Apply some simple optimizations for vanilla section meshing

common/src/main/java/org/embeddedt/modernfix/common/mixin/perf/chunk_meshing/RebuildTaskMixin.java

@@ -0,0 +1,31 @@
+package org.embeddedt.modernfix.common.mixin.perf.chunk_meshing;
+
+import com.llamalad7.mixinextras.sugar.Local;
+import net.minecraft.client.renderer.chunk.RenderChunkRegion;
+import net.minecraft.core.BlockPos;
+import net.minecraft.world.level.block.state.BlockState;
+import org.embeddedt.modernfix.util.blockpos.SectionBlockPosIterator;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Redirect;
+
+@Mixin(targets = { "net/minecraft/client/renderer/chunk/ChunkRenderDispatcher$RenderChunk$RebuildTask"}, priority = 2000)
+public class RebuildTaskMixin {
+    /**
+     * @author embeddedt
+     * @reason Use a much faster iterator implementation than vanilla's Guava-based one.
+     */
+    @Redirect(method = "compile", at = @At(value = "INVOKE", target = "Lnet/minecraft/core/BlockPos;betweenClosed(Lnet/minecraft/core/BlockPos;Lnet/minecraft/core/BlockPos;)Ljava/lang/Iterable;"))
+    private Iterable<BlockPos> fastBetweenClosed(BlockPos firstPos, BlockPos secondPos) {
+        return () -> new SectionBlockPosIterator(firstPos);
+    }
+
+    /**
+     * @author embeddedt
+     * @reason RenderChunkRegion.getBlockState is expensive, avoid calling it multiple times for the same position
+     */
+    @Redirect(method = "compile", at = @At(value = "INVOKE", target = "Lnet/minecraft/client/renderer/chunk/RenderChunkRegion;getBlockState(Lnet/minecraft/core/BlockPos;)Lnet/minecraft/world/level/block/state/BlockState;", ordinal = 1))
+    private BlockState useExistingBlockState(RenderChunkRegion instance, BlockPos pos, @Local(ordinal = 0) BlockState state) {
+        return state;
+    }
+}

common/src/main/java/org/embeddedt/modernfix/util/blockpos/SectionBlockPosIterator.java

@@ -0,0 +1,39 @@
+package org.embeddedt.modernfix.util.blockpos;
+
+import net.minecraft.core.BlockPos;
+
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+
+public class SectionBlockPosIterator implements Iterator<BlockPos> {
+    private final BlockPos.MutableBlockPos pos = new BlockPos.MutableBlockPos();
+    private int index = 0;
+    private final int baseX, baseY, baseZ;
+
+    public SectionBlockPosIterator(int baseX, int baseY, int baseZ) {
+        this.baseX = baseX;
+        this.baseY = baseY;
+        this.baseZ = baseZ;
+    }
+
+    public SectionBlockPosIterator(BlockPos pos) {
+        this(pos.getX(), pos.getY(), pos.getZ());
+    }
+
+    @Override
+    public boolean hasNext() {
+        return index < 4096;
+    }
+
+    @Override
+    public BlockPos next() {
+        int i = index;
+        if (i >= 4096) {
+            throw new NoSuchElementException();
+        }
+        index = i + 1;
+        var pos = this.pos;
+        pos.set(this.baseX + (i & 15), this.baseY + ((i >> 8) & 15), this.baseZ + ((i >> 4) & 15));
+        return pos;
+    }
+}