如何将切片中的数据写入同一切片？

Question

我想将切片的末尾写到同一切片的顶部.

let mut foo = [1, 2, 3, 4, 5];

foo[..2].copy_from_slice(&[4..]); // error: multiple references to same data (mut and not)

assert!(foo, [4, 5, 3, 4, 5]);

我已经看过如何操作2个可变片的Rust数组

我希望尽可能提高性能(例如,使用foo.as_ptr()).

Answer 1

要将数据从切片内的一个范围复制到另一个范围(允许重叠),我们甚至无法使用.split_at_mut().

我会.split_at_mut()主要使用否则.(有没有什么能让你认为边界检查不会被优化出去？另外,你是否复制了足够的数据,相比之下它的影响很小？)

无论如何,这是你如何std::ptr::copy在安全或unsafe功能中包装(允许重叠,即memmove).

use std::ptr::copy;
use std::ops::Range;

/// Copy the range `data[from]` onto the index `to` and following
///
/// **Panics** if `from` or `to` is out of bounds
pub fn move_memory<T: Copy>(data: &mut [T], from: Range<usize>, to: usize) {
    assert!(from.start <= from.end);
    assert!(from.end <= data.len());
    assert!(to <= data.len() - (from.end - from.start));
    unsafe {
        move_memory_unchecked(data, from, to);
    }
}

pub unsafe fn move_memory_unchecked<T: Copy>(data: &mut [T], from: Range<usize>, to: usize) {
    debug_assert!(from.start <= from.end);
    debug_assert!(from.end <= data.len());
    debug_assert!(to <= data.len() - (from.end - from.start));
    let ptr = data.as_mut_ptr();
    copy(ptr.offset(from.start as isize),
         ptr.offset(to as isize),
         from.end - from.start)
}

fn main() {
    let mut data = [0, 1, 2, 3, 4, 5, 6, 7];
    move_memory(&mut data, 2..6, 0);
    println!("{:?}", data);
    move_memory(&mut data, 0..3, 5);
    println!("{:?}", data);
}

游乐场链接

Answer 2

Rust 1.37 (2019-08-15)添加了slice::copy_within完全符合您要求的库函数：

let mut foo = [1, 2, 3, 4, 5];

foo.copy_within(3 .. 5, 0);  // <-- THIS

assert_eq!(foo, [4, 5, 3, 4, 5]);

Answer 3

我找到了一种更好的方法来做我想做的事：

fn main() {
    let mut v = [1, 2, 3, 4, 5, 6];

    // scoped to restrict the lifetime of the borrows
    {
        let (left, right) = v.split_at_mut(3);
        assert!(left == [1, 2, 3]);
        assert!(right == [4, 5, 6]);
        for (l, r) in left.iter_mut().zip(right) {
            *l = *r;
        }
    }

    assert!(v == [4, 5, 6, 4, 5, 6]);
}