Enum im::vector::Focus

pub enum Focus<'a, A> {
    // some variants omitted
}

Focused indexing over a Vector.

By remembering the last tree node accessed through an index lookup and the path we took to get there, we can speed up lookups for adjacent indices tremendously. Lookups on indices in the same node are instantaneous, and lookups on sibling nodes are also very fast.

A Focus can also be used as a restricted view into a vector, using the narrow and split_at methods.

When should I use a Focus for better performance?

Focus is useful when you need to perform a large number of index lookups that are more likely than not to be close to each other. It’s usually worth using a Focus in any situation where you’re batching a lot of index lookups together, even if they’re not obviously adjacent - there’s likely to be some performance gain for even completely random access.

If you’re just iterating forwards or backwards over the Vector in order, you’re better off with a regular iterator, which, in fact, is implemented using a Focus, but provides a simpler interface.

If you’re just doing a very small number of index lookups, the setup cost for the Focus is probably not worth it.

A Focus is never faster than an index lookup on a small Vector with a length below the internal RRB tree’s branching factor of 64.

Examples

This example is contrived, as the better way to iterate forwards or backwards over a vector is with an actual iterator. Even so, the version using a Focus should run nearly an order of magnitude faster than the version using index lookups at a length of 1000. It should also be noted that vector::Iter is actually implemented using a Focus behind the scenes, so the performance of the two should be identical.

let mut vec: Vector<i64> = Vector::from_iter(0..1000);

// Summing a vector, the slow way:
let mut sum = 0;
for i in 0..1000 {
    sum += *vec.get(i).unwrap();
}
assert_eq!(499500, sum);

// Summing a vector faster using a Focus:
let mut sum = 0;
let mut focus = vec.focus();
for i in 0..1000 {
    sum += *focus.get(i).unwrap();
}
assert_eq!(499500, sum);

// And the easy way, for completeness:
let sum: i64 = vec.iter().sum();
assert_eq!(499500, sum);

Implementations

impl<'a, A> Focus<'a, A>

where A: Clone + 'a,

pub fn new(vector: &'a Vector<A>) -> Self

Construct a Focus for a Vector.

pub fn len(&self) -> usize

Get the length of the focused Vector.

pub fn is_empty(&self) -> bool

Test if the focused Vector is empty.

pub fn get(&mut self, index: usize) -> Option<&A>

Get a reference to the value at a given index.

pub fn index(&mut self, index: usize) -> &A

Get a reference to the value at a given index.

Panics if the index is out of bounds.

pub fn chunk_at(&mut self, index: usize) -> (Range<usize>, &[A])

Get the chunk for the given index.

This gives you a reference to the leaf node that contains the index, along with its start and end indices.

pub fn narrow<R>(self, range: R) -> Self

where R: RangeBounds<usize>,

Narrow the focus onto a subslice of the vector.

Focus::narrow(range) has the same effect as &slice[range], without actually modifying the underlying vector.

Panics if the range isn’t fully inside the current focus.

Examples

let vec = Vector::from_iter(0..1000);
let narrowed = vec.focus().narrow(100..200);
let narrowed_vec = narrowed.into_iter().cloned().collect();
assert_eq!(Vector::from_iter(100..200), narrowed_vec);

pub fn split_at(self, index: usize) -> (Self, Self)

Split the focus into two.

Given an index index, consume the focus and produce two new foci, the left onto indices 0..index, and the right onto indices index..N where N is the length of the current focus.

Panics if the index is out of bounds.

This is the moral equivalent of slice::split_at, in that it leaves the underlying data structure unchanged, unlike Vector::split_at.

Examples

let vec = Vector::from_iter(0..1000);
let (left, right) = vec.focus().split_at(500);
let left_vec = left.into_iter().cloned().collect();
let right_vec = right.into_iter().cloned().collect();
assert_eq!(Vector::from_iter(0..500), left_vec);
assert_eq!(Vector::from_iter(500..1000), right_vec);

Trait Implementations

impl<'a, A> Clone for Focus<'a, A>

where A: Clone + 'a,

fn clone(&self) -> Self

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<'a, A> From<FocusMut<'a, A>> for Focus<'a, A>

where A: Clone + 'a,

fn from(f: FocusMut<'a, A>) -> Self

Converts to this type from the input type.

impl<'a, A> IntoIterator for Focus<'a, A>

where A: Clone + 'a,

type Item = &'a A

The type of the elements being iterated over.

type IntoIter = Iter<'a, A>

Which kind of iterator are we turning this into?

fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value.