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use crate::{
component::Component,
entities::Null,
};
use alloc::vec::Vec;
use core::mem::ManuallyDrop;
pub trait Storage {
/// Extend the component columns with the components contained in this heterogeneous list.
///
/// This consumes the entities, moving the components into their appropriate columns.
///
/// The components are stored within the `Vec<C>`s defined by `components` and `length`, using
/// the given `component_map` to determine which column each components should be added to.
///
/// # Safety
/// The components in both the entities and `components` much correspond to the same components
/// in the same order.
///
/// `components`, together with `length`, must define a valid `Vec<C>` for each component.
unsafe fn extend_components(self, components: &mut [(*mut u8, usize)], length: usize);
}
impl Storage for Null {
unsafe fn extend_components(self, _components: &mut [(*mut u8, usize)], _length: usize) {}
}
impl<C, E> Storage for (Vec<C>, E)
where
C: Component,
E: Storage,
{
unsafe fn extend_components(self, components: &mut [(*mut u8, usize)], length: usize) {
// SAFETY: `components` is guaranteed by the safety contract of this method to contain a
// column for component `C` as its first value.
let component_column = unsafe { components.get_unchecked_mut(0) };
// Check both length and capacity to ensure nothing is currently allocated.
if length == 0 && component_column.1 == 0 {
let mut v = ManuallyDrop::new(self.0);
*component_column = (v.as_mut_ptr().cast::<u8>(), v.capacity());
} else {
let mut v = ManuallyDrop::new(
// SAFETY: The `component_column` extracted from `components` is guaranteed to,
// together with `length`, define a valid `Vec<C>` for the current `C`, because the
// `component_column` extracted is guaranteed by the safety contract to correspond
// to the column for component `C`.
unsafe {
Vec::<C>::from_raw_parts(
component_column.0.cast::<C>(),
length,
component_column.1,
)
},
);
v.extend(self.0);
*component_column = (v.as_mut_ptr().cast::<u8>(), v.capacity());
}
// SAFETY: Since `component_map`, `components`, and `length` all meet the safety
// requirements for the current method body, they will meet those same requirements for
// this method call.
unsafe { E::extend_components(self.1, components.get_unchecked_mut(1..), length) };
}
}