Auto merge of #78818 - scottmcm:as_rchunks, r=KodrAus
Add `as_rchunks` (and friends) to slices `@est31` mentioned (https://github.com/rust-lang/rust/issues/76354#issuecomment-717027175) that, for completeness, there needed to be an `as_chunks`-like method that chunks from the end (with the remainder at the beginning) like `rchunks` does. So here's a PR for `as_rchunks: &[T] -> (&[T], &[[T; N]])` and `as_rchunks_mut: &mut [T] -> (&mut [T], &mut [[T; N]])`. But as I was doing this and copy-pasting `from_raw_parts` calls, I thought that I should extract that into an unsafe method. It started out a private helper, but it seemed like `as_chunks_unchecked` could be reasonable as a "real" method, so I added docs and made it public. Let me know if you think it doesn't pull its weight.
This commit is contained in:
bors
commit
49d7889da4
@ -888,6 +888,46 @@ impl<T> [T] {
|
||||
ChunksExactMut::new(self, chunk_size)
|
||||
}
|
||||
|
||||
/// Splits the slice into a slice of `N`-element arrays,
|
||||
/// assuming that there's no remainder.
|
||||
///
|
||||
/// # Safety
|
||||
///
|
||||
/// This may only be called when
|
||||
/// - The slice splits exactly into `N`-element chunks (aka `self.len() % N == 0`).
|
||||
/// - `N != 0`.
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// ```
|
||||
/// #![feature(slice_as_chunks)]
|
||||
/// let slice: &[char] = &['l', 'o', 'r', 'e', 'm', '!'];
|
||||
/// let chunks: &[[char; 1]] =
|
||||
/// // SAFETY: 1-element chunks never have remainder
|
||||
/// unsafe { slice.as_chunks_unchecked() };
|
||||
/// assert_eq!(chunks, &[['l'], ['o'], ['r'], ['e'], ['m'], ['!']]);
|
||||
/// let chunks: &[[char; 3]] =
|
||||
/// // SAFETY: The slice length (6) is a multiple of 3
|
||||
/// unsafe { slice.as_chunks_unchecked() };
|
||||
/// assert_eq!(chunks, &[['l', 'o', 'r'], ['e', 'm', '!']]);
|
||||
///
|
||||
/// // These would be unsound:
|
||||
/// // let chunks: &[[_; 5]] = slice.as_chunks_unchecked() // The slice length is not a multiple of 5
|
||||
/// // let chunks: &[[_; 0]] = slice.as_chunks_unchecked() // Zero-length chunks are never allowed
|
||||
/// ```
|
||||
#[unstable(feature = "slice_as_chunks", issue = "74985")]
|
||||
#[inline]
|
||||
pub unsafe fn as_chunks_unchecked<const N: usize>(&self) -> &[[T; N]] {
|
||||
debug_assert_ne!(N, 0);
|
||||
debug_assert_eq!(self.len() % N, 0);
|
||||
let new_len =
|
||||
// SAFETY: Our precondition is exactly what's needed to call this
|
||||
unsafe { crate::intrinsics::exact_div(self.len(), N) };
|
||||
// SAFETY: We cast a slice of `new_len * N` elements into
|
||||
// a slice of `new_len` many `N` elements chunks.
|
||||
unsafe { from_raw_parts(self.as_ptr().cast(), new_len) }
|
||||
}
|
||||
|
||||
/// Splits the slice into a slice of `N`-element arrays,
|
||||
/// starting at the beginning of the slice,
|
||||
/// and a remainder slice with length strictly less than `N`.
|
||||
@ -912,12 +952,42 @@ impl<T> [T] {
|
||||
assert_ne!(N, 0);
|
||||
let len = self.len() / N;
|
||||
let (multiple_of_n, remainder) = self.split_at(len * N);
|
||||
// SAFETY: We cast a slice of `len * N` elements into
|
||||
// a slice of `len` many `N` elements chunks.
|
||||
let array_slice: &[[T; N]] = unsafe { from_raw_parts(multiple_of_n.as_ptr().cast(), len) };
|
||||
// SAFETY: We already panicked for zero, and ensured by construction
|
||||
// that the length of the subslice is a multiple of N.
|
||||
let array_slice = unsafe { multiple_of_n.as_chunks_unchecked() };
|
||||
(array_slice, remainder)
|
||||
}
|
||||
|
||||
/// Splits the slice into a slice of `N`-element arrays,
|
||||
/// starting at the end of the slice,
|
||||
/// and a remainder slice with length strictly less than `N`.
|
||||
///
|
||||
/// # Panics
|
||||
///
|
||||
/// Panics if `N` is 0. This check will most probably get changed to a compile time
|
||||
/// error before this method gets stabilized.
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// ```
|
||||
/// #![feature(slice_as_chunks)]
|
||||
/// let slice = ['l', 'o', 'r', 'e', 'm'];
|
||||
/// let (remainder, chunks) = slice.as_rchunks();
|
||||
/// assert_eq!(remainder, &['l']);
|
||||
/// assert_eq!(chunks, &[['o', 'r'], ['e', 'm']]);
|
||||
/// ```
|
||||
#[unstable(feature = "slice_as_chunks", issue = "74985")]
|
||||
#[inline]
|
||||
pub fn as_rchunks<const N: usize>(&self) -> (&[T], &[[T; N]]) {
|
||||
assert_ne!(N, 0);
|
||||
let len = self.len() / N;
|
||||
let (remainder, multiple_of_n) = self.split_at(self.len() - len * N);
|
||||
// SAFETY: We already panicked for zero, and ensured by construction
|
||||
// that the length of the subslice is a multiple of N.
|
||||
let array_slice = unsafe { multiple_of_n.as_chunks_unchecked() };
|
||||
(remainder, array_slice)
|
||||
}
|
||||
|
||||
/// Returns an iterator over `N` elements of the slice at a time, starting at the
|
||||
/// beginning of the slice.
|
||||
///
|
||||
@ -952,6 +1022,48 @@ impl<T> [T] {
|
||||
ArrayChunks::new(self)
|
||||
}
|
||||
|
||||
/// Splits the slice into a slice of `N`-element arrays,
|
||||
/// assuming that there's no remainder.
|
||||
///
|
||||
/// # Safety
|
||||
///
|
||||
/// This may only be called when
|
||||
/// - The slice splits exactly into `N`-element chunks (aka `self.len() % N == 0`).
|
||||
/// - `N != 0`.
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// ```
|
||||
/// #![feature(slice_as_chunks)]
|
||||
/// let slice: &mut [char] = &mut ['l', 'o', 'r', 'e', 'm', '!'];
|
||||
/// let chunks: &mut [[char; 1]] =
|
||||
/// // SAFETY: 1-element chunks never have remainder
|
||||
/// unsafe { slice.as_chunks_unchecked_mut() };
|
||||
/// chunks[0] = ['L'];
|
||||
/// assert_eq!(chunks, &[['L'], ['o'], ['r'], ['e'], ['m'], ['!']]);
|
||||
/// let chunks: &mut [[char; 3]] =
|
||||
/// // SAFETY: The slice length (6) is a multiple of 3
|
||||
/// unsafe { slice.as_chunks_unchecked_mut() };
|
||||
/// chunks[1] = ['a', 'x', '?'];
|
||||
/// assert_eq!(slice, &['L', 'o', 'r', 'a', 'x', '?']);
|
||||
///
|
||||
/// // These would be unsound:
|
||||
/// // let chunks: &[[_; 5]] = slice.as_chunks_unchecked_mut() // The slice length is not a multiple of 5
|
||||
/// // let chunks: &[[_; 0]] = slice.as_chunks_unchecked_mut() // Zero-length chunks are never allowed
|
||||
/// ```
|
||||
#[unstable(feature = "slice_as_chunks", issue = "74985")]
|
||||
#[inline]
|
||||
pub unsafe fn as_chunks_unchecked_mut<const N: usize>(&mut self) -> &mut [[T; N]] {
|
||||
debug_assert_ne!(N, 0);
|
||||
debug_assert_eq!(self.len() % N, 0);
|
||||
let new_len =
|
||||
// SAFETY: Our precondition is exactly what's needed to call this
|
||||
unsafe { crate::intrinsics::exact_div(self.len(), N) };
|
||||
// SAFETY: We cast a slice of `new_len * N` elements into
|
||||
// a slice of `new_len` many `N` elements chunks.
|
||||
unsafe { from_raw_parts_mut(self.as_mut_ptr().cast(), new_len) }
|
||||
}
|
||||
|
||||
/// Splits the slice into a slice of `N`-element arrays,
|
||||
/// starting at the beginning of the slice,
|
||||
/// and a remainder slice with length strictly less than `N`.
|
||||
@ -982,13 +1094,48 @@ impl<T> [T] {
|
||||
assert_ne!(N, 0);
|
||||
let len = self.len() / N;
|
||||
let (multiple_of_n, remainder) = self.split_at_mut(len * N);
|
||||
let array_slice: &mut [[T; N]] =
|
||||
// SAFETY: We cast a slice of `len * N` elements into
|
||||
// a slice of `len` many `N` elements chunks.
|
||||
unsafe { from_raw_parts_mut(multiple_of_n.as_mut_ptr().cast(), len) };
|
||||
// SAFETY: We already panicked for zero, and ensured by construction
|
||||
// that the length of the subslice is a multiple of N.
|
||||
let array_slice = unsafe { multiple_of_n.as_chunks_unchecked_mut() };
|
||||
(array_slice, remainder)
|
||||
}
|
||||
|
||||
/// Splits the slice into a slice of `N`-element arrays,
|
||||
/// starting at the end of the slice,
|
||||
/// and a remainder slice with length strictly less than `N`.
|
||||
///
|
||||
/// # Panics
|
||||
///
|
||||
/// Panics if `N` is 0. This check will most probably get changed to a compile time
|
||||
/// error before this method gets stabilized.
|
||||
///
|
||||
/// # Examples
|
||||
///
|
||||
/// ```
|
||||
/// #![feature(slice_as_chunks)]
|
||||
/// let v = &mut [0, 0, 0, 0, 0];
|
||||
/// let mut count = 1;
|
||||
///
|
||||
/// let (remainder, chunks) = v.as_rchunks_mut();
|
||||
/// remainder[0] = 9;
|
||||
/// for chunk in chunks {
|
||||
/// *chunk = [count; 2];
|
||||
/// count += 1;
|
||||
/// }
|
||||
/// assert_eq!(v, &[9, 1, 1, 2, 2]);
|
||||
/// ```
|
||||
#[unstable(feature = "slice_as_chunks", issue = "74985")]
|
||||
#[inline]
|
||||
pub fn as_rchunks_mut<const N: usize>(&mut self) -> (&mut [T], &mut [[T; N]]) {
|
||||
assert_ne!(N, 0);
|
||||
let len = self.len() / N;
|
||||
let (remainder, multiple_of_n) = self.split_at_mut(self.len() - len * N);
|
||||
// SAFETY: We already panicked for zero, and ensured by construction
|
||||
// that the length of the subslice is a multiple of N.
|
||||
let array_slice = unsafe { multiple_of_n.as_chunks_unchecked_mut() };
|
||||
(remainder, array_slice)
|
||||
}
|
||||
|
||||
/// Returns an iterator over `N` elements of the slice at a time, starting at the
|
||||
/// beginning of the slice.
|
||||
///
|
||||
|
||||
33
src/test/codegen/slice-as_chunks.rs
Normal file
33
src/test/codegen/slice-as_chunks.rs
Normal file
@ -0,0 +1,33 @@
|
||||
// no-system-llvm
|
||||
// compile-flags: -O
|
||||
// only-64bit (because the LLVM type of i64 for usize shows up)
|
||||
// ignore-debug: the debug assertions get in the way
|
||||
|
||||
#![crate_type = "lib"]
|
||||
#![feature(slice_as_chunks)]
|
||||
|
||||
// CHECK-LABEL: @chunks4
|
||||
#[no_mangle]
|
||||
pub fn chunks4(x: &[u8]) -> &[[u8; 4]] {
|
||||
// CHECK-NEXT: start:
|
||||
// CHECK-NEXT: lshr i64 %x.1, 2
|
||||
// CHECK-NOT: shl
|
||||
// CHECK-NOT: mul
|
||||
// CHECK-NOT: udiv
|
||||
// CHECK-NOT: urem
|
||||
// CHECK: ret
|
||||
x.as_chunks().0
|
||||
}
|
||||
|
||||
// CHECK-LABEL: @chunks4_with_remainder
|
||||
#[no_mangle]
|
||||
pub fn chunks4_with_remainder(x: &[u8]) -> (&[[u8; 4]], &[u8]) {
|
||||
// CHECK: and i64 %x.1, -4
|
||||
// CHECK: and i64 %x.1, 3
|
||||
// CHECK: lshr exact
|
||||
// CHECK-NOT: mul
|
||||
// CHECK-NOT: udiv
|
||||
// CHECK-NOT: urem
|
||||
// CHECK: ret
|
||||
x.as_chunks()
|
||||
}
|
||||
Loading…
Reference in New Issue
Block a user