How do I filter a vector in place in Move?

[banool]

Describe your question in detail.

Let's say I have a vector like this:

v = [1, 4, 6, 8, 9]

I want a function like this:

/// Edit a vector in place, retaining only items that match the predicate.
public inline fun filter_in_place<Element:drop>(
    v: &mut vector<Element>,
    p: |&Element|bool
): vector<Element> { }

I would call it like this:

filter_in_place(&mut v, |e| e > 6)

Afterwards, v would look like this:

[8, 9]

filter mostly works how I want but it allocates a new vector, it doesn't operate in place.

I have this at the moment:

public inline fun filter_in_place<Element:drop>(
    v: &mut vector<Element>,
    p: |&Element|bool
) {
    let write_idx = 0;
    let read_idx = 0;
    let len = vector::length(v);

    while (read_idx < len) {
        if (p(vector::borrow(v, read_idx))) {
            if (write_idx != read_idx) {
                vector::swap(v, write_idx, read_idx);
            };
            write_idx = write_idx + 1;
        };
        read_idx = read_idx + 1;
    };

    while (len > write_idx) {
        vector::pop_back(v);
        len = len - 1;
    };
}

Is there a better, more obvious way that I'm missing?

Overall I think the vector functions need to be more clearly separated by those that create copies vs those that work in place.

What error, if any, are you getting?

No response

What have you tried or looked at? Or how can we reproduce the error?

No response

Which operating system are you using?

macOS

Which SDK or tool are you using? (if any)

N/A

Describe your environment or tooling in detail

No response

[MoonShiesty]

there shouldn't be any difference in performance between pass-by-value and pass-by-reference here

the "borrow-checker" should be able to determine if the value is borrowed later, if it isn't it won't allocate memory

aptos also has vector::trim so you can free all the memory in a single operation here