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use crate::{
HeteroSizedVec,
pushable::HeteroSizedPush,
};
use std::{
mem::{
size_of,
transmute_copy,
ManuallyDrop,
MaybeUninit,
},
ptr,
};
/// Size of a thin pointer.
const THIN_PTR_SIZE: usize = size_of::<usize>();
/// Size of a fat pointer.
const FAT_PTR_SIZE: usize = size_of::<usize>() * 2;
/// Whether a pointer to a type is a fat pointer.
fn pointer_is_fat<T: ?Sized>() -> bool {
match size_of::<&T>() {
FAT_PTR_SIZE => true,
THIN_PTR_SIZE => false,
_ => panic!("unexpected pointer size: {}", size_of::<&T>())
}
}
/// Decomposed memory representation of a fat pointer.
#[repr(C)]
#[derive(Copy, Clone)]
struct FatPtr<DataPtr> {
data: DataPtr,
meta: usize,
}
type FatPtrConst = FatPtr<*const u8>;
type FatPtrMut = FatPtr<*mut u8>;
impl FatPtrConst {
#[inline(always)]
unsafe fn transmute_to_ref<'a, T: ?Sized>(self) -> &'a T {
transmute_copy::<FatPtrConst, &T>(&self)
}
#[inline(always)]
unsafe fn transmute_from_ptr<T: ?Sized>(p: *const T) -> Self {
transmute_copy::<*const T, FatPtrConst>(&p)
}
}
impl FatPtrMut {
#[inline(always)]
unsafe fn transmute_to_mut<'a, T: ?Sized>(self) -> &'a mut T {
transmute_copy::<FatPtrMut, &mut T>(&self)
}
#[inline(always)]
unsafe fn transmute_to_box<T: ?Sized>(self) -> Box<T> {
transmute_copy::<FatPtrMut, Box<T>>(&self)
}
}
impl<T: ?Sized> HeteroSizedVec<T> {
/// Push an element onto the vector.
pub fn push<E: HeteroSizedPush<T>>(&mut self, elem: E) {
unsafe {
// prevent double-free in panic
let mut elem = ManuallyDrop::new(elem);
let elem_size: usize = elem.elem_size();
let elem_align: usize = elem.elem_align();
let elem_ptr: *const T = elem.elem_ptr();
let elem_drop_handler: fn(*mut u8, usize) = elem.elem_drop_handler();
// push the fat pointer meta,
// handle the case that the pointer isn't actually fat
let elem_data_ptr: *const u8 = match pointer_is_fat::<T>() {
true => {
let parts = FatPtrConst::transmute_from_ptr::<T>(elem_ptr);
self.ptr_meta.push(parts.meta);
parts.data
},
false => transmute_copy::<*const T, *const u8>(&elem_ptr),
};
// determine the start position in the elements storage
// and the amount of padding bytes to place before-hand
let mut to_reserve: usize = elem_size;
let mut offset: usize = self.storage.len();
if self.storage.len() % elem_align != 0 {
let padding: usize = (
elem_align - (self.storage.len() % elem_align)
);
offset += padding;
to_reserve += padding;
}
// add mem index and elem len
self.mem_indices.push(offset);
self.elems_size.push(elem_size);
// write element to storage memory
// 1. this is really unsafe
// 2. it leaves the padding bytes as uninitialized
// 3. but i don't think that will actually trigged UB
// 4. because of... vector implementation details... :/
self.storage.reserve(to_reserve);
ptr::copy_nonoverlapping::<u8>(
// src:
elem_data_ptr,
// dst:
self.storage.as_mut_ptr().offset(offset as isize),
// len:
elem_size,
);
self.storage.set_len(
self.storage.len() + to_reserve
);
// once we've finished reading from the elem, we can call
// `outer_drop` on the wrapper, which will invalidate
// `elem_data_ptr`.
elem.outer_drop();
// add drop handler, now that all other state is properly created
self.drop_handlers.push(elem_drop_handler);
}
}
/// Get element by index as reference.
pub fn get(&self, index: usize) -> Option<&T> {
unsafe {
let offset: usize = match self.mem_indices.get(index) {
Some(&offset) => offset,
None => return None,
};
let raw_ptr: *const u8 = self.storage.as_ptr()
.offset(offset as isize);
if pointer_is_fat::<T>() {
Some(FatPtrConst {
data: raw_ptr,
meta: *self.ptr_meta.get_unchecked(index),
}.transmute_to_ref::<T>())
} else {
Some(transmute_copy::<*const u8, &T>(&raw_ptr))
}
}
}
/// Get element by index as mutable reference.
pub fn get_mut(&mut self, index: usize) -> Option<&mut T> {
unsafe {
let offset: usize = match self.mem_indices.get(index) {
Some(&offset) => offset,
None => return None,
};
let raw_ptr: *mut u8 = self.storage.as_mut_ptr()
.offset(offset as isize);
if pointer_is_fat::<T>() {
Some(FatPtrMut {
data: raw_ptr,
meta: *self.ptr_meta.get_unchecked(index),
}.transmute_to_mut::<T>())
} else {
Some(transmute_copy::<*mut u8, &mut T>(&raw_ptr))
}
}
}
/// Convert into a vector of boxes.
pub fn into_box_vec(self) -> Vec<Box<T>> {
unsafe {
if pointer_is_fat::<T>() {
self.into_boxed_vec_fat()
} else {
self.into_boxed_vec_thin()
}
}
}
// `into_boxed_vec` case where `&T` is a fat pointer.
unsafe fn into_boxed_vec_fat(mut self) -> Vec<Box<T>> {
// clear the drop handlers now to prevent double-free in panic
self.drop_handlers.clear();
let mut boxed_vec: Vec<Box<T>> =
Vec::with_capacity(self.mem_indices.len());
// iterate through elements
for (offset, (size, meta)) in Iterator::zip(
self.mem_indices.iter().copied(),
Iterator::zip(
self.elems_size.iter().copied(),
self.ptr_meta.iter().copied(),
),
) {
// make the heap allocation
let heap_alloc: Box<[MaybeUninit<u8>]> =
Box::new_uninit_slice(size);
// lower it into a raw heap data pointer
let heap_ptr: *mut u8 =
Box::into_raw(heap_alloc) as *mut u8;
// copy the element onto the heap
ptr::copy_nonoverlapping::<u8>(
// src:
self.storage.as_mut_ptr().offset(offset as isize),
// dst:
heap_ptr,
// len:
size,
);
// produce the fat heap pointer as a box
let fat_box: Box<T> = FatPtrMut {
data: heap_ptr,
meta,
}.transmute_to_box::<T>();
// push to the vec
boxed_vec.push(fat_box);
}
// done
boxed_vec
}
// `into_boxed_vec` case where `&T` is a thin pointer.
unsafe fn into_boxed_vec_thin(mut self) -> Vec<Box<T>> {
// clear the drop handlers now to prevent double-free in panic
self.drop_handlers.clear();
let mut boxed_vec: Vec<Box<T>> =
Vec::with_capacity(self.mem_indices.len());
// iterate through elements
for (offset, size) in Iterator::zip(
self.mem_indices.iter().copied(),
self.elems_size.iter().copied(),
) {
// make the heap allocation
let heap_alloc: Box<[MaybeUninit<u8>]> =
Box::new_uninit_slice(size);
// lower it into a raw heap data pointer
let heap_ptr: *mut u8 =
Box::into_raw(heap_alloc) as *mut u8;
// copy the element onto the heap
ptr::copy_nonoverlapping::<u8>(
// src:
self.storage.as_mut_ptr().offset(offset as isize),
// dst:
heap_ptr,
// len:
size,
);
// make the heap data pointer typed
let thin_box: Box<T> = transmute_copy::<*mut u8, Box<T>>(&heap_ptr);
// push to the vec
boxed_vec.push(thin_box);
}
boxed_vec
}
}
impl<T: ?Sized> Drop for HeteroSizedVec<T> {
fn drop(&mut self) {
// drop elements
unsafe {
if pointer_is_fat::<T>() {
for (offset, (destructor, meta)) in Iterator::zip(
self.mem_indices.iter().copied(),
Iterator::zip(
self.drop_handlers.iter().copied(),
self.ptr_meta.iter().copied(),
),
) {
let ptr: *mut u8 = self.storage.as_mut_ptr()
.offset(offset as isize);
destructor(ptr, meta);
}
} else {
for (offset, destructor) in Iterator::zip(
self.mem_indices.iter().copied(),
self.drop_handlers.iter().copied(),
) {
let ptr: *mut u8 = self.storage.as_mut_ptr()
.offset(offset as isize);
destructor(ptr, 0);
}
}
}
}
}