VoxelEngine/World/WorldGen/CaveCarver.cs

using OpenTK.Mathematics;
using VoxelEngine.Core;
using VoxelEngine.World.Blocks;
using VoxelChunk = VoxelEngine.World.Chunk.Chunk;

namespace VoxelEngine.World.WorldGen;

// Carves caves into an already-terrain-filled chunk. Stateless after
// construction; the carve methods are pure functions of (chunk, chunkPos).
// Safe to share across worker threads.
public sealed class CaveCarver
{
    private readonly int _seed;
    private readonly BlockRegistry _blocks;
    private readonly Configuration _cfg;
    private readonly FastNoiseLite _caveNoise;
    private readonly FastNoiseLite _wormDirNoise;

    public CaveCarver(int seed, BlockRegistry blocks, Configuration cfg)
    {
        this._seed = seed;
        this._blocks = blocks;
        this._cfg = cfg;

        this._caveNoise = new FastNoiseLite(seed + 100);
        this._caveNoise.SetNoiseType(FastNoiseLite.NoiseType.OpenSimplex2);
        this._caveNoise.SetFrequency(cfg.CaveNoiseFrequency);

        this._wormDirNoise = new FastNoiseLite(seed + 200);
        this._wormDirNoise.SetNoiseType(FastNoiseLite.NoiseType.OpenSimplex2);
        this._wormDirNoise.SetFrequency(0.05f);
    }

    public void Carve(VoxelChunk chunk, Vector3i chunkPos)
    {
        this.CarveNoiseCaves(chunk, chunkPos);
        this.CarveWormCaves(chunk, chunkPos);
    }

    // Per-voxel 3D simplex threshold. Public for direct testability,
    // matching the pattern of WorldGenerator.BlockAt.
    public void CarveNoiseCaves(VoxelChunk chunk, Vector3i chunkPos)
    {
        int baseX = chunkPos.X * VoxelChunk.Size;
        int baseY = chunkPos.Y * VoxelChunk.Size;
        int baseZ = chunkPos.Z * VoxelChunk.Size;

        for (int x = 0; x < VoxelChunk.Size; x++)
        for (int y = 0; y < VoxelChunk.Size; y++)
        for (int z = 0; z < VoxelChunk.Size; z++)
        {
            int worldY = baseY + y;
            if (worldY <= 0) continue;

            ushort id = chunk[x, y, z];
            if (id == this._blocks.Air.Id) continue;
            if (id == this._blocks.Water.Id) continue;

            float density = this._caveNoise.GetNoise(baseX + x, worldY, baseZ + z);
            if (density < this._cfg.CaveNoiseThreshold)
            {
                chunk[x, y, z] = this._blocks.Air.Id;
            }
        }
    }

    public void CarveWormCaves(VoxelChunk chunk, Vector3i chunkPos)
    {
        int cellSize = this._cfg.WormCellSize;
        int radius = (int)Math.Ceiling((double)this._cfg.WormMaxLength / cellSize) + 1;

        int chunkBaseX = chunkPos.X * VoxelChunk.Size;
        int chunkBaseY = chunkPos.Y * VoxelChunk.Size;
        int chunkBaseZ = chunkPos.Z * VoxelChunk.Size;

        int chunkCellX = CaveCarver.FloorDiv(chunkBaseX, cellSize);
        int chunkCellY = CaveCarver.FloorDiv(chunkBaseY, cellSize);
        int chunkCellZ = CaveCarver.FloorDiv(chunkBaseZ, cellSize);

        for (int cdx = -radius; cdx <= radius; cdx++)
        for (int cdy = -radius; cdy <= radius; cdy++)
        for (int cdz = -radius; cdz <= radius; cdz++)
        {
            int cellX = chunkCellX + cdx;
            int cellY = chunkCellY + cdy;
            int cellZ = chunkCellZ + cdz;

            uint h = WormHash.Hash(cellX, cellY, cellZ, this._seed);
            if ((h % 1000) >= (uint)this._cfg.WormSpawnProbabilityPermille) continue;

            // Hash bit layout: [ox:6][oy:6][oz:6][theta:8][phi:6] = 32 bits
            float ox = (h         & 0x3F) / 64f * cellSize;
            float oy = ((h >> 6)  & 0x3F) / 64f * cellSize;
            float oz = ((h >> 12) & 0x3F) / 64f * cellSize;
            Vector3 pos = new(cellX * cellSize + ox,
                              cellY * cellSize + oy,
                              cellZ * cellSize + oz);

            float theta = ((h >> 18) & 0xFF) / 255f * MathF.Tau;
            float phi   = (((h >> 26) & 0x3F) / 63f * 2f - 1f) * 0.3f * MathF.PI;
            Vector3 dir = new(
                MathF.Cos(theta) * MathF.Cos(phi),
                MathF.Sin(phi),
                MathF.Sin(theta) * MathF.Cos(phi));
            dir = Vector3.Normalize(dir);

            this.TraceAndCarveWorm(chunk, chunkPos, pos, dir);
        }
    }

    private void TraceAndCarveWorm(VoxelChunk chunk, Vector3i chunkPos,
                                   Vector3 startPos, Vector3 startDir)
    {
        int maxLen = this._cfg.WormMaxLength;
        float minR = this._cfg.WormMinRadius;
        float maxR = this._cfg.WormMaxRadius;

        Vector3 pos = startPos;
        Vector3 dir = startDir;

        for (int step = 0; step < maxLen; step++)
        {
            float t = step / (float)maxLen;
            float bulgeT = 0.5f + 0.5f * MathF.Sin(t * MathF.Tau * 3f);
            float r = minR + (maxR - minR) * bulgeT;

            this.CarveSphere(chunk, chunkPos, pos, r);

            float px = this._wormDirNoise.GetNoise(pos.X,        pos.Y, pos.Z);
            float py = this._wormDirNoise.GetNoise(pos.X + 100f, pos.Y, pos.Z);
            float pz = this._wormDirNoise.GetNoise(pos.X,        pos.Y, pos.Z + 100f);
            Vector3 perturbation = new(px, py, pz);
            dir = Vector3.Normalize(dir + perturbation * 0.3f);

            dir = new Vector3(dir.X, dir.Y * 0.7f, dir.Z);
            dir = Vector3.Normalize(dir);

            pos += dir;
        }
    }

    private void CarveSphere(VoxelChunk chunk, Vector3i chunkPos, Vector3 center, float r)
    {
        int baseX = chunkPos.X * VoxelChunk.Size;
        int baseY = chunkPos.Y * VoxelChunk.Size;
        int baseZ = chunkPos.Z * VoxelChunk.Size;

        int xMin = Math.Max(0, (int)MathF.Floor(center.X - r) - baseX);
        int xMax = Math.Min(VoxelChunk.Size - 1, (int)MathF.Ceiling(center.X + r) - baseX);
        int yMin = Math.Max(0, (int)MathF.Floor(center.Y - r) - baseY);
        int yMax = Math.Min(VoxelChunk.Size - 1, (int)MathF.Ceiling(center.Y + r) - baseY);
        int zMin = Math.Max(0, (int)MathF.Floor(center.Z - r) - baseZ);
        int zMax = Math.Min(VoxelChunk.Size - 1, (int)MathF.Ceiling(center.Z + r) - baseZ);

        float r2 = r * r;

        for (int x = xMin; x <= xMax; x++)
        for (int y = yMin; y <= yMax; y++)
        for (int z = zMin; z <= zMax; z++)
        {
            int worldY = baseY + y;
            if (worldY <= 0) continue;

            float vx = baseX + x + 0.5f;
            float vy = baseY + y + 0.5f;
            float vz = baseZ + z + 0.5f;
            float dx = vx - center.X, dy = vy - center.Y, dz = vz - center.Z;
            if (dx * dx + dy * dy + dz * dz > r2) continue;

            ushort id = chunk[x, y, z];
            if (id == this._blocks.Air.Id) continue;
            if (id == this._blocks.Water.Id) continue;
            chunk[x, y, z] = this._blocks.Air.Id;
        }
    }

    private static int FloorDiv(int a, int b) => a >= 0 ? a / b : -(((-a) + b - 1) / b);
}