Fix block placement below players (#767)

src/main/java/net/minestom/server/collision/BlockCollision.java

@@ -3,11 +3,17 @@ package net.minestom.server.collision;
 import net.minestom.server.coordinate.Point;
 import net.minestom.server.coordinate.Pos;
 import net.minestom.server.coordinate.Vec;
+import net.minestom.server.entity.Entity;
+import net.minestom.server.entity.EntityType;
+import net.minestom.server.entity.metadata.other.ArmorStandMeta;
+import net.minestom.server.instance.Instance;
 import net.minestom.server.instance.block.Block;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
-final class BlockCollision {
+import java.util.Collection;
+
+public final class BlockCollision {
     // Minimum move amount, minimum final velocity
     private static final double MIN_DELTA = 0.001;
 
@@ -325,6 +331,36 @@ final class BlockCollision {
                 Vec.ZERO, finalResult.collidedShapePosition, finalResult.blockType);
     }
 
+    static boolean canPlaceBlockAt(Instance instance, Point blockPos, Block b) {
+        final Collection<Entity> entities = instance.getNearbyEntities(blockPos, 3);
+
+        return entities
+                .stream()
+                .filter(entity -> entity.getEntityType() != EntityType.ITEM)
+                .filter(entity -> {
+                    // Marker Armor Stands should not prevent block placement
+                    if (entity.getEntityMeta() instanceof ArmorStandMeta armorStandMeta) {
+                        return !armorStandMeta.isMarker();
+                    }
+                    return true;
+                })
+                .noneMatch(entity -> {
+                    boolean intersects;
+
+                    if (entity.getEntityType() == EntityType.PLAYER) {
+                        // Need to move player slightly away from block we're placing.
+                        // If player is at block 40 we cannot place a block at block 39 with side length 1 because the block will be in [39, 40]
+                        // For this reason we subtract a small amount from the player position
+                        Point playerPos = entity.getPosition().add(entity.getPosition().sub(blockPos).mul(0.01));
+                        intersects = b.registry().collisionShape().intersectBox(playerPos.sub(blockPos), entity.getBoundingBox());
+                    } else {
+                        intersects = b.registry().collisionShape().intersectBox(entity.getPosition().sub(blockPos), entity.getBoundingBox());
+                    }
+
+                    return intersects;
+                });
+    }
+
     /**
      * Check if a moving entity will collide with a block. Updates finalResult
      *

src/main/java/net/minestom/server/collision/CollisionUtils.java

@@ -1,5 +1,6 @@
 package net.minestom.server.collision;
 
+import net.minestom.server.coordinate.Point;
 import net.minestom.server.coordinate.Pos;
 import net.minestom.server.coordinate.Vec;
 import net.minestom.server.entity.Entity;
@@ -41,6 +42,10 @@ public final class CollisionUtils {
         return handlePhysics(entity, entityVelocity, null);
     }
 
+    public static boolean canPlaceBlockAt(Instance instance, Point blockPos, Block b) {
+        return BlockCollision.canPlaceBlockAt(instance, blockPos, b);
+    }
+
     /**
      * Applies world border collision.
      *

src/main/java/net/minestom/server/listener/BlockPlacementListener.java

@@ -1,12 +1,10 @@
 package net.minestom.server.listener;
 
 import net.minestom.server.MinecraftServer;
+import net.minestom.server.collision.CollisionUtils;
 import net.minestom.server.coordinate.Point;
-import net.minestom.server.entity.Entity;
-import net.minestom.server.entity.EntityType;
 import net.minestom.server.entity.GameMode;
 import net.minestom.server.entity.Player;
-import net.minestom.server.entity.metadata.other.ArmorStandMeta;
 import net.minestom.server.event.EventDispatcher;
 import net.minestom.server.event.player.PlayerBlockInteractEvent;
 import net.minestom.server.event.player.PlayerBlockPlaceEvent;
@@ -26,8 +24,6 @@ import net.minestom.server.network.packet.server.play.BlockChangePacket;
 import net.minestom.server.utils.chunk.ChunkUtils;
 import net.minestom.server.utils.validate.Check;
 
-import java.util.Collection;
-
 public class BlockPlacementListener {
     private static final BlockManager BLOCK_MANAGER = MinecraftServer.getBlockManager();
 
@@ -109,24 +105,7 @@ public class BlockPlacementListener {
         }
 
         final Block placedBlock = useMaterial.block();
-        final Collection<Entity> entities = instance.getNearbyEntities(placementPosition, 5);
-
-        // Check if the player is trying to place a block in an entity
-        boolean intersectPlayer = placedBlock.registry().collisionShape().intersectBox(player.getPosition().sub(placementPosition), player.getBoundingBox());
-
-        boolean hasIntersect = intersectPlayer || entities
-                .stream()
-                .filter(entity -> entity.getEntityType() != EntityType.ITEM)
-                .filter(entity -> {
-                    // Marker Armor Stands should not prevent block placement
-                    if (entity.getEntityMeta() instanceof ArmorStandMeta armorStandMeta) {
-                        return !armorStandMeta.isMarker();
-                    }
-                    return true;
-                })
-                .anyMatch(entity -> placedBlock.registry().collisionShape().intersectBox(entity.getPosition().sub(placementPosition), entity.getBoundingBox()));
-
-        if (hasIntersect) {
+        if (!CollisionUtils.canPlaceBlockAt(instance, placementPosition, placedBlock)) {
             refresh(player, chunk);
             return;
         }

src/test/java/net/minestom/server/collision/PlacementCollisionIntegrationTest.java

@@ -0,0 +1,44 @@
+package net.minestom.server.collision;
+
+import net.minestom.server.api.Env;
+import net.minestom.server.api.EnvTest;
+import net.minestom.server.coordinate.Pos;
+import net.minestom.server.coordinate.Vec;
+import net.minestom.server.entity.Entity;
+import net.minestom.server.entity.EntityType;
+import net.minestom.server.instance.block.Block;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+@EnvTest
+public class PlacementCollisionIntegrationTest {
+
+    @Test
+    public void empty(Env env) {
+        var instance = env.createFlatInstance();
+        assertTrue(BlockCollision.canPlaceBlockAt(instance, new Vec(0, 40, 0), Block.STONE));
+    }
+
+    @Test
+    public void entityBlock(Env env) {
+        var instance = env.createFlatInstance();
+        new Entity(EntityType.ZOMBIE).setInstance(instance, new Pos(0, 40, 0)).join();
+        assertFalse(BlockCollision.canPlaceBlockAt(instance, new Vec(0, 40, 0), Block.STONE));
+    }
+
+    @Test
+    public void slab(Env env) {
+        var instance = env.createFlatInstance();
+        new Entity(EntityType.ZOMBIE).setInstance(instance, new Pos(0, 40.75, 0)).join();
+        assertTrue(BlockCollision.canPlaceBlockAt(instance, new Vec(0, 40, 0), Block.STONE_SLAB));
+    }
+
+    @Test
+    public void belowPlayer(Env env) {
+        var instance = env.createFlatInstance();
+        env.createPlayer(instance, new Pos(5.7, -8, 6.389));
+        assertTrue(BlockCollision.canPlaceBlockAt(instance, new Vec(5, -9, 6), Block.STONE));
+    }
+}