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utils.rs
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utils.rs
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use core::cmp::{Ordering, PartialOrd};
use core::intrinsics::{assume, ptr_offset_from};
use core::mem::{size_of, MaybeUninit};
use crate::StaticVec;
/// An internal function for calculating pointer offsets as usizes, while accounting
/// directly for possible ZSTs. This is used specifically in the iterator implementations.
#[inline(always)]
pub(crate) const fn distance_between<T>(dest: *const T, origin: *const T) -> usize {
// Safety: this function is used strictly with linear inputs where `dest` is known to come after
// `origin`.
match size_of::<T>() {
// If `T` is a ZST, we cannot use typed pointers as it would either return an incorrect value
// or in some cases segfault.
0 => unsafe { ptr_offset_from(dest as *const u8, origin as *const u8) as usize },
// For all other sizes of `T`, however typed pointers are just fine.
_ => unsafe { ptr_offset_from(dest, origin) as usize },
}
}
/// A `const fn` compatible `min` function specifically for usizes. Not being generic allows it to
/// actually work in the const contexts we need it to.
#[inline(always)]
pub(crate) const fn const_min(lhs: usize, rhs: usize) -> usize {
if lhs < rhs {
lhs
} else {
rhs
}
}
/// A simple reversal function that returns a new array, called in
/// [`StaticVec::reversed`](crate::StaticVec::reversed).
#[inline]
pub(crate) const fn reverse_copy<T, const N: usize>(
length: usize,
this: &MaybeUninit<[T; N]>,
) -> MaybeUninit<[T; N]>
where
T: Copy,
{
let mut i = length;
let src = StaticVec::first_ptr(this);
let mut res = StaticVec::new_data_uninit();
let mut dest = StaticVec::first_ptr_mut(&mut res);
// We know these are valid pointers based on how and where this
// function is called from, so these are safe hints to give to the
// optimizer.
unsafe {
assume(!src.is_null());
// Curiously, the explicit typecast to `*mut T` on the next line
// is necessary to get it to compile. Without the typecast, `rustc` can't figure out
// what the type is supposed to be for some reason.
assume(!(dest as *mut T).is_null());
}
while i > 0 {
unsafe {
src.add(i - 1).copy_to_nonoverlapping(dest, 1);
dest = dest.offset(1);
i -= 1;
}
}
res
}
/// Previously this was what one of the forms of the [`staticvec!`] macro used internally. Currently
/// it's not used at all, and may be removed if I don't think of another use for it in the next
/// little while.
#[allow(dead_code)]
#[inline(always)]
pub fn new_from_value<T, const COUNT: usize>(value: T) -> StaticVec<T, COUNT>
where T: Copy {
StaticVec {
data: {
unsafe {
let mut data = StaticVec::new_data_uninit();
for i in 0..COUNT {
// Can't use `first_ptr_mut` here as the type inference doesn't work
// in this context for some reason.
(data.as_mut_ptr() as *mut T).add(i).write(value);
}
data
}
},
length: COUNT,
}
}
/// An internal convenience function for incrementing immutable ZST pointers by usize offsets.
#[inline(always)]
pub(crate) const fn zst_ptr_add<T>(ptr: *const T, count: usize) -> *const T {
debug_assert!(size_of::<T>() == 0, "`zst_ptr_add` called on a non-ZST!");
(ptr as *const u8).wrapping_add(count) as *const T
}
/// An internal convenience function for incrementing mutable ZST pointers by usize offsets.
#[inline(always)]
pub(crate) const fn zst_ptr_add_mut<T>(ptr: *mut T, count: usize) -> *mut T {
debug_assert!(
size_of::<T>() == 0,
"`zst_ptr_add_mut` called on a non-ZST!"
);
(ptr as *mut u8).wrapping_add(count) as *mut T
}
/// A version of the default `partial_cmp` implementation with a more flexible function signature.
#[inline]
pub(crate) fn partial_compare<T1, T2: PartialOrd<T1>>(
this: &[T2],
other: &[T1],
) -> Option<Ordering> {
let min_length = this.len().min(other.len());
unsafe {
let left = this.get_unchecked(0..min_length);
let right = other.get_unchecked(0..min_length);
for i in 0..min_length {
match left.get_unchecked(i).partial_cmp(right.get_unchecked(i)) {
Some(Ordering::Equal) => (),
non_eq => return non_eq,
}
}
}
this.len().partial_cmp(&other.len())
}
/// A simple quicksort function for internal use, called in
/// ['quicksorted_unstable`](crate::StaticVec::quicksorted_unstable).
#[inline]
pub(crate) const fn quicksort_internal<T: Copy + ~const PartialOrd>(
values: *mut T,
mut low: isize,
mut high: isize,
) {
// We call this function from exactly one place where `low` and `high` are known to be within an
// appropriate range before getting passed into it, so there's no need to check them again here.
// We also know that `values` will never be null, so we can safely give an optimizer hint here.
unsafe { assume(!values.is_null()) };
loop {
let mut i = low;
let mut j = high;
unsafe {
let p = *values.offset(low + ((high - low) >> 1));
loop {
while *values.offset(i) < p {
i += 1;
}
while *values.offset(j) > p {
j -= 1;
}
if i <= j {
if i != j {
let q = *values.offset(i);
*values.offset(i) = *values.offset(j);
*values.offset(j) = q;
}
i += 1;
j -= 1;
}
if i > j {
break;
}
}
}
if j - low < high - i {
if low < j {
quicksort_internal(values, low, j);
}
low = i;
} else {
if i < high {
quicksort_internal(values, i, high)
}
high = j;
}
if low >= high {
break;
}
}
}