison is load-bearing and must stay complete.** `S.Key != ColumnKey || S.X != X ||
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S.Y != Y` — the full key is compared on every touch precisely so a hash collision can only cost a
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recompute and never return **someone else's column**. Do not shorten it to feed a counter.
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4. **Zero cost when `STATS == 0`.** No value computed outside the macros.
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5. `FCaveCliffMod` and the overhang read this same memo through
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`Column->GetColumn(...)` (~line 864). They are legitimate traffic and must be counted, not
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excluded — they are part of why a miss is expensive.
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## Acceptance — the prediction is now numeric (tightened 2026-08-16 from the real grid dimensions)
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The hand-wavy "20–30 %" band this section used to carry has been replaced by an arithmetic
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prediction, because the inputs are all statically knowable and were read out of the source:
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- `CHUNK_SIZE = 32`, `CellsPerAxis = 32`, `GridDim = 33`, and the pre-sample loops run
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`g ∈ [-1, GridDim]` per axis ⇒ **35 × 35 = 1225 distinct columns per tile**, over **35 Z planes**
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(the mesher's own buffer comment, "35³ floats", confirms the dimension).
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- 1225 keys in 4096 slots is a load factor of **0.299**. Expected slots holding exactly one key
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`= 4096 · np(1-p)^(n-1) ≈ 908`, so **~317 columns (25.9 %) share a slot with another** and evict
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each other on every plane.
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- ⇒ op path ≈ `1225 + 34 × 317` ≈ **12 000** column computations per tile.
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Original path (`GSurfColCache`, direct-indexed, `Computed[CI]` persists) = **1225**.
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**≈ 9.8×.**
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### How to read the result
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⚠️ **Compare the RATIO OF THE TWO HYPOTHESES, not an absolute percentage.** The overhang and cliff
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modifiers call `GetColumn` again at the same XY (~line 864); every extra consumer adds **hits** and
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no misses, so it inflates the denominator and drags the miss *rate* down without changing the
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verdict. What does not move is the ~10× gap between the two outcomes.
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| observation | verdict |
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|---|---|
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| misses ≈ **8–10×** the hit-path baseline (single-consumer: ~28 % of lookups) | **CONFIRMED** — the table evicts on collision every Z plane. The fix gets its own task, with this run as its "before". |
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| misses ≈ **1 per distinct column** (single-consumer: ~3 %, hit rate ≥ 97 %) | **hypothesis WRONG.** The table behaves like the box, the ~9.8× does not exist, and the perf cost is suspect 2 (19 virtual calls per voxel). |
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*A negative result here is a real result.* It retires the most-suspected cause and is worth the build
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either way; it must be written into `OPSTACK-PROGRESS.md`, not quietly dropped.
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Report **both raw numbers**, never the ratio alone — a ratio cannot distinguish "few lookups" from
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"many", and the absolute miss count is what the fix would be reducing.
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## Notes for the reviewer (Claude)
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- Confirm the miss counter sits inside the `if`, and the hit counter in an `else` — not computed
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from a subtraction, which would silently agree with itself.
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- Confirm `GetColumn`'s early-out path (if any is added later) cannot skip both counters.
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- Confirm nothing else in `VoxelDensityOpStack.cpp` changed. `git diff --stat` should show one file
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and a handful of lines.
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