lyra-engine/lyra-ecs/src/world.rs

use std::{any::TypeId, collections::HashMap, ptr::NonNull};

use atomic_refcell::{AtomicRef, AtomicRefMut};

use crate::{archetype::{Archetype, ArchetypeId}, bundle::Bundle, query::{dynamic::DynamicView, AsQuery, Query, ViewIter, ViewOne, ViewState}, resource::ResourceData, ComponentInfo, DynTypeId, DynamicBundle, Entities, Entity, ResourceObject, Tick, TickTracker};

/// The id of the entity for the Archetype.
/// 
/// The Archetype uses this as the index in the component columns
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub struct ArchetypeEntityId(pub u64);

#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub struct Record {
    pub id: ArchetypeId,
    pub index: ArchetypeEntityId,
}

#[derive(Clone)]
pub struct World {
    pub(crate) archetypes: HashMap<ArchetypeId, Archetype>,
    next_archetype_id: ArchetypeId,
    resources: HashMap<TypeId, ResourceData>,
    tracker: TickTracker,
    pub(crate) entities: Entities,
}

impl Default for World {
    fn default() -> Self {
        Self {
            archetypes: HashMap::new(),
            next_archetype_id: ArchetypeId(0),
            resources: HashMap::new(),
            tracker: TickTracker::new(),
            entities: Entities::default(),
        }
    }
}

impl World {
    pub fn new() -> Self {
        Self::default()
    }

    /// Reserves an entity in the world
    pub fn reserve_entity(&mut self) -> Entity {
        self.entities.reserve()
    }

    pub fn spawn<B>(&mut self, bundle: B) -> Entity
    where
        B: Bundle
    {
        let new_entity = self.reserve_entity();
        self.spawn_into(new_entity, bundle);
        new_entity
    }

    /// Spawn the components into a reserved entity. Only do this with entities that
    /// were reserved with [`World::reserve_entity`].
    ///
    /// # Safety
    /// Do not use this method with an entity that is currently alive, it WILL cause undefined behavior.
    pub fn spawn_into<B>(&mut self, entity: Entity, bundle: B)
    where
        B: Bundle
    {
        let bundle_types = bundle.type_ids();

        let tick = self.tick();

        // try to find an archetype
        let archetype = self.archetypes
            .values_mut()
            .find(|a| a.is_archetype_for(&bundle_types));

        if let Some(archetype) = archetype {
            // make at just one check to ensure you're not spawning twice
            debug_assert!(!archetype.entity_ids.contains_key(&entity),
                "You attempted to spawn components into an entity that already exists!");

            let arche_idx = archetype.add_entity(entity, bundle, &tick);

            // Create entity record and store it
            let record = Record {
                id: archetype.id(),
                index: arche_idx,
            };
            
            self.entities.insert_entity_record(entity, record);
        }
        // create a new archetype if one isn't found
        else {
            // create archetype
            let new_arch_id = self.next_archetype_id.increment();
            let mut archetype = Archetype::from_bundle_info(new_arch_id, bundle.info());
            let entity_arch_id = archetype.add_entity(entity, bundle, &tick);

            // store archetype
            self.archetypes.insert(new_arch_id, archetype);

            // Create entity record and store it
            let record = Record {
                id: new_arch_id,
                // this is the first entity in the archetype
                index: entity_arch_id,
            };

            self.entities.insert_entity_record(entity, record);
        }
    }

    /// Despawn an entity from the World
    pub fn despawn(&mut self, entity: Entity) {
        // Tick the tracker if the entity is spawned. This is done here instead of the `if let`
        // below due to the borrow checker complaining about multiple mutable borrows to self.
        let tick = if self.entities.arch_index.contains_key(&entity.id) {
            Some(self.tick())
        } else { None };

        if let Some(record) = self.entities.arch_index.get_mut(&entity.id) {
            let tick = tick.unwrap();
            let arch = self.archetypes.get_mut(&record.id).unwrap();

            if let Some((moved, new_index)) = arch.remove_entity(entity, &tick) {
                // replace the archetype index of the moved index with its new index.
                self.entities.arch_index.get_mut(&moved.id).unwrap().index = new_index;
            }
        }
    }

    /// Insert a component bundle into an existing entity.
    /// 
    /// If the components are already existing on the entity, they will be updated, else the
    /// entity will be moved to a different Archetype that can store the entity. That may
    /// involve creating a new Archetype.
    pub fn insert<B>(&mut self, entity: Entity, bundle: B)
    where
        B: Bundle
    {
        let tick = self.tick();

        let record = self.entities.entity_record(entity).unwrap();
        let current_arch = self.archetypes.get(&record.id).unwrap();
        let current_arch_len = current_arch.len();

        let mut contains_all = true;
        for id in bundle.type_ids() {
            contains_all = contains_all && current_arch.get_column(id).is_some();
        }

        if contains_all {
            let current_arch = self.archetypes.get_mut(&record.id).unwrap();
            let entry_idx = *current_arch.entity_indexes()
                .get(&entity).unwrap();

            bundle.take(|ptr, id, _info| {
                let col = current_arch.get_column_mut(id).unwrap();
                unsafe { col.set_at(entry_idx.0 as _, ptr, tick) };
            });

            return;
        }

        // contains the type ids for the old component columns + the ids for the new components
        let mut combined_column_types: Vec<DynTypeId> = current_arch.columns.iter().map(|c| c.info.type_id()).collect();
        combined_column_types.extend(bundle.type_ids());
        
        // contains the ComponentInfo for the old component columns + the info for the new components
        let mut combined_column_infos: Vec<ComponentInfo> = current_arch.columns.iter().map(|c| c.info).collect();
        combined_column_infos.extend(bundle.info());

        // pointers only for the old columns
        let old_columns: Vec<(NonNull<u8>, ComponentInfo)> = current_arch.columns.iter()
            .map(|c| unsafe { (NonNull::new_unchecked(c.borrow_ptr().as_ptr()), c.info) })
            .collect();

        if let Some(arch) = self.archetypes.values_mut().find(|a| a.is_archetype_for(&combined_column_types)) {
            let mut dbun = DynamicBundle::new();
            // move old entity components into new archetype columns
            for (col_ptr, col_info) in old_columns.into_iter() {
                unsafe {
                    let ptr = NonNull::new_unchecked(col_ptr.as_ptr()
                        .add(record.index.0 as usize * col_info.layout().size()));
                    dbun.push_unknown(ptr, col_info);
                }
            }
            dbun.push_bundle(bundle);

            let res_index = arch.add_entity(entity, dbun, &tick);
            arch.ensure_synced();
            
            let new_record = Record {
                id: arch.id(),
                index: res_index,
            };
            self.entities.insert_entity_record(entity, new_record);
        } else {
            if current_arch_len == 1 {
                // if this entity is the only entity for this archetype, add more columns to it
                let current_arch = self.archetypes.get_mut(&record.id).unwrap();
                current_arch.extend(&tick, vec![bundle]);
                return;
            }

            let new_arch_id = self.next_archetype_id.increment();
            let mut archetype = Archetype::from_bundle_info(new_arch_id, combined_column_infos);
            
            let mut dbun = DynamicBundle::new();
            for (column_ptr, column_info) in old_columns.into_iter() {
                unsafe {
                    // ptr of component for the entity
                    let comp_ptr = NonNull::new_unchecked(column_ptr.as_ptr()
                        .add(record.index.0 as usize * column_info.layout().size()));
                    dbun.push_unknown(comp_ptr, column_info);
                }
            }
            dbun.push_bundle(bundle);

            let entity_arch_id = archetype.add_entity(entity, dbun, &tick);

            self.archetypes.insert(new_arch_id, archetype);

            // Create entity record and store it
            let record = Record {
                id: new_arch_id,
                index: entity_arch_id,
            };

            self.entities.insert_entity_record(entity, record);
        }

        let current_arch = self.archetypes.get_mut(&record.id).unwrap();
        if let Some((en, enar)) = current_arch.remove_entity(entity, &tick) {
            let rec = Record {
                id: current_arch.id(),
                index: enar
            };
            self.entities.insert_entity_record(en, rec);
        }   

        current_arch.ensure_synced();
    }

    /// A method used for debugging implementation details of the ECS.
    /// 
    /// Here's an example of the output:
    /// ```nobuild
    /// Entities
    ///   1 in archetype 0 at 0
    ///   0 in archetype 1 at 0
    ///   2 in archetype 0 at 1
    ///   3 in archetype 2 at 0
    /// Arch 1 -- 1 entities
    ///   Col 175564825027445222460146453544114453753
    ///     0: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 128 63 0 0 128 63 0 0 128 63 0 0 128 63 78 86 0 0
    ///   Col 162279302565774655543278578489329315472
    ///     0: 0 0 32 65 0 0 32 65 0 0 32 65 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 128 63 0 0 128 63 0 0 128 63 0 0 128 63 0 0 0 0
    ///   Col 24291284537013640759061027938209843602
    ///     0: 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    /// Arch 2 -- 1 entities
    ///   Col 175564825027445222460146453544114453753
    ///     0: 0 0 0 0 0 0 0 0 0 0 0 0 237 127 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 128 63 0 0 128 63 0 0 128 63 0 0 128 63 78 86 0 0
    ///   Col 162279302565774655543278578489329315472
    ///     0: 0 0 76 66 0 0 170 66 0 0 136 65 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 128 63 0 0 128 63 0 0 128 63 0 0 128 63 0 0 0 0
    ///   Col 142862377085187052737282554588643015580
    ///     0: 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    /// Arch 0 -- 2 entities
    ///   Col 175564825027445222460146453544114453753
    ///     0: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 32 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 128 63 0 0 128 63 0 0 128 63 0 0 128 63 0 0 0 0
    ///     1: 0 0 0 0 0 0 0 0 0 0 0 0 237 127 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 128 63 0 0 128 63 0 0 128 63 0 0 128 63 78 86 0 0
    ///   Col 162279302565774655543278578489329315472
    ///     0: 0 0 112 65 0 0 112 65 0 0 112 65 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 128 63 0 0 128 63 0 0 128 63 0 0 128 63 0 0 0 0
    ///     1: 0 0 27 67 0 0 184 65 0 0 192 64 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 128 63 0 0 128 63 0 0 128 63 0 0 128 63 0 0 0 0
    ///   Col 142862377085187052737282554588643015580
    ///     0: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    ///     1: 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    ///   Col 24291284537013640759061027938209843602
    ///     0: 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    ///     1: 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    /// Arch 3 -- 0 entities
    ///   Col 175564825027445222460146453544114453753
    ///   Col 162279302565774655543278578489329315472
    /// ```
    /// 
    /// This output prints all Entity ids, the archetype they're in, and the index that they're
    /// in inside of the archetype. Additionally, the archetypes are printing, including their
    /// columns type ids, and the contents of the type ids. This output can be used to debug
    /// the contents of entities inside the archetypes.
    /// 
    /// Below is a template of the output:
    /// ```nobuild
    /// Entities
    ///   %ENTITY_ID% in archetype %ARCHETYPE_ID% at %INDEX%
    /// Arch ID -- %ARCHETYPE_LEN% entities
    ///   %FOR EACH COL%
    ///   Col COLUMN_COMPONENT_TYPE_ID
    ///     %FOR EACH ENTITY%
    ///     %ENTITY_INDEX%: %COMPONENT_BYTES%
    /// ```
    /// If the template above doesn't help you in understanding the output, read the source code
    /// of the function. The source code is pretty simple.
    pub fn debug_print_world(&self) {
        println!("Entities");
        for (en, rec) in &self.entities.arch_index {
            println!("  {} in archetype {} at {}", en.0, rec.id.0, rec.index.0);
        }

        for arch in self.archetypes.values() {
            println!("Arch {} -- {} entities", arch.id().0, arch.len());

            for col in &arch.columns {
                // no clue if doing this is stable, but this is a debug function so :shrug:
                let tyid: u128 = unsafe { std::mem::transmute(col.info.type_id().as_rust()) };
                println!("  Col {}", tyid);

                for en in 0..col.len {
                    // get the ptr starting at the component
                    let p = col.borrow_ptr();
                    let p = unsafe { p.as_ptr().add(en * col.info.layout().size()) };

                    print!("    {}: ", en);
                    // print each byte of the component
                    for i in 0..col.info.layout().size() {
                        let d = unsafe { *p.add(i) };
                        print!("{} ", d);
                    }
                    println!();
                }
            }
        }
    }

    pub fn entity_archetype(&self, entity: Entity) -> Option<&Archetype> {
        self.entities.entity_record(entity)
            .and_then(|record| self.archetypes.get(&record.id))
    }

    pub fn entity_archetype_mut(&mut self, entity: Entity) -> Option<&mut Archetype> {
        self.entities.entity_record(entity)
            .and_then(|record| self.archetypes.get_mut(&record.id))
    }

    /// View into the world for a set of entities that satisfy the queries.
    pub fn view<Q: AsQuery>(&self) -> ViewState<Q::Query, ()> {
        self.filtered_view::<Q, ()>()
    }

    /// View into the world for a set of entities that satisfy the query and the filter.
    pub fn filtered_view<Q: AsQuery, F: AsQuery>(&self) -> ViewState<Q::Query, F::Query> {
        let archetypes = self.archetypes.values().collect();
        ViewState::<Q::Query, F::Query>::new(self, Q::Query::new(), F::Query::new(), archetypes)
    }

    /// View into the world for a set of entities that satisfy the queries.
    pub fn view_iter<Q: AsQuery>(&self) -> ViewIter<Q::Query, ()> {
        let archetypes = self.archetypes.values().collect();
        let v = ViewState::new(self, Q::Query::new(), (), archetypes);
        v.into_iter()
    }

    /// View into the world for a set of entities that satisfy the queries.
    pub fn filtered_view_iter<Q: AsQuery, F: AsQuery>(&self) -> ViewIter<Q::Query, F::Query> {
        let archetypes = self.archetypes.values().collect();
        let v = ViewState::new(self, Q::Query::new(), F::Query::new(), archetypes);
        v.into_iter()
    }

    pub fn dynamic_view(&self) -> DynamicView {
        DynamicView::new(self)
    }

    pub fn view_one<T: AsQuery>(&self, entity: Entity) -> ViewOne<T::Query> {
        ViewOne::new(self, entity.id, T::Query::new())
    }

    //pub fn view_one(&self, entity: EntityId) -> 

    pub fn add_resource<T: ResourceObject>(&mut self, data: T) {
        self.resources.insert(TypeId::of::<T>(), ResourceData::new(data));
    }

    pub fn add_resource_default<T: ResourceObject + Default>(&mut self) {
        self.resources.insert(TypeId::of::<T>(), ResourceData::new(T::default()));
    }

    /// Get a resource from the world, or insert it into the world with the provided
    /// `fn` and return it.
    pub fn get_resource_or_else<T: ResourceObject, F>(&mut self, f: F) -> AtomicRefMut<T>
    where
        F: Fn() -> T + 'static
    {
        self.resources.entry(TypeId::of::<T>())
            .or_insert_with(|| ResourceData::new(f()))
            .get_mut()
    }

    /// Get a resource from the world, or insert it into the world as its default.
    pub fn get_resource_or_default<T: ResourceObject + Default>(&mut self) -> AtomicRefMut<T>
    {
        self.resources.entry(TypeId::of::<T>())
            .or_insert_with(|| ResourceData::new(T::default()))
            .get_mut()
    }

    /// Gets a resource from the World.
    /// 
    /// Will panic if the resource is not in the world. See [`World::try_get_resource`] for
    /// a function that returns an option.
    pub fn get_resource<T: ResourceObject>(&self) -> AtomicRef<T> {
        self.resources.get(&TypeId::of::<T>())
            .expect(&format!("World is missing resource of type '{}'", std::any::type_name::<T>()))
            .get()
    }

    /// Returns boolean indicating if the World contains a resource of type `T`.
    pub fn has_resource<T: ResourceObject>(&self) -> bool {
        self.resources.contains_key(&TypeId::of::<T>())
    }

    /// Attempts to get a resource from the World.
    /// 
    /// Returns `None` if the resource was not found.
    pub fn try_get_resource<T: ResourceObject>(&self) -> Option<AtomicRef<T>> {
        self.resources.get(&TypeId::of::<T>())
            .and_then(|r| r.try_get())
    }

    /// Gets a mutable borrow of a resource from the World.
    /// 
    /// Will panic if the resource is not in the world. See [`World::try_get_resource_mut`] for
    /// a function that returns an option.
    pub fn get_resource_mut<T: ResourceObject>(&self) -> AtomicRefMut<T> {
        self.resources.get(&TypeId::of::<T>())
            .expect(&format!("World is missing resource of type '{}'", std::any::type_name::<T>()))
            .get_mut()
    }

    /// Attempts to get a mutable borrow of a resource from the World.
    /// 
    /// Returns `None` if the resource was not found.
    pub fn try_get_resource_mut<T: ResourceObject>(&self) -> Option<AtomicRefMut<T>> {
        self.resources.get(&TypeId::of::<T>())
            .and_then(|r| r.try_get_mut())
    }

    pub fn try_get_resource_data<T: ResourceObject>(&self) -> Option<ResourceData> {
        self.resources.get(&TypeId::of::<T>())
            .map(|r| r.clone())
    }

    /// Increments the TickTracker which is used for tracking changes to components.
    /// 
    /// Most users wont need to call this manually, its done for you through queries and views.
    pub fn tick(&self) -> Tick {
        self.tracker.tick()
    }

    /// Gets the current tick that the world is at.
    /// 
    /// See [`TickTracker`]
    pub fn current_tick(&self) -> Tick {
        self.tracker.current()
    }

    pub fn tick_tracker(&self) -> &TickTracker {
        &self.tracker
    }

    /// Attempts to find a resource in the world and returns a NonNull pointer to it
    pub unsafe fn try_get_resource_ptr<T: ResourceObject>(&self) -> Option<NonNull<T>> {
        self.resources.get(&TypeId::of::<T>())
            .map(|d| unsafe { NonNull::new_unchecked(d.data.as_ptr() as *mut T) })
    }

    pub fn archetype_count(&self) -> usize {
        self.archetypes.len()
    }
}

// TODO: Ensure that all non-send resources are only accessible on the main thread.
unsafe impl Send for World {}
unsafe impl Sync for World {}

#[cfg(test)]
mod tests {
    use crate::{query::TickOf, tests::{Vec2, Vec3}, Entity};

    use super::World;

    struct SimpleCounter(i32);

    #[test]
    fn spawning_entity() {
        let mut world = World::new();
        let _e = world.spawn((Vec2 {
            x: 10.0,
            y: 15.0,
        }, ));
    }

    #[test]
    fn world_view_entities() {
        let mut world = World::new();
        world.spawn((Vec2 {
            x: 10.0,
            y: 15.0,
        }, ));
        world.spawn((Vec2 {
            x: 152.0,
            y: 3585.0,
        }, ));
        world.spawn((Vec2 {
            x: 235.0,
            y: 734.0,
        }, ));

        let mut count = 0;
        for pos in world.view_iter::<&Vec2>() {
            println!("Found entity at {:?}", pos);
            count += 1;
        }
        assert!(count == 3);
    }

    #[test]
    fn despawn_entity() {
        let mut world = World::new();
        world.spawn((Vec2::rand(),));
        let middle_en = world.spawn((Vec2::rand(),));
        let last_en = world.spawn((Vec2::rand(),));

        world.despawn(middle_en);

        let record = world.entities.entity_record(last_en).unwrap();
        assert_eq!(record.index.0, 1);
    }

    #[test]
    fn simple_resource() {
        let mut world = World::new();
        {
            let counter = SimpleCounter(0);
            world.add_resource(counter);
        }

        let counter = world.get_resource::<SimpleCounter>();
        assert_eq!(counter.0, 0);
        drop(counter);

        let mut counter = world.get_resource_mut::<SimpleCounter>();
        counter.0 += 4582;
        drop(counter);

        assert!(world.try_get_resource::<u32>().is_none());
    }

    #[test]
    fn resource_multi_borrow() {
        let mut world = World::new();
        let counter = SimpleCounter(4582);
        world.add_resource(counter);

        // test multiple borrows at the same time
        let counter = world.get_resource::<SimpleCounter>();
        assert_eq!(counter.0, 4582);
        let counter2 = world.get_resource::<SimpleCounter>();
        assert_eq!(counter.0, 4582);
        assert_eq!(counter2.0, 4582);
    }

    #[test]
    fn resource_one_mutable_borrow() {
        let mut world = World::new();
        {
            let counter = SimpleCounter(4582);
            world.add_resource(counter);
        }

        // test that its only possible to get a single mutable borrow
        let counter = world.get_resource_mut::<SimpleCounter>();
        assert_eq!(counter.0, 4582);
        assert!(world.try_get_resource_mut::<SimpleCounter>().is_none());
        assert_eq!(counter.0, 4582);
    }

    #[test]
    fn insert_into_existing_archetype() {
        let mut world = World::new();
        let e = world.spawn((Vec2::rand(),));
        world.spawn((Vec2::rand(),Vec3::rand()));

        world.insert(e, (Vec3::rand(),));

        assert!(world.view_one::<&Vec3>(e).get().is_some())
    }

    #[test]
    fn insert_into_new_archetype() {
        let mut world = World::new();
        let e = world.spawn((Vec2::rand(),));

        world.insert(e, (Vec3::rand(),));

        assert!(world.view_one::<&Vec3>(e).get().is_some())
    }

    #[test]
    fn insert_multiple_times() {
        let v2s = &[Vec2::rand(), Vec2::rand(), Vec2::rand()];
        let v3s = &[Vec3::rand(), Vec3::rand(), Vec3::rand()];

        let mut world = World::new();
        let e1 = world.spawn(v2s[0]);
        let e2 = world.spawn(v2s[1]);
        let e3 = world.spawn(v2s[2]);
        println!("Spawned entities");

        let ev2 = world.view_one::<&Vec2>(e2).get()
            .expect("Failed to find Vec2 and Vec3 on inserted entity!");
        assert_eq!(*ev2, v2s[1]);
        drop(ev2);

        let insert_and_assert = |world: &mut World, e: Entity, v2: Vec2, v3: Vec3| {
            println!("inserting entity");
            world.insert(e, (v3,));
            println!("inserted entity");

            let (ev2, ev3) = world.view_one::<(&Vec2, &Vec3)>(e).get()
                .expect("Failed to find Vec2 and Vec3 on inserted entity!");
            assert_eq!(*ev2, v2);
            assert_eq!(*ev3, v3);
        };

        insert_and_assert(&mut world, e2, v2s[1], v3s[1]);
        println!("Entity 2 is good");
        insert_and_assert(&mut world, e3, v2s[2], v3s[2]);
        println!("Entity 3 is good");
        assert_eq!(world.archetypes.len(), 2);
        println!("No extra archetypes were created");

        insert_and_assert(&mut world, e1, v2s[0], v3s[0]);
        println!("Entity 1 is good");
    }

    #[test]
    fn view_one() {
        let v = Vec2::rand();

        let mut world = World::new();
        let e = world.spawn((v,));

        let view = world.view_one::<&Vec2>(e);
        assert_eq!(*view.get().unwrap(), v);
    }

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

        println!("spawning");
        world.spawn((Vec2::new(10.0, 10.0),));
        world.spawn((Vec2::new(5.0, 5.0),));
        println!("spawned");

        for mut v in world.view_iter::<&mut Vec2>() {
            v.y += 50.0;
            println!("Moved v to {:?}", v);
        }

        let world_tick = world.current_tick();
        println!("The world tick is {}", *world_tick);
        for (v, tick) in world.view_iter::<(&Vec2, TickOf<Vec2>)>() {
            println!("Is at {:?}, it was changed at {}", v, *tick);
            assert!(v.y > 50.0);
            assert!(tick >= world_tick);
        }
    }

    /// Tests replacing components using World::insert
    #[test]
    fn entity_insert_replace() {
        let mut world = World::new();
        let first = world.spawn((Vec2::new(10.0, 10.0),));
        let second = world.spawn((Vec2::new(5.0, 5.0),));

        world.insert(first, Vec2::new(50.0, 50.0));

        let pos = world.view_one::<&mut Vec2>(first).get().unwrap();
        assert_eq!(*pos, Vec2::new(50.0, 50.0));
        drop(pos);

        let pos = world.view_one::<&mut Vec2>(second).get().unwrap();
        assert_eq!(*pos, Vec2::new(5.0, 5.0));
    }
}