Add ITerrainGenerator with GenerationUnit fork support for cross-chunk modifications

Minecraft/Implementations/Server/Terrain/ChunkGenerationModifier.cs

@@ -0,0 +1,76 @@
+using Minecraft.Data.Blocks;
+using Minecraft.Schemas.Chunks;
+using Minecraft.Schemas.Vec;
+
+namespace Minecraft.Implementations.Server.Terrain;
+
+/// <summary>
+/// A modifier that modifies blocks within a single chunk.
+/// </summary>
+internal class ChunkGenerationModifier : IGenerationModifier {
+    private readonly ChunkData _chunk;
+    private readonly int _chunkAbsoluteX;
+    private readonly int _chunkAbsoluteZ;
+    private readonly int _minY;
+    private readonly int _maxY;
+
+    public ChunkGenerationModifier(ChunkData chunk, int minY) {
+        _chunk = chunk;
+        _chunkAbsoluteX = chunk.ChunkX * ChunkSection.Size;
+        _chunkAbsoluteZ = chunk.ChunkZ * ChunkSection.Size;
+        _minY = minY;
+        _maxY = minY + chunk.WorldHeight;
+    }
+
+    public void SetBlock(Vec3<int> position, IBlock block) {
+        // Check if the position is within this chunk
+        if (!IsInChunk(position)) {
+            return;
+        }
+
+        // Convert to chunk-local coordinates using bitwise AND (ChunkSection.Size is 16, a power of 2)
+        int localX = position.X & (ChunkSection.Size - 1);
+        int localZ = position.Z & (ChunkSection.Size - 1);
+        int localY = position.Y - _minY;
+
+        if (localY < 0 || localY >= _chunk.WorldHeight) {
+            return;
+        }
+
+        _chunk.SetBlock(localX, localY, localZ, block.StateId);
+    }
+
+    public void Fill(Vec3<int> start, Vec3<int> end, IBlock block) {
+        // Clamp to chunk boundaries
+        int startX = Math.Max(start.X, _chunkAbsoluteX);
+        int startZ = Math.Max(start.Z, _chunkAbsoluteZ);
+        int startY = Math.Max(start.Y, _minY);
+
+        int endX = Math.Min(end.X, _chunkAbsoluteX + ChunkSection.Size);
+        int endZ = Math.Min(end.Z, _chunkAbsoluteZ + ChunkSection.Size);
+        int endY = Math.Min(end.Y, _maxY);
+
+        for (int x = startX; x < endX; x++) {
+            for (int y = startY; y < endY; y++) {
+                for (int z = startZ; z < endZ; z++) {
+                    SetBlock(new Vec3<int>(x, y, z), block);
+                }
+            }
+        }
+    }
+
+    public void FillHeight(int minY, int maxY, IBlock block) {
+        Fill(
+            new Vec3<int>(_chunkAbsoluteX, minY, _chunkAbsoluteZ),
+            new Vec3<int>(_chunkAbsoluteX + ChunkSection.Size, maxY, _chunkAbsoluteZ + ChunkSection.Size),
+            block
+        );
+    }
+
+    private bool IsInChunk(Vec3<int> position) {
+        int localX = position.X - _chunkAbsoluteX;
+        int localZ = position.Z - _chunkAbsoluteZ;
+        return localX >= 0 && localX < ChunkSection.Size &&
+               localZ >= 0 && localZ < ChunkSection.Size;
+    }
+}