bevy/crates/bevy_ecs/src/lib.rs

#![warn(clippy::undocumented_unsafe_blocks)]
#![warn(missing_docs)]
#![allow(clippy::type_complexity)]
#![doc = include_str!("../README.md")]

#[cfg(target_pointer_width = "16")]
compile_error!("bevy_ecs cannot safely compile for a 16-bit platform.");

pub mod archetype;
pub mod bundle;
pub mod change_detection;
pub mod component;
pub mod entity;
pub mod event;
pub mod query;
#[cfg(feature = "bevy_reflect")]
pub mod reflect;
pub mod removal_detection;
pub mod schedule;
pub mod storage;
pub mod system;
pub mod world;

use std::any::TypeId;

pub use bevy_ptr as ptr;

/// Most commonly used re-exported types.
pub mod prelude {
    #[doc(hidden)]
    #[cfg(feature = "bevy_reflect")]
    pub use crate::reflect::{AppTypeRegistry, ReflectComponent, ReflectResource};
    #[doc(hidden)]
    pub use crate::{
        bundle::Bundle,
        change_detection::{DetectChanges, DetectChangesMut, Mut, Ref},
        component::Component,
        entity::Entity,
        event::{Event, EventReader, EventWriter, Events},
        query::{Added, AnyOf, Changed, Has, Or, QueryState, With, Without},
        removal_detection::RemovedComponents,
        schedule::{
            apply_deferred, apply_state_transition, common_conditions::*, Condition,
            IntoSystemConfigs, IntoSystemSet, IntoSystemSetConfig, IntoSystemSetConfigs, NextState,
            OnEnter, OnExit, OnTransition, Schedule, Schedules, State, States, SystemSet,
        },
        system::{
            adapter as system_adapter,
            adapter::{dbg, error, ignore, info, unwrap, warn},
            Commands, Deferred, In, IntoSystem, Local, NonSend, NonSendMut, ParallelCommands,
            ParamSet, Query, ReadOnlySystem, Res, ResMut, Resource, System, SystemParamFunction,
        },
        world::{EntityRef, FromWorld, World},
    };
}

pub use bevy_utils::all_tuples;

/// A specialized hashmap type with Key of [`TypeId`]
type TypeIdMap<V> = rustc_hash::FxHashMap<TypeId, V>;

#[cfg(test)]
mod tests {
    use crate as bevy_ecs;
    use crate::prelude::Or;
    use crate::{
        bundle::Bundle,
        change_detection::Ref,
        component::{Component, ComponentId},
        entity::Entity,
        query::{Added, Changed, FilteredAccess, ReadOnlyWorldQuery, With, Without},
        system::Resource,
        world::{EntityRef, Mut, World},
    };
    use bevy_tasks::{ComputeTaskPool, TaskPool};
    use std::{
        any::TypeId,
        marker::PhantomData,
        sync::{
            atomic::{AtomicUsize, Ordering},
            Arc, Mutex,
        },
    };

    #[derive(Component, Resource, Debug, PartialEq, Eq, Clone, Copy)]
    struct A(usize);
    #[derive(Component, Debug, PartialEq, Eq, Clone, Copy)]
    struct B(usize);
    #[derive(Component, Debug, PartialEq, Eq, Clone, Copy)]
    struct C;

    #[derive(Default)]
    struct NonSendA(usize, PhantomData<*mut ()>);

    #[derive(Component, Clone, Debug)]
    struct DropCk(Arc<AtomicUsize>);
    impl DropCk {
        fn new_pair() -> (Self, Arc<AtomicUsize>) {
            let atomic = Arc::new(AtomicUsize::new(0));
            (DropCk(atomic.clone()), atomic)
        }
    }

    impl Drop for DropCk {
        fn drop(&mut self) {
            self.0.as_ref().fetch_add(1, Ordering::Relaxed);
        }
    }

    #[derive(Component, Clone, Debug)]
    #[component(storage = "SparseSet")]
    struct DropCkSparse(DropCk);

    #[derive(Component, Copy, Clone, PartialEq, Eq, Debug)]
    #[component(storage = "Table")]
    struct TableStored(&'static str);
    #[derive(Component, Copy, Clone, PartialEq, Eq, Debug)]
    #[component(storage = "SparseSet")]
    struct SparseStored(u32);

    #[test]
    fn random_access() {
        let mut world = World::new();

        let e = world.spawn((TableStored("abc"), SparseStored(123))).id();
        let f = world
            .spawn((TableStored("def"), SparseStored(456), A(1)))
            .id();
        assert_eq!(world.get::<TableStored>(e).unwrap().0, "abc");
        assert_eq!(world.get::<SparseStored>(e).unwrap().0, 123);
        assert_eq!(world.get::<TableStored>(f).unwrap().0, "def");
        assert_eq!(world.get::<SparseStored>(f).unwrap().0, 456);

        // test archetype get_mut()
        world.get_mut::<TableStored>(e).unwrap().0 = "xyz";
        assert_eq!(world.get::<TableStored>(e).unwrap().0, "xyz");

        // test sparse set get_mut()
        world.get_mut::<SparseStored>(f).unwrap().0 = 42;
        assert_eq!(world.get::<SparseStored>(f).unwrap().0, 42);
    }

    #[test]
    fn bundle_derive() {
        let mut world = World::new();

        #[derive(Bundle, PartialEq, Debug)]
        struct FooBundle {
            x: TableStored,
            y: SparseStored,
        }
        let mut ids = Vec::new();
        <FooBundle as Bundle>::component_ids(
            &mut world.components,
            &mut world.storages,
            &mut |id| {
                ids.push(id);
            },
        );

        assert_eq!(
            ids,
            &[
                world.init_component::<TableStored>(),
                world.init_component::<SparseStored>(),
            ]
        );

        let e1 = world
            .spawn(FooBundle {
                x: TableStored("abc"),
                y: SparseStored(123),
            })
            .id();
        let e2 = world
            .spawn((TableStored("def"), SparseStored(456), A(1)))
            .id();
        assert_eq!(world.get::<TableStored>(e1).unwrap().0, "abc");
        assert_eq!(world.get::<SparseStored>(e1).unwrap().0, 123);
        assert_eq!(world.get::<TableStored>(e2).unwrap().0, "def");
        assert_eq!(world.get::<SparseStored>(e2).unwrap().0, 456);

        // test archetype get_mut()
        world.get_mut::<TableStored>(e1).unwrap().0 = "xyz";
        assert_eq!(world.get::<TableStored>(e1).unwrap().0, "xyz");

        // test sparse set get_mut()
        world.get_mut::<SparseStored>(e2).unwrap().0 = 42;
        assert_eq!(world.get::<SparseStored>(e2).unwrap().0, 42);

        assert_eq!(
            world.entity_mut(e1).take::<FooBundle>().unwrap(),
            FooBundle {
                x: TableStored("xyz"),
                y: SparseStored(123),
            }
        );

        #[derive(Bundle, PartialEq, Debug)]
        struct NestedBundle {
            a: A,
            foo: FooBundle,
            b: B,
        }

        let mut ids = Vec::new();
        <NestedBundle as Bundle>::component_ids(
            &mut world.components,
            &mut world.storages,
            &mut |id| {
                ids.push(id);
            },
        );

        assert_eq!(
            ids,
            &[
                world.init_component::<A>(),
                world.init_component::<TableStored>(),
                world.init_component::<SparseStored>(),
                world.init_component::<B>(),
            ]
        );

        let e3 = world
            .spawn(NestedBundle {
                a: A(1),
                foo: FooBundle {
                    x: TableStored("ghi"),
                    y: SparseStored(789),
                },
                b: B(2),
            })
            .id();

        assert_eq!(world.get::<TableStored>(e3).unwrap().0, "ghi");
        assert_eq!(world.get::<SparseStored>(e3).unwrap().0, 789);
        assert_eq!(world.get::<A>(e3).unwrap().0, 1);
        assert_eq!(world.get::<B>(e3).unwrap().0, 2);
        assert_eq!(
            world.entity_mut(e3).take::<NestedBundle>().unwrap(),
            NestedBundle {
                a: A(1),
                foo: FooBundle {
                    x: TableStored("ghi"),
                    y: SparseStored(789),
                },
                b: B(2),
            }
        );

        #[derive(Default, Component, PartialEq, Debug)]
        struct Ignored;

        #[derive(Bundle, PartialEq, Debug)]
        struct BundleWithIgnored {
            c: C,
            #[bundle(ignore)]
            ignored: Ignored,
        }

        let mut ids = Vec::new();
        <BundleWithIgnored as Bundle>::component_ids(
            &mut world.components,
            &mut world.storages,
            &mut |id| {
                ids.push(id);
            },
        );

        assert_eq!(ids, &[world.init_component::<C>(),]);

        let e4 = world
            .spawn(BundleWithIgnored {
                c: C,
                ignored: Ignored,
            })
            .id();

        assert_eq!(world.get::<C>(e4).unwrap(), &C);
        assert_eq!(world.get::<Ignored>(e4), None);

        assert_eq!(
            world.entity_mut(e4).take::<BundleWithIgnored>().unwrap(),
            BundleWithIgnored {
                c: C,
                ignored: Ignored,
            }
        );
    }

    #[test]
    fn despawn_table_storage() {
        let mut world = World::new();
        let e = world.spawn((TableStored("abc"), A(123))).id();
        let f = world.spawn((TableStored("def"), A(456))).id();
        assert_eq!(world.entities.len(), 2);
        assert!(world.despawn(e));
        assert_eq!(world.entities.len(), 1);
        assert!(world.get::<TableStored>(e).is_none());
        assert!(world.get::<A>(e).is_none());
        assert_eq!(world.get::<TableStored>(f).unwrap().0, "def");
        assert_eq!(world.get::<A>(f).unwrap().0, 456);
    }

    #[test]
    fn despawn_mixed_storage() {
        let mut world = World::new();

        let e = world.spawn((TableStored("abc"), SparseStored(123))).id();
        let f = world.spawn((TableStored("def"), SparseStored(456))).id();
        assert_eq!(world.entities.len(), 2);
        assert!(world.despawn(e));
        assert_eq!(world.entities.len(), 1);
        assert!(world.get::<TableStored>(e).is_none());
        assert!(world.get::<SparseStored>(e).is_none());
        assert_eq!(world.get::<TableStored>(f).unwrap().0, "def");
        assert_eq!(world.get::<SparseStored>(f).unwrap().0, 456);
    }

    #[test]
    fn query_all() {
        let mut world = World::new();
        let e = world.spawn((TableStored("abc"), A(123))).id();
        let f = world.spawn((TableStored("def"), A(456))).id();

        let ents = world
            .query::<(Entity, &A, &TableStored)>()
            .iter(&world)
            .map(|(e, &i, &s)| (e, i, s))
            .collect::<Vec<_>>();
        assert_eq!(
            ents,
            &[
                (e, A(123), TableStored("abc")),
                (f, A(456), TableStored("def"))
            ]
        );
    }

    #[test]
    fn query_all_for_each() {
        let mut world = World::new();
        let e = world.spawn((TableStored("abc"), A(123))).id();
        let f = world.spawn((TableStored("def"), A(456))).id();

        let mut results = Vec::new();
        world
            .query::<(Entity, &A, &TableStored)>()
            .for_each(&world, |(e, &i, &s)| results.push((e, i, s)));
        assert_eq!(
            results,
            &[
                (e, A(123), TableStored("abc")),
                (f, A(456), TableStored("def"))
            ]
        );
    }

    #[test]
    fn query_single_component() {
        let mut world = World::new();
        let e = world.spawn((TableStored("abc"), A(123))).id();
        let f = world.spawn((TableStored("def"), A(456), B(1))).id();
        let ents = world
            .query::<(Entity, &A)>()
            .iter(&world)
            .map(|(e, &i)| (e, i))
            .collect::<Vec<_>>();
        assert_eq!(ents, &[(e, A(123)), (f, A(456))]);
    }

    #[test]
    fn stateful_query_handles_new_archetype() {
        let mut world = World::new();
        let e = world.spawn((TableStored("abc"), A(123))).id();
        let mut query = world.query::<(Entity, &A)>();

        let ents = query.iter(&world).map(|(e, &i)| (e, i)).collect::<Vec<_>>();
        assert_eq!(ents, &[(e, A(123))]);

        let f = world.spawn((TableStored("def"), A(456), B(1))).id();
        let ents = query.iter(&world).map(|(e, &i)| (e, i)).collect::<Vec<_>>();
        assert_eq!(ents, &[(e, A(123)), (f, A(456))]);
    }

    #[test]
    fn query_single_component_for_each() {
        let mut world = World::new();
        let e = world.spawn((TableStored("abc"), A(123))).id();
        let f = world.spawn((TableStored("def"), A(456), B(1))).id();
        let mut results = Vec::new();
        world
            .query::<(Entity, &A)>()
            .for_each(&world, |(e, &i)| results.push((e, i)));
        assert_eq!(results, &[(e, A(123)), (f, A(456))]);
    }

    #[test]
    fn par_for_each_dense() {
        ComputeTaskPool::init(TaskPool::default);
        let mut world = World::new();
        let e1 = world.spawn(A(1)).id();
        let e2 = world.spawn(A(2)).id();
        let e3 = world.spawn(A(3)).id();
        let e4 = world.spawn((A(4), B(1))).id();
        let e5 = world.spawn((A(5), B(1))).id();
        let results = Arc::new(Mutex::new(Vec::new()));
        world
            .query::<(Entity, &A)>()
            .par_iter(&world)
            .for_each(|(e, &A(i))| {
                results.lock().unwrap().push((e, i));
            });
        results.lock().unwrap().sort();
        assert_eq!(
            &*results.lock().unwrap(),
            &[(e1, 1), (e2, 2), (e3, 3), (e4, 4), (e5, 5)]
        );
    }

    #[test]
    fn par_for_each_sparse() {
        ComputeTaskPool::init(TaskPool::default);
        let mut world = World::new();
        let e1 = world.spawn(SparseStored(1)).id();
        let e2 = world.spawn(SparseStored(2)).id();
        let e3 = world.spawn(SparseStored(3)).id();
        let e4 = world.spawn((SparseStored(4), A(1))).id();
        let e5 = world.spawn((SparseStored(5), A(1))).id();
        let results = Arc::new(Mutex::new(Vec::new()));
        world
            .query::<(Entity, &SparseStored)>()
            .par_iter(&world)
            .for_each(|(e, &SparseStored(i))| results.lock().unwrap().push((e, i)));
        results.lock().unwrap().sort();
        assert_eq!(
            &*results.lock().unwrap(),
            &[(e1, 1), (e2, 2), (e3, 3), (e4, 4), (e5, 5)]
        );
    }

    #[test]
    fn query_missing_component() {
        let mut world = World::new();
        world.spawn((TableStored("abc"), A(123)));
        world.spawn((TableStored("def"), A(456)));
        assert!(world.query::<(&B, &A)>().iter(&world).next().is_none());
    }

    #[test]
    fn query_sparse_component() {
        let mut world = World::new();
        world.spawn((TableStored("abc"), A(123)));
        let f = world.spawn((TableStored("def"), A(456), B(1))).id();
        let ents = world
            .query::<(Entity, &B)>()
            .iter(&world)
            .map(|(e, &b)| (e, b))
            .collect::<Vec<_>>();
        assert_eq!(ents, &[(f, B(1))]);
    }

    #[test]
    fn query_filter_with() {
        let mut world = World::new();
        world.spawn((A(123), B(1)));
        world.spawn(A(456));
        let result = world
            .query_filtered::<&A, With<B>>()
            .iter(&world)
            .cloned()
            .collect::<Vec<_>>();
        assert_eq!(result, vec![A(123)]);
    }

    #[test]
    fn query_filter_with_for_each() {
        let mut world = World::new();
        world.spawn((A(123), B(1)));
        world.spawn(A(456));

        let mut results = Vec::new();
        world
            .query_filtered::<&A, With<B>>()
            .for_each(&world, |i| results.push(*i));
        assert_eq!(results, vec![A(123)]);
    }

    #[test]
    fn query_filter_with_sparse() {
        let mut world = World::new();

        world.spawn((A(123), SparseStored(321)));
        world.spawn(A(456));
        let result = world
            .query_filtered::<&A, With<SparseStored>>()
            .iter(&world)
            .cloned()
            .collect::<Vec<_>>();
        assert_eq!(result, vec![A(123)]);
    }

    #[test]
    fn query_filter_with_sparse_for_each() {
        let mut world = World::new();

        world.spawn((A(123), SparseStored(321)));
        world.spawn(A(456));
        let mut results = Vec::new();
        world
            .query_filtered::<&A, With<SparseStored>>()
            .for_each(&world, |i| results.push(*i));
        assert_eq!(results, vec![A(123)]);
    }

    #[test]
    fn query_filter_without() {
        let mut world = World::new();
        world.spawn((A(123), B(321)));
        world.spawn(A(456));
        let result = world
            .query_filtered::<&A, Without<B>>()
            .iter(&world)
            .cloned()
            .collect::<Vec<_>>();
        assert_eq!(result, vec![A(456)]);
    }

    #[test]
    fn query_optional_component_table() {
        let mut world = World::new();
        let e = world.spawn((TableStored("abc"), A(123))).id();
        let f = world.spawn((TableStored("def"), A(456), B(1))).id();
        // this should be skipped
        world.spawn(TableStored("abc"));
        let ents = world
            .query::<(Entity, Option<&B>, &A)>()
            .iter(&world)
            .map(|(e, b, &i)| (e, b.copied(), i))
            .collect::<Vec<_>>();
        assert_eq!(ents, &[(e, None, A(123)), (f, Some(B(1)), A(456))]);
    }

    #[test]
    fn query_optional_component_sparse() {
        let mut world = World::new();

        let e = world.spawn((TableStored("abc"), A(123))).id();
        let f = world
            .spawn((TableStored("def"), A(456), SparseStored(1)))
            .id();
        // this should be skipped
        // world.spawn(SparseStored(1));
        let ents = world
            .query::<(Entity, Option<&SparseStored>, &A)>()
            .iter(&world)
            .map(|(e, b, &i)| (e, b.copied(), i))
            .collect::<Vec<_>>();
        assert_eq!(
            ents,
            &[(e, None, A(123)), (f, Some(SparseStored(1)), A(456))]
        );
    }

    #[test]
    fn query_optional_component_sparse_no_match() {
        let mut world = World::new();

        let e = world.spawn((TableStored("abc"), A(123))).id();
        let f = world.spawn((TableStored("def"), A(456))).id();
        // // this should be skipped
        world.spawn(TableStored("abc"));
        let ents = world
            .query::<(Entity, Option<&SparseStored>, &A)>()
            .iter(&world)
            .map(|(e, b, &i)| (e, b.copied(), i))
            .collect::<Vec<_>>();
        assert_eq!(ents, &[(e, None, A(123)), (f, None, A(456))]);
    }

    #[test]
    fn add_remove_components() {
        let mut world = World::new();
        let e1 = world.spawn((A(1), B(3), TableStored("abc"))).id();
        let e2 = world.spawn((A(2), B(4), TableStored("xyz"))).id();

        assert_eq!(
            world
                .query::<(Entity, &A, &B)>()
                .iter(&world)
                .map(|(e, &i, &b)| (e, i, b))
                .collect::<Vec<_>>(),
            &[(e1, A(1), B(3)), (e2, A(2), B(4))]
        );

        assert_eq!(world.entity_mut(e1).take::<A>(), Some(A(1)));
        assert_eq!(
            world
                .query::<(Entity, &A, &B)>()
                .iter(&world)
                .map(|(e, &i, &b)| (e, i, b))
                .collect::<Vec<_>>(),
            &[(e2, A(2), B(4))]
        );
        assert_eq!(
            world
                .query::<(Entity, &B, &TableStored)>()
                .iter(&world)
                .map(|(e, &B(b), &TableStored(s))| (e, b, s))
                .collect::<Vec<_>>(),
            &[(e2, 4, "xyz"), (e1, 3, "abc")]
        );
        world.entity_mut(e1).insert(A(43));
        assert_eq!(
            world
                .query::<(Entity, &A, &B)>()
                .iter(&world)
                .map(|(e, &i, &b)| (e, i, b))
                .collect::<Vec<_>>(),
            &[(e2, A(2), B(4)), (e1, A(43), B(3))]
        );
        world.entity_mut(e1).insert(C);
        assert_eq!(
            world
                .query::<(Entity, &C)>()
                .iter(&world)
                .map(|(e, &f)| (e, f))
                .collect::<Vec<_>>(),
            &[(e1, C)]
        );
    }

    #[test]
    fn table_add_remove_many() {
        let mut world = World::default();
        #[cfg(miri)]
        let (mut entities, to) = {
            let to = 10;
            (Vec::with_capacity(to), to)
        };
        #[cfg(not(miri))]
        let (mut entities, to) = {
            let to = 10_000;
            (Vec::with_capacity(to), to)
        };

        for _ in 0..to {
            entities.push(world.spawn(B(0)).id());
        }

        for (i, entity) in entities.iter().cloned().enumerate() {
            world.entity_mut(entity).insert(A(i));
        }

        for (i, entity) in entities.iter().cloned().enumerate() {
            assert_eq!(world.entity_mut(entity).take::<A>(), Some(A(i)));
        }
    }

    #[test]
    fn sparse_set_add_remove_many() {
        let mut world = World::default();

        let mut entities = Vec::with_capacity(1000);
        for _ in 0..4 {
            entities.push(world.spawn(A(2)).id());
        }

        for (i, entity) in entities.iter().cloned().enumerate() {
            world.entity_mut(entity).insert(SparseStored(i as u32));
        }

        for (i, entity) in entities.iter().cloned().enumerate() {
            assert_eq!(
                world.entity_mut(entity).take::<SparseStored>(),
                Some(SparseStored(i as u32))
            );
        }
    }

    #[test]
    fn remove_missing() {
        let mut world = World::new();
        let e = world.spawn((TableStored("abc"), A(123))).id();
        assert!(world.entity_mut(e).take::<B>().is_none());
    }

    #[test]
    fn spawn_batch() {
        let mut world = World::new();
        world.spawn_batch((0..100).map(|x| (A(x), TableStored("abc"))));
        let values = world
            .query::<&A>()
            .iter(&world)
            .map(|v| v.0)
            .collect::<Vec<_>>();
        let expected = (0..100).collect::<Vec<_>>();
        assert_eq!(values, expected);
    }

    #[test]
    fn query_get() {
        let mut world = World::new();
        let a = world.spawn((TableStored("abc"), A(123))).id();
        let b = world.spawn((TableStored("def"), A(456))).id();
        let c = world.spawn((TableStored("ghi"), A(789), B(1))).id();

        let mut i32_query = world.query::<&A>();
        assert_eq!(i32_query.get(&world, a).unwrap().0, 123);
        assert_eq!(i32_query.get(&world, b).unwrap().0, 456);

        let mut i32_bool_query = world.query::<(&A, &B)>();
        assert!(i32_bool_query.get(&world, a).is_err());
        assert_eq!(i32_bool_query.get(&world, c).unwrap(), (&A(789), &B(1)));
        assert!(world.despawn(a));
        assert!(i32_query.get(&world, a).is_err());
    }

    #[test]
    fn query_get_works_across_sparse_removal() {
        // Regression test for: https://github.com/bevyengine/bevy/issues/6623
        let mut world = World::new();
        let a = world.spawn((TableStored("abc"), SparseStored(123))).id();
        let b = world.spawn((TableStored("def"), SparseStored(456))).id();
        let c = world
            .spawn((TableStored("ghi"), SparseStored(789), B(1)))
            .id();

        let mut query = world.query::<&TableStored>();
        assert_eq!(query.get(&world, a).unwrap(), &TableStored("abc"));
        assert_eq!(query.get(&world, b).unwrap(), &TableStored("def"));
        assert_eq!(query.get(&world, c).unwrap(), &TableStored("ghi"));

        world.entity_mut(b).remove::<SparseStored>();
        world.entity_mut(c).remove::<SparseStored>();

        assert_eq!(query.get(&world, a).unwrap(), &TableStored("abc"));
        assert_eq!(query.get(&world, b).unwrap(), &TableStored("def"));
        assert_eq!(query.get(&world, c).unwrap(), &TableStored("ghi"));
    }

    #[test]
    fn remove_tracking() {
        let mut world = World::new();

        let a = world.spawn((SparseStored(0), A(123))).id();
        let b = world.spawn((SparseStored(1), A(123))).id();

        world.entity_mut(a).despawn();
        assert_eq!(
            world.removed::<A>().collect::<Vec<_>>(),
            &[a],
            "despawning results in 'removed component' state for table components"
        );
        assert_eq!(
            world.removed::<SparseStored>().collect::<Vec<_>>(),
            &[a],
            "despawning results in 'removed component' state for sparse set components"
        );

        world.entity_mut(b).insert(B(1));
        assert_eq!(
            world.removed::<A>().collect::<Vec<_>>(),
            &[a],
            "archetype moves does not result in 'removed component' state"
        );

        world.entity_mut(b).remove::<A>();
        assert_eq!(
            world.removed::<A>().collect::<Vec<_>>(),
            &[a, b],
            "removing a component results in a 'removed component' state"
        );

        world.clear_trackers();
        assert_eq!(
            world.removed::<A>().collect::<Vec<_>>(),
            &[],
            "clearing trackers clears removals"
        );
        assert_eq!(
            world.removed::<SparseStored>().collect::<Vec<_>>(),
            &[],
            "clearing trackers clears removals"
        );
        assert_eq!(
            world.removed::<B>().collect::<Vec<_>>(),
            &[],
            "clearing trackers clears removals"
        );

        // TODO: uncomment when world.clear() is implemented
        // let c = world.spawn(("abc", 123)).id();
        // let d = world.spawn(("abc", 123)).id();
        // world.clear();
        // assert_eq!(
        //     world.removed::<i32>(),
        //     &[c, d],
        //     "world clears result in 'removed component' states"
        // );
        // assert_eq!(
        //     world.removed::<&'static str>(),
        //     &[c, d, b],
        //     "world clears result in 'removed component' states"
        // );
        // assert_eq!(
        //     world.removed::<f64>(),
        //     &[b],
        //     "world clears result in 'removed component' states"
        // );
    }

    #[test]
    fn added_tracking() {
        let mut world = World::new();
        let a = world.spawn(A(123)).id();

        assert_eq!(world.query::<&A>().iter(&world).count(), 1);
        assert_eq!(
            world.query_filtered::<(), Added<A>>().iter(&world).count(),
            1
        );
        assert_eq!(world.query::<&A>().iter(&world).count(), 1);
        assert_eq!(
            world.query_filtered::<(), Added<A>>().iter(&world).count(),
            1
        );
        assert!(world.query::<&A>().get(&world, a).is_ok());
        assert!(world
            .query_filtered::<(), Added<A>>()
            .get(&world, a)
            .is_ok());
        assert!(world.query::<&A>().get(&world, a).is_ok());
        assert!(world
            .query_filtered::<(), Added<A>>()
            .get(&world, a)
            .is_ok());

        world.clear_trackers();

        assert_eq!(world.query::<&A>().iter(&world).count(), 1);
        assert_eq!(
            world.query_filtered::<(), Added<A>>().iter(&world).count(),
            0
        );
        assert_eq!(world.query::<&A>().iter(&world).count(), 1);
        assert_eq!(
            world.query_filtered::<(), Added<A>>().iter(&world).count(),
            0
        );
        assert!(world.query::<&A>().get(&world, a).is_ok());
        assert!(world
            .query_filtered::<(), Added<A>>()
            .get(&world, a)
            .is_err());
        assert!(world.query::<&A>().get(&world, a).is_ok());
        assert!(world
            .query_filtered::<(), Added<A>>()
            .get(&world, a)
            .is_err());
    }

    #[test]
    fn added_queries() {
        let mut world = World::default();
        let e1 = world.spawn(A(0)).id();

        fn get_added<Com: Component>(world: &mut World) -> Vec<Entity> {
            world
                .query_filtered::<Entity, Added<Com>>()
                .iter(world)
                .collect::<Vec<Entity>>()
        }

        assert_eq!(get_added::<A>(&mut world), vec![e1]);
        world.entity_mut(e1).insert(B(0));
        assert_eq!(get_added::<A>(&mut world), vec![e1]);
        assert_eq!(get_added::<B>(&mut world), vec![e1]);

        world.clear_trackers();
        assert!(get_added::<A>(&mut world).is_empty());
        let e2 = world.spawn((A(1), B(1))).id();
        assert_eq!(get_added::<A>(&mut world), vec![e2]);
        assert_eq!(get_added::<B>(&mut world), vec![e2]);

        let added = world
            .query_filtered::<Entity, (Added<A>, Added<B>)>()
            .iter(&world)
            .collect::<Vec<Entity>>();
        assert_eq!(added, vec![e2]);
    }

    #[test]
    fn changed_trackers() {
        let mut world = World::default();
        let e1 = world.spawn((A(0), B(0))).id();
        let e2 = world.spawn((A(0), B(0))).id();
        let e3 = world.spawn((A(0), B(0))).id();
        world.spawn((A(0), B(0)));

        world.clear_trackers();

        for (i, mut a) in world.query::<&mut A>().iter_mut(&mut world).enumerate() {
            if i % 2 == 0 {
                a.0 += 1;
            }
        }

        fn get_filtered<F: ReadOnlyWorldQuery>(world: &mut World) -> Vec<Entity> {
            world
                .query_filtered::<Entity, F>()
                .iter(world)
                .collect::<Vec<Entity>>()
        }

        assert_eq!(get_filtered::<Changed<A>>(&mut world), vec![e1, e3]);

        // ensure changing an entity's archetypes also moves its changed state
        world.entity_mut(e1).insert(C);

        assert_eq!(get_filtered::<Changed<A>>(&mut world), vec![e3, e1], "changed entities list should not change (although the order will due to archetype moves)");

        // spawning a new A entity should not change existing changed state
        world.entity_mut(e1).insert((A(0), B(0)));
        assert_eq!(
            get_filtered::<Changed<A>>(&mut world),
            vec![e3, e1],
            "changed entities list should not change"
        );

        // removing an unchanged entity should not change changed state
        assert!(world.despawn(e2));
        assert_eq!(
            get_filtered::<Changed<A>>(&mut world),
            vec![e3, e1],
            "changed entities list should not change"
        );

        // removing a changed entity should remove it from enumeration
        assert!(world.despawn(e1));
        assert_eq!(
            get_filtered::<Changed<A>>(&mut world),
            vec![e3],
            "e1 should no longer be returned"
        );

        world.clear_trackers();

        assert!(get_filtered::<Changed<A>>(&mut world).is_empty());

        let e4 = world.spawn_empty().id();

        world.entity_mut(e4).insert(A(0));
        assert_eq!(get_filtered::<Changed<A>>(&mut world), vec![e4]);
        assert_eq!(get_filtered::<Added<A>>(&mut world), vec![e4]);

        world.entity_mut(e4).insert(A(1));
        assert_eq!(get_filtered::<Changed<A>>(&mut world), vec![e4]);

        world.clear_trackers();

        // ensure inserting multiple components set changed state for all components and set added
        // state for non existing components even when changing archetype.
        world.entity_mut(e4).insert((A(0), B(0)));

        assert!(get_filtered::<Added<A>>(&mut world).is_empty());
        assert_eq!(get_filtered::<Changed<A>>(&mut world), vec![e4]);
        assert_eq!(get_filtered::<Added<B>>(&mut world), vec![e4]);
        assert_eq!(get_filtered::<Changed<B>>(&mut world), vec![e4]);
    }

    #[test]
    fn changed_trackers_sparse() {
        let mut world = World::default();
        let e1 = world.spawn(SparseStored(0)).id();
        let e2 = world.spawn(SparseStored(0)).id();
        let e3 = world.spawn(SparseStored(0)).id();
        world.spawn(SparseStored(0));

        world.clear_trackers();

        for (i, mut a) in world
            .query::<&mut SparseStored>()
            .iter_mut(&mut world)
            .enumerate()
        {
            if i % 2 == 0 {
                a.0 += 1;
            }
        }

        fn get_filtered<F: ReadOnlyWorldQuery>(world: &mut World) -> Vec<Entity> {
            world
                .query_filtered::<Entity, F>()
                .iter(world)
                .collect::<Vec<Entity>>()
        }

        assert_eq!(
            get_filtered::<Changed<SparseStored>>(&mut world),
            vec![e1, e3]
        );

        // ensure changing an entity's archetypes also moves its changed state
        world.entity_mut(e1).insert(C);

        assert_eq!(get_filtered::<Changed<SparseStored>>(&mut world), vec![e3, e1], "changed entities list should not change (although the order will due to archetype moves)");

        // spawning a new SparseStored entity should not change existing changed state
        world.entity_mut(e1).insert(SparseStored(0));
        assert_eq!(
            get_filtered::<Changed<SparseStored>>(&mut world),
            vec![e3, e1],
            "changed entities list should not change"
        );

        // removing an unchanged entity should not change changed state
        assert!(world.despawn(e2));
        assert_eq!(
            get_filtered::<Changed<SparseStored>>(&mut world),
            vec![e3, e1],
            "changed entities list should not change"
        );

        // removing a changed entity should remove it from enumeration
        assert!(world.despawn(e1));
        assert_eq!(
            get_filtered::<Changed<SparseStored>>(&mut world),
            vec![e3],
            "e1 should no longer be returned"
        );

        world.clear_trackers();

        assert!(get_filtered::<Changed<SparseStored>>(&mut world).is_empty());

        let e4 = world.spawn_empty().id();

        world.entity_mut(e4).insert(SparseStored(0));
        assert_eq!(get_filtered::<Changed<SparseStored>>(&mut world), vec![e4]);
        assert_eq!(get_filtered::<Added<SparseStored>>(&mut world), vec![e4]);

        world.entity_mut(e4).insert(A(1));
        assert_eq!(get_filtered::<Changed<SparseStored>>(&mut world), vec![e4]);

        world.clear_trackers();

        // ensure inserting multiple components set changed state for all components and set added
        // state for non existing components even when changing archetype.
        world.entity_mut(e4).insert(SparseStored(0));

        assert!(get_filtered::<Added<SparseStored>>(&mut world).is_empty());
        assert_eq!(get_filtered::<Changed<SparseStored>>(&mut world), vec![e4]);
    }

    #[test]
    fn empty_spawn() {
        let mut world = World::default();
        let e = world.spawn_empty().id();
        let mut e_mut = world.entity_mut(e);
        e_mut.insert(A(0));
        assert_eq!(e_mut.get::<A>().unwrap(), &A(0));
    }

    #[test]
    fn reserve_and_spawn() {
        let mut world = World::default();
        let e = world.entities().reserve_entity();
        world.flush();
        let mut e_mut = world.entity_mut(e);
        e_mut.insert(A(0));
        assert_eq!(e_mut.get::<A>().unwrap(), &A(0));
    }

    #[test]
    fn changed_query() {
        let mut world = World::default();
        let e1 = world.spawn((A(0), B(0))).id();

        fn get_changed(world: &mut World) -> Vec<Entity> {
            world
                .query_filtered::<Entity, Changed<A>>()
                .iter(world)
                .collect::<Vec<Entity>>()
        }
        assert_eq!(get_changed(&mut world), vec![e1]);
        world.clear_trackers();
        assert_eq!(get_changed(&mut world), vec![]);
        *world.get_mut(e1).unwrap() = A(1);
        assert_eq!(get_changed(&mut world), vec![e1]);
    }

    #[test]
    fn resource() {
        use crate::system::Resource;

        #[derive(Resource, PartialEq, Debug)]
        struct Num(i32);

        #[derive(Resource, PartialEq, Debug)]
        struct BigNum(u64);

        let mut world = World::default();
        assert!(world.get_resource::<Num>().is_none());
        assert!(!world.contains_resource::<Num>());
        assert!(!world.is_resource_added::<Num>());
        assert!(!world.is_resource_changed::<Num>());

        world.insert_resource(Num(123));
        let resource_id = world
            .components()
            .get_resource_id(TypeId::of::<Num>())
            .unwrap();
        let archetype_component_id = world.storages().resources.get(resource_id).unwrap().id();

        assert_eq!(world.resource::<Num>().0, 123);
        assert!(world.contains_resource::<Num>());
        assert!(world.is_resource_added::<Num>());
        assert!(world.is_resource_changed::<Num>());

        world.insert_resource(BigNum(456));
        assert_eq!(world.resource::<BigNum>().0, 456u64);

        world.insert_resource(BigNum(789));
        assert_eq!(world.resource::<BigNum>().0, 789);

        {
            let mut value = world.resource_mut::<BigNum>();
            assert_eq!(value.0, 789);
            value.0 = 10;
        }

        assert_eq!(
            world.resource::<BigNum>().0,
            10,
            "resource changes are preserved"
        );

        assert_eq!(
            world.remove_resource::<BigNum>(),
            Some(BigNum(10)),
            "removed resource has the correct value"
        );
        assert_eq!(
            world.get_resource::<BigNum>(),
            None,
            "removed resource no longer exists"
        );
        assert_eq!(
            world.remove_resource::<BigNum>(),
            None,
            "double remove returns nothing"
        );

        world.insert_resource(BigNum(1));
        assert_eq!(
            world.get_resource::<BigNum>(),
            Some(&BigNum(1)),
            "re-inserting resources works"
        );

        assert_eq!(
            world.get_resource::<Num>(),
            Some(&Num(123)),
            "other resources are unaffected"
        );

        let current_resource_id = world
            .components()
            .get_resource_id(TypeId::of::<Num>())
            .unwrap();
        assert_eq!(
            resource_id, current_resource_id,
            "resource id does not change after removing / re-adding"
        );

        let current_archetype_component_id =
            world.storages().resources.get(resource_id).unwrap().id();

        assert_eq!(
            archetype_component_id, current_archetype_component_id,
            "resource archetype component id does not change after removing / re-adding"
        );
    }

    #[test]
    fn remove() {
        let mut world = World::default();
        let e1 = world.spawn((A(1), B(1), TableStored("a"))).id();

        let mut e = world.entity_mut(e1);
        assert_eq!(e.get::<TableStored>(), Some(&TableStored("a")));
        assert_eq!(e.get::<A>(), Some(&A(1)));
        assert_eq!(e.get::<B>(), Some(&B(1)));
        assert_eq!(
            e.get::<C>(),
            None,
            "C is not in the entity, so it should not exist"
        );

        e.remove::<(A, B, C)>();
        assert_eq!(
            e.get::<TableStored>(),
            Some(&TableStored("a")),
            "TableStored is not in the removed bundle, so it should exist"
        );
        assert_eq!(
            e.get::<A>(),
            None,
            "Num is in the removed bundle, so it should not exist"
        );
        assert_eq!(
            e.get::<B>(),
            None,
            "f64 is in the removed bundle, so it should not exist"
        );
        assert_eq!(
            e.get::<C>(),
            None,
            "usize is in the removed bundle, so it should not exist"
        );
    }

    #[test]
    fn take() {
        let mut world = World::default();
        world.spawn((A(1), B(1), TableStored("1")));
        let e2 = world.spawn((A(2), B(2), TableStored("2"))).id();
        world.spawn((A(3), B(3), TableStored("3")));

        let mut query = world.query::<(&B, &TableStored)>();
        let results = query
            .iter(&world)
            .map(|(a, b)| (a.0, b.0))
            .collect::<Vec<_>>();
        assert_eq!(results, vec![(1, "1"), (2, "2"), (3, "3"),]);

        let removed_bundle = world.entity_mut(e2).take::<(B, TableStored)>().unwrap();
        assert_eq!(removed_bundle, (B(2), TableStored("2")));

        let results = query
            .iter(&world)
            .map(|(a, b)| (a.0, b.0))
            .collect::<Vec<_>>();
        assert_eq!(results, vec![(1, "1"), (3, "3"),]);

        let mut a_query = world.query::<&A>();
        let results = a_query.iter(&world).map(|a| a.0).collect::<Vec<_>>();
        assert_eq!(results, vec![1, 3, 2]);

        let entity_ref = world.entity(e2);
        assert_eq!(
            entity_ref.get::<A>(),
            Some(&A(2)),
            "A is not in the removed bundle, so it should exist"
        );
        assert_eq!(
            entity_ref.get::<B>(),
            None,
            "B is in the removed bundle, so it should not exist"
        );
        assert_eq!(
            entity_ref.get::<TableStored>(),
            None,
            "TableStored is in the removed bundle, so it should not exist"
        );
    }

    #[test]
    fn non_send_resource() {
        let mut world = World::default();
        world.insert_non_send_resource(123i32);
        world.insert_non_send_resource(456i64);
        assert_eq!(*world.non_send_resource::<i32>(), 123);
        assert_eq!(*world.non_send_resource_mut::<i64>(), 456);
    }

    #[test]
    fn non_send_resource_points_to_distinct_data() {
        let mut world = World::default();
        world.insert_resource(A(123));
        world.insert_non_send_resource(A(456));
        assert_eq!(*world.resource::<A>(), A(123));
        assert_eq!(*world.non_send_resource::<A>(), A(456));
    }

    #[test]
    #[should_panic]
    fn non_send_resource_panic() {
        let mut world = World::default();
        world.insert_non_send_resource(0i32);
        std::thread::spawn(move || {
            let _ = world.non_send_resource_mut::<i32>();
        })
        .join()
        .unwrap();
    }

    #[test]
    fn exact_size_query() {
        let mut world = World::default();
        world.spawn((A(0), B(0)));
        world.spawn((A(0), B(0)));
        world.spawn((A(0), B(0), C));
        world.spawn(C);

        let mut query = world.query::<(&A, &B)>();
        assert_eq!(query.iter(&world).len(), 3);
    }

    #[test]
    #[should_panic]
    fn duplicate_components_panic() {
        let mut world = World::new();
        world.spawn((A(1), A(2)));
    }

    #[test]
    #[should_panic]
    fn ref_and_mut_query_panic() {
        let mut world = World::new();
        world.query::<(&A, &mut A)>();
    }

    #[test]
    #[should_panic]
    fn entity_ref_and_mut_query_panic() {
        let mut world = World::new();
        world.query::<(EntityRef, &mut A)>();
    }

    #[test]
    #[should_panic]
    fn mut_and_ref_query_panic() {
        let mut world = World::new();
        world.query::<(&mut A, &A)>();
    }

    #[test]
    #[should_panic]
    fn mut_and_entity_ref_query_panic() {
        let mut world = World::new();
        world.query::<(&mut A, EntityRef)>();
    }

    #[test]
    #[should_panic]
    fn mut_and_mut_query_panic() {
        let mut world = World::new();
        world.query::<(&mut A, &mut A)>();
    }

    #[test]
    #[should_panic]
    fn multiple_worlds_same_query_iter() {
        let mut world_a = World::new();
        let world_b = World::new();
        let mut query = world_a.query::<&A>();
        query.iter(&world_a);
        query.iter(&world_b);
    }

    #[test]
    fn query_filters_dont_collide_with_fetches() {
        let mut world = World::new();
        world.query_filtered::<&mut A, Changed<A>>();
    }

    #[test]
    fn filtered_query_access() {
        let mut world = World::new();
        let query = world.query_filtered::<&mut A, Changed<B>>();

        let mut expected = FilteredAccess::<ComponentId>::default();
        let a_id = world.components.get_id(TypeId::of::<A>()).unwrap();
        let b_id = world.components.get_id(TypeId::of::<B>()).unwrap();
        expected.add_write(a_id);
        expected.add_read(b_id);
        assert!(
            query.component_access.eq(&expected),
            "ComponentId access from query fetch and query filter should be combined"
        );
    }

    #[test]
    #[should_panic]
    fn multiple_worlds_same_query_get() {
        let mut world_a = World::new();
        let world_b = World::new();
        let mut query = world_a.query::<&A>();
        let _ = query.get(&world_a, Entity::from_raw(0));
        let _ = query.get(&world_b, Entity::from_raw(0));
    }

    #[test]
    #[should_panic]
    fn multiple_worlds_same_query_for_each() {
        let mut world_a = World::new();
        let world_b = World::new();
        let mut query = world_a.query::<&A>();
        query.for_each(&world_a, |_| {});
        query.for_each(&world_b, |_| {});
    }

    #[test]
    fn resource_scope() {
        let mut world = World::default();
        world.insert_resource(A(0));
        world.resource_scope(|world: &mut World, mut value: Mut<A>| {
            value.0 += 1;
            assert!(!world.contains_resource::<A>());
        });
        assert_eq!(world.resource::<A>().0, 1);
    }

    #[test]
    #[should_panic(
        expected = "Attempted to access or drop non-send resource bevy_ecs::tests::NonSendA from thread"
    )]
    fn non_send_resource_drop_from_different_thread() {
        let mut world = World::default();
        world.insert_non_send_resource(NonSendA::default());

        let thread = std::thread::spawn(move || {
            // Dropping the non-send resource on a different thread
            // Should result in a panic
            drop(world);
        });

        if let Err(err) = thread.join() {
            std::panic::resume_unwind(err);
        }
    }

    #[test]
    fn non_send_resource_drop_from_same_thread() {
        let mut world = World::default();
        world.insert_non_send_resource(NonSendA::default());
        drop(world);
    }

    #[test]
    fn insert_overwrite_drop() {
        let (dropck1, dropped1) = DropCk::new_pair();
        let (dropck2, dropped2) = DropCk::new_pair();
        let mut world = World::default();
        world.spawn(dropck1).insert(dropck2);
        assert_eq!(dropped1.load(Ordering::Relaxed), 1);
        assert_eq!(dropped2.load(Ordering::Relaxed), 0);
        drop(world);
        assert_eq!(dropped1.load(Ordering::Relaxed), 1);
        assert_eq!(dropped2.load(Ordering::Relaxed), 1);
    }

    #[test]
    fn insert_overwrite_drop_sparse() {
        let (dropck1, dropped1) = DropCk::new_pair();
        let (dropck2, dropped2) = DropCk::new_pair();
        let mut world = World::default();

        world
            .spawn(DropCkSparse(dropck1))
            .insert(DropCkSparse(dropck2));
        assert_eq!(dropped1.load(Ordering::Relaxed), 1);
        assert_eq!(dropped2.load(Ordering::Relaxed), 0);
        drop(world);
        assert_eq!(dropped1.load(Ordering::Relaxed), 1);
        assert_eq!(dropped2.load(Ordering::Relaxed), 1);
    }

    #[test]
    fn clear_entities() {
        let mut world = World::default();

        world.insert_resource(A(0));
        world.spawn(A(1));
        world.spawn(SparseStored(1));

        let mut q1 = world.query::<&A>();
        let mut q2 = world.query::<&SparseStored>();

        assert_eq!(q1.iter(&world).len(), 1);
        assert_eq!(q2.iter(&world).len(), 1);
        assert_eq!(world.entities().len(), 2);

        world.clear_entities();

        assert_eq!(
            q1.iter(&world).len(),
            0,
            "world should not contain table components"
        );
        assert_eq!(
            q2.iter(&world).len(),
            0,
            "world should not contain sparse set components"
        );
        assert_eq!(
            world.entities().len(),
            0,
            "world should not have any entities"
        );
        assert_eq!(
            world.resource::<A>().0,
            0,
            "world should still contain resources"
        );
    }

    #[test]
    fn test_is_archetypal_size_hints() {
        let mut world = World::default();
        macro_rules! query_min_size {
            ($query:ty, $filter:ty) => {
                world
                    .query_filtered::<$query, $filter>()
                    .iter(&world)
                    .size_hint()
                    .0
            };
        }

        world.spawn((A(1), B(1), C));
        world.spawn((A(1), C));
        world.spawn((A(1), B(1)));
        world.spawn((B(1), C));
        world.spawn(A(1));
        world.spawn(C);
        assert_eq!(2, query_min_size![(), (With<A>, Without<B>)],);
        assert_eq!(3, query_min_size![&B, Or<(With<A>, With<C>)>],);
        assert_eq!(1, query_min_size![&B, (With<A>, With<C>)],);
        assert_eq!(1, query_min_size![(&A, &B), With<C>],);
        assert_eq!(4, query_min_size![&A, ()], "Simple Archetypal");
        assert_eq!(4, query_min_size![Ref<A>, ()],);
        // All the following should set minimum size to 0, as it's impossible to predict
        // how many entities the filters will trim.
        assert_eq!(0, query_min_size![(), Added<A>], "Simple Added");
        assert_eq!(0, query_min_size![(), Changed<A>], "Simple Changed");
        assert_eq!(0, query_min_size![(&A, &B), Changed<A>],);
        assert_eq!(0, query_min_size![&A, (Changed<A>, With<B>)],);
        assert_eq!(0, query_min_size![(&A, &B), Or<(Changed<A>, Changed<B>)>],);
    }

    #[test]
    fn reserve_entities_across_worlds() {
        let mut world_a = World::default();
        let mut world_b = World::default();

        let e1 = world_a.spawn(A(1)).id();
        let e2 = world_a.spawn(A(2)).id();
        let e3 = world_a.entities().reserve_entity();
        world_a.flush();

        let world_a_max_entities = world_a.entities().len();
        world_b.entities.reserve_entities(world_a_max_entities);
        world_b.entities.flush_as_invalid();

        let e4 = world_b.spawn(A(4)).id();
        assert_eq!(
            e4,
            Entity::new(3, 0),
            "new entity is created immediately after world_a's max entity"
        );
        assert!(world_b.get::<A>(e1).is_none());
        assert!(world_b.get_entity(e1).is_none());

        assert!(world_b.get::<A>(e2).is_none());
        assert!(world_b.get_entity(e2).is_none());

        assert!(world_b.get::<A>(e3).is_none());
        assert!(world_b.get_entity(e3).is_none());

        world_b.get_or_spawn(e1).unwrap().insert(B(1));
        assert_eq!(
            world_b.get::<B>(e1),
            Some(&B(1)),
            "spawning into 'world_a' entities works"
        );

        world_b.get_or_spawn(e4).unwrap().insert(B(4));
        assert_eq!(
            world_b.get::<B>(e4),
            Some(&B(4)),
            "spawning into existing `world_b` entities works"
        );
        assert_eq!(
            world_b.get::<A>(e4),
            Some(&A(4)),
            "spawning into existing `world_b` entities works"
        );

        let e4_mismatched_generation = Entity::new(3, 1);
        assert!(
            world_b.get_or_spawn(e4_mismatched_generation).is_none(),
            "attempting to spawn on top of an entity with a mismatched entity generation fails"
        );
        assert_eq!(
            world_b.get::<B>(e4),
            Some(&B(4)),
            "failed mismatched spawn doesn't change existing entity"
        );
        assert_eq!(
            world_b.get::<A>(e4),
            Some(&A(4)),
            "failed mismatched spawn doesn't change existing entity"
        );

        let high_non_existent_entity = Entity::new(6, 0);
        world_b
            .get_or_spawn(high_non_existent_entity)
            .unwrap()
            .insert(B(10));
        assert_eq!(
            world_b.get::<B>(high_non_existent_entity),
            Some(&B(10)),
            "inserting into newly allocated high / non-continuous entity id works"
        );

        let high_non_existent_but_reserved_entity = Entity::new(5, 0);
        assert!(
            world_b.get_entity(high_non_existent_but_reserved_entity).is_none(),
            "entities between high-newly allocated entity and continuous block of existing entities don't exist"
        );

        let reserved_entities = vec![
            world_b.entities().reserve_entity(),
            world_b.entities().reserve_entity(),
            world_b.entities().reserve_entity(),
            world_b.entities().reserve_entity(),
        ];

        assert_eq!(
            reserved_entities,
            vec![
                Entity::new(5, 0),
                Entity::new(4, 0),
                Entity::new(7, 0),
                Entity::new(8, 0),
            ],
            "space between original entities and high entities is used for new entity ids"
        );
    }

    #[test]
    fn insert_or_spawn_batch() {
        let mut world = World::default();
        let e0 = world.spawn(A(0)).id();
        let e1 = Entity::from_raw(1);

        let values = vec![(e0, (B(0), C)), (e1, (B(1), C))];

        world.insert_or_spawn_batch(values).unwrap();

        assert_eq!(
            world.get::<A>(e0),
            Some(&A(0)),
            "existing component was preserved"
        );
        assert_eq!(
            world.get::<B>(e0),
            Some(&B(0)),
            "pre-existing entity received correct B component"
        );
        assert_eq!(
            world.get::<B>(e1),
            Some(&B(1)),
            "new entity was spawned and received correct B component"
        );
        assert_eq!(
            world.get::<C>(e0),
            Some(&C),
            "pre-existing entity received C component"
        );
        assert_eq!(
            world.get::<C>(e1),
            Some(&C),
            "new entity was spawned and received C component"
        );
    }

    #[test]
    fn insert_or_spawn_batch_invalid() {
        let mut world = World::default();
        let e0 = world.spawn(A(0)).id();
        let e1 = Entity::from_raw(1);
        let e2 = world.spawn_empty().id();
        let invalid_e2 = Entity::new(e2.index(), 1);

        let values = vec![(e0, (B(0), C)), (e1, (B(1), C)), (invalid_e2, (B(2), C))];

        let result = world.insert_or_spawn_batch(values);

        assert_eq!(
            result,
            Err(vec![invalid_e2]),
            "e2 failed to be spawned or inserted into"
        );

        assert_eq!(
            world.get::<A>(e0),
            Some(&A(0)),
            "existing component was preserved"
        );
        assert_eq!(
            world.get::<B>(e0),
            Some(&B(0)),
            "pre-existing entity received correct B component"
        );
        assert_eq!(
            world.get::<B>(e1),
            Some(&B(1)),
            "new entity was spawned and received correct B component"
        );
        assert_eq!(
            world.get::<C>(e0),
            Some(&C),
            "pre-existing entity received C component"
        );
        assert_eq!(
            world.get::<C>(e1),
            Some(&C),
            "new entity was spawned and received C component"
        );
    }
}