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use std::alloc;
use std::alloc::Layout;
use std::mem;
use std::ops::Deref;
use std::ptr;
struct Inner<T: ?Sized> {
start: usize,
len: usize,
buf: T,
}
pub struct Static<T, const N: usize> {
inner: Inner<[T; N]>,
}
pub struct Dynamic<T> {
inner: Box<Inner<[T]>>,
}
#[repr(transparent)]
pub struct Ref<T> {
inner: Inner<[T]>,
}
impl<T, const N: usize> Deref for Static<T, N> {
type Target = Ref<T>;
fn deref(&self) -> &Self::Target {
let inner: &Inner<[T]> = &self.inner;
unsafe { mem::transmute::<&Inner<[T]>, &Ref<T>>(inner) }
}
}
impl<T> Dynamic<T> {
#[inline]
fn layout_for(capacity: usize) -> Layout {
Layout::new::<Inner<()>>()
.extend(Layout::array::<T>(capacity).expect("..."))
.expect("...")
.0
}
#[inline]
fn ptr_to_inner(ptr: *mut u8, capacity: usize) -> *mut Inner<[T]> {
assert!(!ptr.is_null());
ptr::slice_from_raw_parts_mut(ptr.cast::<T>(), capacity) as *mut Inner<[T]>
}
pub fn new(capacity: usize) -> Self {
let layout = Self::layout_for(capacity);
let inner = unsafe {
let ptr = Self::ptr_to_inner(alloc::alloc(layout), capacity);
ptr::addr_of_mut!((*ptr).start).write(0);
ptr::addr_of_mut!((*ptr).len).write(0);
Box::from_raw(ptr)
};
Self { inner }
}
pub fn capacity(&self) -> usize {
self.inner.buf.len()
}
pub fn grow(&mut self, new_capacity: usize) {
assert!(new_capacity >= self.capacity());
let layout = Self::layout_for(self.capacity());
let new_layout = Self::layout_for(new_capacity);
unsafe {
let ptr = Box::into_raw(ptr::addr_of_mut!(self.inner).read()) as *mut u8;
let new_ptr =
Self::ptr_to_inner(alloc::realloc(ptr, layout, new_layout.size()), new_capacity);
let inner = Box::from_raw(new_ptr);
ptr::addr_of_mut!(self.inner).write(inner);
}
}
}
impl<T> Deref for Dynamic<T> {
type Target = Ref<T>;
fn deref(&self) -> &Self::Target {
let inner: &Inner<[T]> = &self.inner;
unsafe { mem::transmute::<&Inner<[T]>, &Ref<T>>(inner) }
}
}
impl<T> Ref<T> {
#[inline]
pub fn len(&self) -> usize {
self.inner.len
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn size() {
// `Dynamic` should have the same size as a fat pointer
assert_eq!(size_of::<Dynamic<i32>>(), size_of::<*mut [i32]>());
assert_eq!(size_of::<Dynamic<i32>>(), size_of::<Box<[i32]>>());
assert_eq!(size_of::<Dynamic<i32>>(), size_of::<[usize; 2]>());
}
#[test]
fn capacity() {
let mut b = Dynamic::<i32>::new(10);
assert_eq!(b.capacity(), 10);
b.grow(20);
assert_eq!(b.capacity(), 20);
}
#[test]
fn deref() {
let b = Dynamic::<i32>::new(0);
assert_eq!(b.len(), 0);
}
}
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