Efficiently insert or replace multiple elements in

Question

Is there any straightforward way to insert or replace multiple elements from &[T] and/or Vec<T> in the middle or at the beginning of a Vec in linear time?

I could only find std::vec::Vec::insert, but that's only for inserting a single element in O(n) time, so I obviously cannot call that in a loop.

I could do a split_off at that index, extend the new elements into the left half of the split, and then extend the second half into the first, but is there a better way?

Answer 1

As of Rust 1.21.0, Vec::splice is available and allows inserting at any point, including fully prepending:

let mut vec = vec![1, 5];
let slice = &[2, 3, 4];

vec.splice(1..1, slice.iter().cloned());

println!("{:?}", vec); // [1, 2, 3, 4, 5]

The docs state:

Note 4: This is optimal if:

• The tail (elements in the vector after range) is empty
• or replace_with yields fewer elements than range’s length
• or the lower bound of its size_hint() is exact.

In this case, the lower bound of the slice's iterator should be exact, so it should perform one memory move.

---

splice is a bit more powerful in that it allows you to remove a range of values (the first argument), insert new values (the second argument), and optionally get the old values (the result of the call).

Replacing a set of items

let mut vec = vec![0, 1, 5];
let slice = &[2, 3, 4];

vec.splice(..2, slice.iter().cloned());

println!("{:?}", vec); // [2, 3, 4, 5]

Getting the previous values

let mut vec = vec![0, 1, 2, 3, 4];
let slice = &[9, 8, 7];

let old: Vec<_> = vec.splice(3.., slice.iter().cloned()).collect();

println!("{:?}", vec); // [0, 1, 2, 9, 8, 7]
println!("{:?}", old); // [3, 4]

Answer 2

Okay, there is no appropriate method in Vec interface (as I can see). But we can always implement the same thing ourselves.

memmove

When T is Copy, probably the most obvious way is to move the memory, like this:

fn push_all_at<T>(v: &mut Vec<T>, offset: usize, s: &[T]) where T: Copy {
    match (v.len(), s.len()) {
        (_, 0) => (),
        (current_len, _) => {
            v.reserve_exact(s.len());
            unsafe {
                v.set_len(current_len + s.len());
                let to_move = current_len - offset;
                let src = v.as_mut_ptr().offset(offset as isize);
                if to_move > 0 {
                    let dst = src.offset(s.len() as isize);
                    std::ptr::copy_memory(dst, src, to_move);
                }
                std::ptr::copy_nonoverlapping_memory(src, s.as_ptr(), s.len());
            }
        },
    }
}

shuffle

If T is not copy, but it implements Clone, we can append given slice to the end of the Vec, and move it to the required position using swaps in linear time:

fn push_all_at<T>(v: &mut Vec<T>, mut offset: usize, s: &[T]) where T: Clone + Default {
    match (v.len(), s.len()) {
        (_, 0) => (),
        (0, _) => { v.push_all(s); },
        (_, _) => {
            assert!(offset <= v.len());
            let pad = s.len() - ((v.len() - offset) % s.len());
            v.extend(repeat(Default::default()).take(pad));
            v.push_all(s);
            let total = v.len();
            while total - offset >= s.len() {
                for i in 0 .. s.len() { v.swap(offset + i, total - s.len() + i); }
                offset += s.len();
            }
            v.truncate(total - pad);
        },
    }
}

iterators concat

Maybe the best choice will be to not modify Vec at all. For example, if you are going to access the result via iterator, we can just build iterators chain from our chunks:

let v: &[usize] = &[0, 1, 2];
let s: &[usize] = &[3, 4, 5, 6];
let offset = 2;
let chain = v.iter().take(offset).chain(s.iter()).chain(v.iter().skip(offset));

let result: Vec<_> = chain.collect();
println!("Result: {:?}", result);