Performing a reverse split_off operation in Rust efficiently - rust

Question

My use case involves two mutable vectors a and b and a usize parameter x. I want to make the following change:
take the elements b[0..x] and append them to the end of a (changing capacity of a as required)
transform b into b[x..], without changing the original capacity of b
Currently I do the following:
while a.len() < x && !b.is_empty() {
a.push(b.pop_front().unwrap());
// here `b` is a VecDeque but I am happy to use a Vec if pop_front was not required
}
Obviously this seems a very slow operation, checking two conditions and calling unwrap at every iteration. It would be great if there was a rev_split_off operation such that:
let mut front = b.rev_split_off(x);
a.append(&mut front);
Here rev_split_off returns a newly allocated vector for the slice b[0..x] and transforms b into the remaining slice with unchanged capacity.
Question: How to perform my use case efficiently, with or without using such a thing as rev_split_off?

Answer 1

Well, I think you will have to implement the rev_split_off yourself (even though I would probably call it split_off_back but it's the same).
Here is how I would implement it:
/// Moves the `i` first elements of `vec` at the end of `buffer`.
fn split_off_back<T>(vec: &mut Vec<T>, i: usize, buffer: &mut Vec<T>) {
// We have to make sure vec has enough elements.
// You could make the function unsafe and ask the caller to ensure
// this condition.
assert!(vec.len() >= i);
// Reserve enough memory in the target buffer
buffer.reserve(i);
// Now we know `buffer.capacity() >= buffer.len() + i`.
unsafe {
// SAFETY:
// * `vec` and `buffer` are two distinct vectors (they come from mutable references)
// so their allocations cannot overlap.
// * `vec` is valid for reads because we have an exclusive reference to it and we
// checked the value of `i`.
// * `buffer` is valid for writes because we ensured we had enough memory to store
// `i` additional elements.
std::ptr::copy_nonoverlapping(vec.as_ptr(), buffer.as_mut_ptr().add(buffer.len()), i);
// Now the memory is moved.
// we are not allowed to use it again from the `vec` vector.
// We just extanded `buffer`, we need to update its length.
// SAFEFY:
// * We ensured that the new length is less than the capacity (with `Vec::reserved`)
// * The vector is initialized for this new length (we moved the values).
buffer.set_len(buffer.len() + i);
// Now we need to update the first vector. The values from index `i` to its end
// need to be moved at the begining of the vector.
// SAFETY:
// * We have an exclusive reference to the vector. It is both valid for reads and writes.
std::ptr::copy(vec.as_ptr().add(i), vec.as_mut_ptr(), i);
// And update the length of `vec`.
// SAFETY: This subtraction is safe because we previously checked that `vec.len() >= i`.
vec.set_len(vec.len() - i);
}
}
Note that I put buffer in the parameters of the function to avoid allocating a vector. If you want the same semantic as split_off, you can just do the following.
fn split_of_back<T>(vec: &mut Vec<T>, i: usize) -> Vec<T> {
assert!(vec.len() >= i);
let mut buffer = Vec::with_capacity(i);
unsafe { /* same thing */ }
buffer
}

Answer 2

This is very simple using drain and extend.
a.extend(b.drain(..x));
If your values are Copy, then you can get optimal speed using as_slice. IIUC, using extend_from_slice should be optional due to specialization, but aids clarity.
a.extend_from_slice(b.drain(..x).as_slice());

Answer 3

I have added some benchmarks to show that the unsafe version of this code is significantly faster.
#![feature(test)]
extern crate test;
#[cfg(test)]
mod tests {
extern crate test;
use test::{black_box, Bencher};
/// Moves the `i` first elements of `vec` at the end of `buffer`.
fn split_off_back<T>(vec: &mut Vec<T>, i: usize, buffer: &mut Vec<T>) {
assert!(vec.len() >= i);
buffer.reserve(i);
unsafe {
std::ptr::copy_nonoverlapping(vec.as_ptr(), buffer.as_mut_ptr().add(buffer.len()), i);
buffer.set_len(buffer.len() + i);
std::ptr::copy(vec.as_ptr().add(i), vec.as_mut_ptr(), i);
vec.set_len(vec.len() - i);
}
}
fn split_off_back_two<T>(vec: &mut Vec<T>, i: usize, buffer: &mut Vec<T>) {
buffer.extend(vec.drain(..i));
}
const VEC_SIZE: usize = 100000;
const SPLIT_POINT: usize = 20000;
#[bench]
fn run_v1(b: &mut Bencher) {
b.iter(|| {
let mut a = black_box(vec![0; VEC_SIZE]);
let mut b = black_box(vec![0; VEC_SIZE]);
split_off_back(&mut a, SPLIT_POINT, &mut b);
});
}
#[bench]
fn run_v2(b: &mut Bencher) {
b.iter(|| {
let mut a = black_box(vec![0; VEC_SIZE]);
let mut b = black_box(vec![0; VEC_SIZE]);
split_off_back_two(&mut a, SPLIT_POINT, &mut b);
});
}
}
This is the output of cargo bench on my machine:
running 2 tests
test tests::run_v1 ... bench: 98,863 ns/iter (+/- 2,058)
test tests::run_v2 ... bench: 230,665 ns/iter (+/- 6,093)
test result: ok. 0 passed; 0 failed; 0 ignored; 2 measured; 0 filtered out; finished in 0.48s