// Entities who went from !on_ground => on_ground

// (Legion for_each closures are not FnMuts; no idea why.)

let landed = RefCell::new(BumpVec::<Entity>::new_in(game.bump()));

// TODO: use parallel iterator (blocked on allocator API)

// (BumpVec isn't Send.)

<(Read<Position>, Read<PreviousPosition>, Write<BlocksFallen>)>::query()

.filter(component::<Health>())

.filter(component::<CanTakeDamage>())

.filter(!component::<Dead>())

.for_each_entities_mut(

world.inner_mut(),

|(entity, (pos, prev_pos, mut blocks_fallen))| {

match (prev_pos.0.map(|pos| pos.on_ground), pos.on_ground) {

(Some(false), false) => {

// In air: update blocks_fallen

blocks_fallen.0 += (prev_pos.0.unwrap().y - pos.y).max(0.0);

}

(Some(true), false) => {

// reset blocks_fallen

blocks_fallen.0 = 0.0;

}

(Some(false), true) => {

// landed

landed.borrow_mut().push(entity);

}

_ => (),

}

},

);

// Damage landed entities

for entity in landed.into_inner() {

let blocks_fallen = world.get::<BlocksFallen>(entity).0;

// https://minecraft.gamepedia.com/Damage#Fall_damage

let damage = (blocks_fallen - 3.0).max(0.0).round() as u32;

if damage != 0 {

game.damage(entity, damage, world);

}

}