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feat: port FloatingIslands — the stack that runs backwards

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6 of 8 archetypes ported. This one starts from VOID and FILLS where the
other four start from ROCK and CARVE, which is what it was worth doing:
neither end of the pile needed a new operator, only the opposite sign.

  FConstantRockSource -> FConstantFieldSource(+/-Base)   AllSolid <-> AllAir
  FSdfCarveOp         -> FSdfConvertOp(Sign = +/-1)      carve    <-> fill
  FSdfRoughnessMod                                       4th archetype, unchanged

Only the island blob source is new. Multiplying by +/-1 is exact in
IEEE-754, so the three already-green ports are bit-for-bit untouched.

ClassifyBox can return AllAir for the first time in the plugin, and an
island strate is by construction mostly empty — the test counts AllSolid
and AllAir separately so an aggregate cannot hide whether that fired.

Two bounds that would have been holes if assumed rather than derived:
the island bound is one-sided (a hairline thread of matter hangs below
each island down its axis, so only the TOP may reject), and the domain
warp displaces X and Y independently, so the pad needs WarpAmp*sqrt(2).

Also: AUDIT C1 was NOT closed. The 2026-07-27 sweep matched `SeedF * K`
and this archetype's warp spells it `(float)S * K`, so one site survived
— at seed 2e9 the warp flattens and every island snaps back to a perfect
circle. Fixed in both paths in one pass so the equivalence test stays a
valid oracle. Expect island silhouettes to change at large seeds.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Fr0zka
2026-07-27 17:50:36 +02:00
parent f5d5a03ad1
commit 96e75abe57
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@@ -400,11 +400,19 @@ Port each archetype **the next time a feature makes you open it anyway**. The sw
Suggested order when there's a free choice — cheapest and least risky first:
✅ `Maze` (P1) → ✅ `FlatPlain`/`CrystalChamber` (one op, two default sets — the first real win: two
archetypes collapse into one; **done**, `BuildSlabStack`, 8 archetypes → 7) → ✅ `SurfaceWorld` (**done**, incl. biomes — needed a whole second op family, `VoxelHeightOp.h`) → ✅ `VerticalShafts` (**done**, 3 ops reused from Maze unchanged) (biggest payoff, biggest care: the T1.a column cache and the exact-
lattice `ClassifyTile` bound must both survive) → `VerticalShafts` → `FloatingIslands` → `TunnelNetwork`
archetypes collapse into one; **done**, `BuildSlabStack`, 8 archetypes → 7) → ✅ `SurfaceWorld`
(**done**, incl. biomes — needed a whole second op family, `VoxelHeightOp.h`; biggest payoff, biggest
care: the T1.a column cache and the exact-lattice `ClassifyTile` bound both survived) →
✅ `VerticalShafts` (**done**, 3 ops reused from Maze unchanged) →
✅ `FloatingIslands` (**done**, `BuildFloatingIslandStack` — the stack that runs **backwards**: void
source + fill instead of rock source + carve, the *same* classes with the opposite sign; only the
blob source is new) → `Underwater` (TunnelNetwork + a flag) → `TunnelNetwork`
(**last** — it owns `BuildChunkCache`'s two-region window-invariance discipline, §8.4, the most delicate
code in the plugin).
**6 of 8 ported.** The two that remain are really one: `Underwater` *is* TunnelNetwork plus
`WaterLevelRelative` (§8), so the switch loses its last two cases in a single port.
Along the way, `FStrateGenerationParams`' 74 fields decompose into per-op structs, which retires the
`VF_STRATE_PARAM_FIELDS` X-macro drift problem for free.