VecDeque missing operations for "front" handling.

[Firstyear]

From the docs of VecDeque:

"""
A double-ended queue implemented with a growable ring buffer.

The “default” usage of this type as a queue is to use push_back to add to the queue, and pop_front to remove from the queue. extend and append push onto the back in this manner, and iterating over VecDeque goes front to back.
"""

From this, we can assume the default way to use VecDeque when visualised is:

 front [ a, b, c, d, e ] back
 # push_back(f)
 front [ a, b, c, d, e, f ] back

VecDeque has a number of operations to reduce the size of the content in the VecDeque such as truncate or split_off, but these assume you want to truncate or split from the back.

 front [ a, b, c, d, e, f ] back
 # truncate(4)
 front [ a, b, c, d ] back

 front [ a, b, c, d, e, f ] back
 # split_off(4)
 front [ a, b, c, d ] back  ->  front [ e, f ] back

However, what is missing is equivalents for operating on the front of the VecDeque

 front [ a, b, c, d, e, f ] back
 # truncate_front(4)
 front [ c, d, e, f ] back

This created confusion for me wanting to use VecDeque as an ordered ringbuffer since VecDeque will iterate front to back, and recommends you push_back, but trying to maintain a fixed capacity requires you to either manually pop_front to len, OR you need to actually store your VecDeque in reverse, and rely on iter().rev() allowing you to use truncate.

Since VecDeque intends to be "double ended" it would be useful and convenient to have a number of the methods given front variants. My non-exhaustive list is:

• truncate
• split_off

[hellow554]

Maybe I don't understand you right, but split_off does exactly what you want. It gives you two independent deques. You can choose yourself what to keep and what to throw away (if you want to).

As an alternative you could also use drain and drop the result.

[Firstyear]

No it doesn't. If I have

fn something(&mut self) {
    self.queue.split_off(4)
}

What happens is that self.queue has it's right hand side (the back) removed similar to truncate. The only way to emulate this with "split off" to remove from the front (left) is:

fn something(&mut self) {
    let rhs = self.queue.split_off(4);
    std::mem::swap(&mut rhs, self.queue);
}

[sfackler]

fn something(&mut self) {
    self.queue = self.queue.split_off(4);
}

[Firstyear]

Given rust's pride in docs and accessibility, I think that methods for front operations should exist, so that others don't have to play a puzzle-solving game to achieve this behaviour.

[the8472]

Improving the documentation might be sufficient.

[Firstyear]

Actually, it's not @the8472. If you look at the source to vecdeque you will see that truncate operates differently to split_off, meaning that it's not sufficient to suggest it as a replacement to truncate. For split_off, retaining the right side, then sure, a documentation improvement would be sufficient. I still think truncate_front is needed.

[Spielerr]

Yup, I agree with @Firstyear , achieving truncate at front of a deque using split_off or drop/drain workarounds is not ideal. A method such as truncate_front is very similar to the existing implementation of truncate, by altering the front and rear pointers, which would be the most optimized.
There are additional methods of deque which can be extended to respect the inherent property of a deque, that is to support operations both on front and back.

[yshui]

Maybe I don't understand you right, but split_off does exactly what you want. It gives you two independent deques. You can choose yourself what to keep and what to throw away (if you want to).
As an alternative you could also use drain and drop the result.

split_off allocates, and using drain would be O(n), neither are replacement of truncate_front

[EdenSegal]

I also think that truncate_front is needed.

[urben1680]

I want to add that I would also use such functions, truncate_front and split_off_front, the latter if I want to split off the other end.

The workaround in this comment is not ideal as the capacity of self.queue is reduced to it's length while the split off part does not necessarily need a capacity as large as it did as a whole.

[esemeniuc]

Would love to see this added as well!

[JulianKnodt]

(sorry for piggybacking off this post)
I was also looking for an equivalent of extend_from_slice or extend for the beginning (see here)

I can add it if it would be acceptable.

[appcypher]

Just adding my voice here in support for corresponding "front" operations.
For me in particular, I need the extend_front method and it is weird that it doesn't exist because some other operations do.
I would also like to contribute towards its addition.

[dachan-harmonicinc]

Please add it. I really need truncate_front

[vkrivopalov]

VecDeque::truncate_front() has been added with #140668