Assert when two entities should not share the same grid position but do

src/bin/tetris/main.rs

@@ -77,6 +77,7 @@ fn main() {
                 adjust_block_lines
                     .run_if(any_component_changed::<Line>)
                     .after(clear_line),
+                assert_grid_position_uniqueness.run_if(any_component_changed::<GridPosition>),
             ),
         )
         // UI systems
@@ -272,6 +273,9 @@ impl Display for ShapeStore {
     }
 }
 
+#[derive(Component)]
+struct GridBackground;
+
 fn init_world(
     mut commands: Commands,
     mut meshes: ResMut<Assets<Mesh>>,
@@ -299,6 +303,7 @@ fn init_world(
                 MeshMaterial2d(grid_material.clone()),
                 GridPosition { x, y },
                 Transform::from_xyz(0.0, 0.0, -1.0),
+                GridBackground,
                 TETRIS,
             ));
         });
@@ -441,13 +446,13 @@ fn init_ui(mut commands: Commands, output_images: Res<OutputImages>, images: Res
             border: UiRect::all(Val::Px(5.0)),
             ..default()
         },
-        BorderColor(BLACK.into()),
+        BorderColor(WHITE.into()),
     )).with_children(|parent| {
         let img = images.get(&output_images.tetris).unwrap();
         parent.spawn((
             Node {
-                width: Val::Px(img.size_f32().x * 0.5),
-                height: Val::Px(img.size_f32().y * 0.5),
+                width: Val::Px(img.size_f32().x * 0.75),
+                height: Val::Px(img.size_f32().y * 0.75),
                 ..default()
             },
             ImageNode {
@@ -465,7 +470,7 @@ fn init_ui(mut commands: Commands, output_images: Res<OutputImages>, images: Res
             border: UiRect::all(Val::Px(5.0)),
             ..default()
         },
-        BorderColor(BLACK.into()),
+        BorderColor(WHITE.into()),
     )).with_children(|parent| {
         let img = images.get(&output_images.battler).unwrap();
         parent.spawn((
@@ -919,7 +924,7 @@ fn clear_line(
 fn adjust_block_lines(
     query: Query<(Entity, &Line), Changed<Line>>,
     parent: Query<&LineBlocks>,
-    mut blocks: Query<&mut GridPosition, With<ShapeBlock>>,
+    mut blocks: Query<&mut GridPosition>,
 ) {
     query.iter().for_each(|(e, Line(i))| {
         parent.iter_descendants(e).for_each(|block| {
@@ -1093,3 +1098,11 @@ fn update_next_shapes(mut buffer: ResMut<ShapesBuffer>) {
         ]);
     }
 }
+
+fn assert_grid_position_uniqueness(
+    grid_positions: Query<&GridPosition, (Without<GridBackground>, Without<Shape>)>
+) {
+    grid_positions.iter_combinations().for_each(|[a, b]| {
+        assert_ne!(a, b, "Two entities are in the same grid position!");
+    });
+}