perf(gen): a thrashing one-slot cache cost 18.7x -- and the world is unchanged

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TunnelNetwork density evaluation was 45x more expensive than Maze for identical
work: 2,097,152 voxels, 64 tiles, same mesher, same machine. Profiling killed
every hypothesis I brought to it -- box classification skipping 0/64 tiles was
true but irrelevant (export bypasses classification), near-surface early-outs
fired at 49.7%, the room cache built 340 entries for 2.74M samples, and the
per-room overrides resolved once per sample rather than once per modifier.

The cause was the wrapper, not the stack. A mesher tile samples an EXPANDED grid
(-1..Cells+1), so one worker crosses several exact chunk coordinates while
meshing a single tile. The generator's cache held only the last coordinate, so
every boundary crossing rebuilt the resolved parameters, the whole 22-operator
stack and the tunnel-core cache. On one representative tile: 3,675 refetches,
38.0s. The entire operator stack was 18.5s of a 4,272s total -- 0.43%.

Replaced with a 128-slot direct-mapped thread_local cache keyed on owner, chunk,
layout version, params fingerprint and generator type. Collisions force a safe
rebuild, never a stale read. No shared mutable state, no order-dependent
reduction.

  TunnelNetwork  344.5s / 164.267 us/vox  ->  18.4s / 8.763 us/vox   (18.7x)
  Maze             7.4s /   3.535 us/vox  ->   1.08s / 0.513 us/vox  ( 6.9x)
  same tile        38.0s                  ->   0.51s ; refetches 3675 -> 27

Output is BYTE-IDENTICAL, before vs after and across repeats, both archetypes:
TunnelNetwork 50d6b4d0..f23126, Maze 364f7489..2fbe99. The permission to change
the field was not needed.

Also in this commit, from the two preceding tasks:

- Explorer renders from the MESH instead of raymarching. It meshes the region
  once through the canonical UVoxelMarchingCubesMesher, then CPU-rasterises via
  a BVH. A viewpoint went from ~16 minutes to 0.155s; 8 views now cost 1.25s of
  drawing. No render pixel samples the density field.
- One sampled grid and player-fit mask per run, shared by render/walk/export,
  instead of the same 368s walk computed twice.
- A wall-clock budget (25 min default, hard 30 max) that stops cleanly and
  records truncated/truncated_during/completed_modes rather than overshooting.
- ParallelFor across the measurement sampling; the suite was ~10% of a 16-thread
  machine.
- -surfaceroughness= on the commandlet, and a surface-relief metric in export
  mode (edge lengths, dihedral distribution, face-vs-smooth normal divergence)
  so "does this look like rock" is a number, not a screenshot argument.
- Smooth vertex normals in the rasteriser: FVoxelMeshData::Normals is the
  density-gradient normal the engine shades with, and the raster path was
  recomputing a face normal over it.
- ScaleDiagnosis: baked candidate indices are now derived from the current
  corpus, so a legitimate field change stops breaking the test.

NOT verified, and not claimed:
- TunnelNetworkSpineEquivalence exceeded the 30-minute cap and was stopped.
  Inconclusive, not passing. The identical hashes are strong evidence the
  op-stack/legacy equivalence holds, but the test has not confirmed it.
- CrossPlatformDigest passes but reports 2 samples within 1e-6 of the
  isosurface; FMath::Sin/Cos leaves Windows/Linux equivalence unproven.
  Pre-existing, not caused by this change.
- The cache's memory footprint (128 entries x ~16 workers) is unmeasured.

What the roughness sweep settled, for the record: SurfaceRoughness 0 -> 8 moves
cost by 0.3% (163.2 / 163.2 / 163.7 us), so roughness was never the cost. It
does reach the surface (dihedral p50 1.1 -> 2.2 -> 5.9), which kills my claim
that the density-channel operator contributed nothing. TunnelNetwork's hard
seams are present at roughness 0 (4.4% of edges past 40 deg vs Maze's 1.0%), so
they are structural -- in how primitives are unioned -- not a roughness
deficiency. More roughness makes them worse, not better.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012aheo2crrZmHuYtPpdY2Cb

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Fr0zkaandClaude Opus 5 committed 2026-09-08 20:31:56 +02:00

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