VoxelForge/Standalone/Core/API.md

# VoxelForgeCore API reference

Public header: `Core/include/VoxelForgeCore/VoxelForgeCore.h` (C++20, standard library only).
Build the static library and viewer with `Build.ps1 -Target All`, or only the library with
`Build.ps1 -Target Core`.

The public header uses only standard C++ types and can be included after Unreal's `CoreMinimal.h`.
`Core/verification/UnrealHeaderCoexistence.cpp` is the compile smoke check against the installed
Unreal 5.7 headers. Copied generator and compatibility declarations stay in the private
`vfcore::detail` implementation and do not appear in this API.

## Units and ownership

- `Vec3d` query positions and `Vec3i` voxel coordinates are in **voxels**.
- `TileKey.coordinate` is a tile coordinate; one coordinate step spans `32 << lod` voxels.
- Mesh vertex positions are **centimetres**. UVs, RGBA8 colors, normals and 32-bit indices are
  returned per vertex / triangle in owned `std::vector`s.
- `MeshGroups::groundTriangleCount` and `ceilingTriangleCount` describe the contiguous index runs
  emitted by the mesher. Ceiling is the final run. `skirtTriangleCount` is a subset of the ground
  and ceiling runs. `surfaceVertexCount` ends before skirt vertices.
- A `World` owns its compiled field and edit log. The snapshot can be shared as immutable input.
  Serialize compile/edit changes against query and tile-generation calls on that same `World`.

## Load and compile

`ResolvedWorldSnapshot::FromJson` accepts the normalized UTF-8 schema-v1 owner export used by the
standalone loader. It returns `std::nullopt` and fills `error` when parsing or asset resolution
fails. Read `configuration()` for the resolved scalar settings.

Implement `JobScheduler::parallel_for` in the host. It must run each index from zero to `count - 1`
exactly once and return only after all submitted work is complete. The library creates no threads.
`ShapeBuildRequest` supplies the detail and clip bounds in voxels, worker limit, detail variant,
far-world inclusion, and optional tunnel/detail profiles.

```cpp
std::string error;
auto snapshot = VoxelForgeCore::ResolvedWorldSnapshot::FromJson(json, error);
if (!snapshot) { /* report error */ }

VoxelForgeCore::World world(std::move(*snapshot));
VoxelForgeCore::ShapeBuildRequest build;
build.minVoxel = {-256, -768, -384};
build.maxVoxel = { 256, -256,  128};
build.clipMinVoxel = {-57344, -57344, -8192};
build.clipMaxVoxel = { 49152,  49152,  8192};
build.workerLimit = 8;
build.includeFarWorld = true;
if (!world.compile(build, hostScheduler, error)) { /* report error */ }
```

For a streamed shape world, compile the initial camera region once, then add deterministic detail
pages before generating any newly desired fine tile outside that region:

```cpp
const auto entering = world.desiredTiles(newCameraVoxels);
VoxelForgeCore::TileGenerationOptions options;
std::size_t pageCoveredTiles = 0;
if (!world.compileTileRegion(entering, 8, hostScheduler, error, &pageCoveredTiles)) {
    /* report error */
}
for (const auto& key : entering) {
    if (auto mesh = world.generateTile(key, options, error)) { /* upload and retain */ }
}
world.retainTileRegions(entering);
```

By default, the Core groups missing keys by LOD and builds one analytic primitive/bin region for
each entering LOD slab. The region bounds come from the requested absolute tile coordinates, with
an 8-voxel sampling apron; no dense lattice is allocated for unused parts of the slab. A tile's
samples therefore depend on its world coordinates and the world snapshot, not the camera path or
the order in which tiles arrive. `compileTileRegion` skips LODs at or above
`fullResolutionLodCount`, keys covered by the initial `ShapeBuildRequest`, and keys already
resident. Its optional output counts requested keys that received a new region. After the regions
needed by the next desired set compile, call `retainTileRegions` with that set to release regions
that are no longer needed. The Core owns region lifetime and mesh data; the host keeps still-visible
meshes resident until replacement tiles are generated and uploaded.

Pass `analyticOnly = false` to retain the reference packed-page compiler. Packed pages use absolute
parent-cell coordinates and an 8-voxel apron; each covers a 128-voxel cube or one larger coarse
tile. This path remains available for parity checks and workloads that need the packed lattice.

## World assembler manifest v1 (evaluator v1–v4)

`ResolvedWorldSnapshot::FromJson` accepts an optional `world_assembly` object alongside the owner
snapshot. The object is the shipped-plan payload shape: a fixed-scale recipe and finite-strate plan
with hashes, no generated mesh or voxel samples. The current review payload is written to
[`round4/world_assembly.json`](../Research/WorldAssemblerM1/round4/world_assembly.json).
Evaluator 1–3 payloads remain readable with their original hashes and field semantics. Evaluator 4
adds a minimum two-voxel shelf thickness and a versioned route support contract; it requires at
least two route waypoints.

Every coordinate and scalar uses signed integer units of 1/256 voxel (`units_per_voxel: 256`),
independent of the plugin's 25 cm Core voxel and of tile size. The schema version is 1; evaluator
versions 1 and 2 are accepted, with evaluator 2 used for new recipes. Evaluator 1 retains its
original evaluation and hash semantics. JSON integer fields are canonicalized in declared field
order and hashed as 4- or 8-byte little-endian words with FNV-1a 64. Hashes are emitted as 16
lowercase hexadecimal characters; JSON key order and whitespace do not affect them. The root,
recipe, plan and each stratum carry a content hash. The recipe stores its seed, warped biome-map
settings and one to eight biome profiles; the plan stores finite-strate extents, cap settings,
stable-ID features, fixed-season water, route points and (for evaluator 4) player-capsule, grade,
support and sweep-spacing constraints. Route points are fixed-scale capsule-center locations with
stable IDs and a protected automatic-change corridor radius. Bounds, feature count, route count, route
IDs, corridor widths and finite-strate capsule extents are validated before a manifest is accepted.

Evaluator 2 adds climate-based biome assignment, output-space blending of both ground and cap
profiles, per-biome relief and height warp, beach width, cliff limits, and optional per-biome cap
profiles. Feature types 1, 2 and 3 represent a cliff plane, a bounded 3D overhang box and an authored
cap feature. An overhang feature has a stable ID and a finite AABB clipped to its stratum; intervals
outside the box can use its certified influence bound, while intersecting intervals remain on the
evaluation path. Water surface and fill are separate sample outputs. The season water plane is fixed;
this block does not generate a water mesh.

```cpp
std::string error;
auto manifest = VoxelForgeCore::WorldAssemblyManifest::FromJson(payload, error);
if (!manifest || !manifest->validate(error)) { /* reject the payload */ }
const auto* loaded = snapshot.worldAssembly(); // null when the owner snapshot has no plan
```

`Heightfield2D` is the first field block. `point(xQ, yQ, zQ)` evaluates the biome map and blended
height profiles at fixed-scale coordinates, returning the ground and cap heights, primary and
neighbor biome IDs, Q16 blend weight and signed density. Evaluator 2 also returns the fixed water
surface and water-fill sample values. Negative density is solid. Its contract reads `biome` and
writes `height`, `rock`, `water` and `waterSurface`; route and `surfaceTags` remain declared channels
for later blocks. The detail envelope reports maximum absolute displacement and minimum feature
size. Evaluator 4 defines the shelf's minimum solid thickness as two Core voxels (0.5 m) and applies
its feather to the horizontal footprint so it cannot thin the roof-to-floor section.
`AuditHeightfieldShelfThickness` samples shelf sections against the pre-feature field and includes
the previous sub-voxel taper as a negative control. Influence is global in XY and limited to the
finite stratum's Z extent, except for the explicitly bounded feature boxes.

`interval(minXQ, minYQ, minZQ, maxXQ, maxYQ, maxZQ)` returns a conservative density range or
`known == false`; callers must keep evaluating when it is unknown. The current heightfield range
uses conservative profile, beach, cliff, cap and bounded-feature envelopes. The public
`AuditHeightfieldBounds` samples points in randomized boxes and targeted biome-border, cliff, cap,
negative-coordinate and overhang boxes, and includes a deliberately false one-sided range as a
negative control. A pass means no sampled point escaped and the negative control was rejected; it
is an audit of this block implementation, not a proof for future blocks.

`AuditHeightfieldRoute` sweeps the recorded route through the final composed field using the
versioned player capsule (0.34 m radius, 1.76 m height), checks a 15-degree maximum grade, samples
capsule clearance at no more than half-voxel intervals, and probes support below five foot locations
at each swept center. Automatic changes must be composed first and pass this audit before the plan is
accepted. Player carve/fill edits remain unrestricted. This validates analytic field collision and
support; it does not cook an Unreal physics mesh.

## Floating-island feature family v1 (standalone Core)

`FloatingIslandRecipeV1` and `FloatingIslandManifestV1` define a separate, reusable finite 3D feature
family. Coordinates, dimensions, and density values use signed 1/256-voxel units; negative density
is solid. The recipe carries its seed, fixed vertical layout, site density, island count and spacing,
radius range, top flattening, taper, bounded contour warp, roughness, blend, connector dimensions and
route constraints. `PlanFloatingIslandCluster` emits stable island and connector IDs, a connected
cluster graph, conservative influence AABBs, and a centerline route. At least the primary chain is
always planned; `islandDensityQ16` controls up to two optional side sites. Connectors carry a sharp-tag
ready bit for a later hybrid-mesher adapter. The schema accepts geometry version 1 and 2. Manifests
without a geometry version retain version-1 evaluation and canonical hashes.

`FloatingIslandFieldV1::densitySubvoxels` evaluates the canonical island and connector field.
`interval` returns unknown for boxes overlapping any influence AABB and a certified all-air range
outside them. Geometry version 2 builds a flat walkable crust plus deterministic lobed rock roots,
keeps bounded oblique surface detail inside its certified envelope, and arches the connector
undersides. AABBs include root depth, contour displacement, detail, arch sag and smoothing reach.
`AuditFloatingIslandBounds` checks sampled boxes and void beside connectors and includes a deliberately
shrunken island-bound negative control. `AuditFloatingIslandThickness` reports each island's
`(topZ - bottomZ) / (2 * radius)` and checks a caller-supplied minimum; `AuditFloatingIslandRoute`
samples the player capsule and support along the serialized route. M3 round 2 passed at a minimum
depth-to-diameter ratio of 0.75 against a 0.70 threshold, while a 0.30 thin-pad control failed.

The family is still separate from `WorldAssemblyManifest` and the Unreal adapter. M3 rounds 1–2 prove
the standalone Core representation, bounded geometry, route, determinism and matched legacy timing.
The frozen owner evaluator is a visual and timing oracle, not a hash target. Unreal import and cooked
collision are planned for M3 round 3.

## Queries and tiles

`densityAt(position)` returns signed density; negative is solid and nonnegative is air. Server and
authority code can compile/query a `World` and run `AuditHeightfieldRoute` without calling
`generateTile`; generated render meshes are not part of this query path. Unreal physics collision
cooking remains the adapter's separate readiness step.
`isSolidAt` wraps that sign test. `isSolidAlongSegment(start, end, spacing)` samples evenly along
the segment with no interval wider than `spacing` voxels; it returns true if any sample is solid.
`classifyBox(origin, step, cells)` returns `allSolid`, `allAir` or `mixed` for mesher tile
classification.

`desiredTiles(cameraVoxels)` returns the snapshot's clipmap set. Pass it to
`prioritizeTiles(tiles, centerChunk)` when submitting work. Generate independent keys concurrently
on host workers:

```cpp
VoxelForgeCore::TileGenerationOptions options;
auto mesh = world.generateTile(key, options, error);
if (!mesh) { /* report error */ }
// mesh->vertices, mesh->indices and mesh->groups are owned by this result.
```

The optional `hashDensity` flag fills deterministic density, viewer-mesh, payload and topology
hashes. `captureDensitySamples` is a diagnostic option and retains the sampled lattice values.

## Deterministic edits

Build an `EditTransaction` with a monotonically increasing authority sequence and an ordered brush
array (`sphere`, `capsule`, `roundedBox`; `carve`, `addRock`, `densityOffset`). `carve` and
`addRock` apply hard signed-distance CSG. `densityOffset` applies `strength` inside a sphere or
inside/around a capsule or box with its `falloff` band; negative strength carves and positive
strength adds rock. This operation maps the plugin's soft `FVoxelModification` brushes into the
shared field. Apply the transaction against the currently desired keys:

```cpp
VoxelForgeCore::EditTransaction edit;
edit.sequence = 12;
VoxelForgeCore::EditBrush brush;
brush.shape = VoxelForgeCore::EditBrush::Shape::capsule;
brush.operation = VoxelForgeCore::EditBrush::Operation::densityOffset;
brush.a = {0.0, 0.0, 0.0};
brush.b = {12.0, 0.0, 0.0};
brush.radius = 3.0;
brush.strength = -10.0f;
brush.falloff = 2.0f;
edit.brushes.push_back(brush);
const auto affected = world.applyEditTransaction(edit, currentTiles, error);
if (affected) {
    for (const auto& lodTiles : affected->affectedByLod)
        for (const auto& key : lodTiles)
            submitHostJob([&, key] {
                std::string tileError;
                return world.generateTile(key, options, tileError);
            });
}
```

The transaction digest covers the sequence and each brush field in order. Replaying the same
sequence and values returns the original result; reusing a sequence with different values or
applying an older sequence is rejected. Each client must deserialize identical brush values and
apply transactions in the same sequence. `affectedByLod` is limited to the desired keys passed to
the call.

## Other outputs

`farConnectorSegments()` returns owned connector geometry, while `farConnectorCandidates(min,max)`
returns candidate indices overlapping a voxel-space box. The audit methods expose shape detail,
surface-family and camera-look checks for host diagnostics. All returned values are standard C++;
the host owns conversion to engine mesh, collision or renderer objects.