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USE WITH CAUTION
As of now this crate has not yet been battle tested or benchmarked to an extend where the author would recommend general production usage. Please file bugs, suggestions or enhancements to the issue tracker of this repository.
A vector-like data structure that organizes its elements into a set of buckets of fixed-capacity in order to guarantee that mutations to the bucket vector never moves elements and thus invalidates references to them.
This is comparable to a Vec<Box<T>> but a lot more efficient.
The BucketVecConfig trait allows to customize the internal structure of your
BucketVec. This allows users to fine-tune their BucketVec for particular
use cases.
The trait mainly controls the capacity of the first bucket and the growth rate of the capacity of new buckets.
The default DefaultConfig tries to balance out the different interests
between start capacity and growth rate.
The BucketVec is really just a vector of Bucket instances.
Whenever an element is pushed to the BucketVec the element is pushed onto
the last Bucket if it isn't filled, yet.
If the last Bucket is filled a new Bucket is pushed onto the BucketVec
with a new capacity determined by the used bucket vector configuration.
This way the BucketVec never moves elements around upon inserting new elements
in order to preserve references. When a normal Vec is modified it can potentially
invalidate references because of reallocation of the internal buffer which
might cause severe bugs if references to the internal elements are stored
outside the Vec. Note that normally Rust prevents such situations so the
BucketVec is mainly used in the area of unsafe Rust where a developer
actively decides that they want or need pinned references into another data
structure.
For the same reasons as stated above the BucketVec does not allow to remove
or swap elements.
Looking at an example BucketVec<i32> with the following configuration:
start_capacity := 1growth_rate := 2
We have already pushed the elements A,.., K onto it.
[ [A], [B, C], [D, E, F, G], [H, I, J, K, _, _, _, _] ]
Where _ refers to a vacant bucket entry.
Pushing another L,.., O onto the same BucketVec results in:
[ [A], [B, C], [D, E, F, G], [H, I, J, K, L, M, N, O] ]
So we are full on capacity for all buckets.
The next time we push another element onto the BucketVec it will create a new Bucket with a capacity of 16 since growth_rate == 2 and our current latest bucket already has a capacity of 8.
[ [A], [B, C], [D, E, F, G], [H, I, J, K, L, M, N, O], [P, 15 x _] ]
Where 15 x _ denotes 15 consecutive vacant entries.
BucketVec fullfills its role in being a replacement for situations where
a Vec<Box<T>> is the naive go-to solution.
The benchmark suite is still small and not super expressive but already provides
some insights in where BucketVec already performs pretty well and where it
could improve.
Benchmarks have been run on a Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz.
Note that for every of the benchmark groups (push, get and iter) the
most efficient configuration for the BucketVec has been chosen.
Some benchmarks (get) have shown significant difference between configs.
The following is the output of a benchmark run each operating on 10k elements.
bucket_vec::push time: [43.647 us 43.861 us 44.108 us]
bucket_vec::push_get time: [48.872 us 49.396 us 49.834 us]
vec_box::push time: [405.37 us 410.91 us 417.20 us]
vec_value::push time: [25.826 us 25.915 us 26.020 us]
bucket_vec::get (fast config) time: [17.732 us 17.782 us 17.840 us]
bucket_vec::get (mid config) time: [243.95 us 244.75 us 245.66 us]
bucket_vec::get (slow config) time: [341.06 us 350.02 us 361.15 us]
vec_box::get time: [14.446 us 14.485 us 14.537 us]
vec_value::get time: [8.7939 us 8.8105 us 8.8300 us]
bucket_vec::iter time: [4.4195 us 4.4316 us 4.4454 us]
vec_box::iter time: [9.5925 us 9.6246 us 9.6610 us]
vec_value::iter time: [3.5955 us 3.6043 us 3.6142 us]
bucket_vec::iter_back time: [3.9804 us 3.9957 us 4.0144 us]
vec_box::iter_back time: [9.9677 us 9.9980 us 10.033 us]
vec_value::iter_back time: [3.5827 us 3.5944 us 3.6080 us]
bucket_vec::iter_mut time: [5.0533 us 5.0710 us 5.0909 us]
vec_box::iter_mut time: [13.425 us 13.845 us 14.203 us]
vec_value::iter_mut time: [4.0172 us 4.0473 us 4.0820 us]
It can be seen that BucketVec greatly outperforms Vec<Box<_>> on
push, iter and iter_back benchmarks.
However, for some configurations BucketVec::get is a lot slower than
Vec<Box<T>>::get. The configurations used in the benchmark are:
fastconfig:STARTING_CAPACITY = 16; GROWTH_RATE = 1.0;midconfig:STARTING_CAPACITY = 4; GROWTH_RATE = 2.0;slowconfig:STARTING_CAPACITY = 5; GROWTH_RATE = 1.5
For other benchmarked operations the difference in performance has not been as significant as for the get operation.
Also BucketVec is approximately 50% slower than the Vec<_> which is the
theoretical optimum that unfortunately doesn't solve the underlying problem.
Before you use this data structure make sure that you are really in need of it.
The only problem it solves is that BucketVec::{push, push_get} guarantee that
elements stored inside the BucketVec are never moved around.
- This can also be achieved by
Vec<Box<T>>although performance is generally worse for the majority of operations. - Note that this is not a solution to store trait objects in a
Vec! - Also note that under certain curcumstances it is possible to instead of a
standard Rust
Vec<T>and make sure that elements won't move upon apushoperation by usingVec::reserveorVec::reserve_exactwith appropriate arguments for the use case.
If none of the above alternative solutions is applicable you might consider
using BucketVec.
Author: Robin Freyler (github.com/Robbepop)
Special thanks to Niklas Tittjung (github.com/lugino-emeritus) who helped me a lot with some internal formulae.
Licensed under either of
- Apache license, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
- MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)
at your option.
