Creating an empty iterator of a certain type in Rust

2018-12-15

I am working these days on the development of offst's Index server. I needed to implement a basic directed graph structure, allowing to run the BFS algorithm to find routes with a certain amount of capacity.

The Index server has the job of collecting capacity reports from nodes in the network. Nodes can then query the Index server for routes of certain capacities.

Whenever a node A wants to send k credits to node B, the node A should first send a request to the Index server, requesting a route from A to B of capacity at least k credits. When A obtains a response from the Index server containing a route, A can push credits along the provided route all the way to B.

Example:

A requests a route:

RouteRequest  (A -> IndexServer):  (A -> B, capacity >= 20)
RouteResponse (IndexServer -> A):  (A -- M1 -- M2 -- M3 -- B, capacity = 35)

Note that in the example A requested a route of send capacity at least 20 credits, and he obtained as response a route with capacity 35, which is more than what A asked for.

Now A can use the obtained route A -- M1 -- M2 -- M3 -- B to push credits all the way to B.

Returning an empty iterator

During the work on the Index server I had the problem of wanting to return an empty iterator in an early flow of a function.

Let's begin with some context to the problem. The graph we am working with has the following structure:

pub struct SimpleCapacityGraph<N> {
    nodes: HashMap<N, HashMap<N,(u128, u128)>>,
}

This is a map, mapping every node in the graph to a map of his neighbors. The map of his neighbors maps each neighbor to a pair of capacities, send capacity and receive capacity.

Example:

The graph:

 0 -- 1 
 |
 |
 2

Could be described with the following map:

0 -> {1 -> (10, 20), 2 -> (30, 15)}
1 -> {0 -> (20, 10)}
2 -> {0 -> (15, 35)}

In the example above, note that the node 0 reported a send capacity of 10 credits to the node 1 and a receive capacity of 20 credits to the node 1. The report of node 1 about capacities to node 0 matches the report of node 0.

However, note that this will not always be the case. For example, node 2 reported receive capacity of 35 credits from node 0, but node 0 reported send capacity of 30 credits to node 2.

The details about the send and receive capacities are not very relevant to the discussion here, but I felt obligated to explain the non matching numbers in the example above.

Going back to the code, somewhere inside the implementation of SimpleCapacityGraph there is the neighbors_with_send_capacity(&self, a: N, capacity: u128). This function returns all the neighbors of the node a with capacity which is at least the given capacity argument:

impl<N> SimpleCapacityGraph<N> 
where
    N: cmp::Eq + hash::Hash + Clone + std::fmt::Debug,
{
    /* ... */

    fn neighbors_with_send_capacity(&self, a: N, capacity: u128) -> ??? {
        // ???
    }

    /* ... */
}

Returning a Vec

The steps we should follow are roughly as follows:

  1. Try to obtain the node a's map, using self.nodes.get(&a). If this node is not present, a has no matching neighbors.

  2. Iterate over a_map, the map of a's neighbors. For every neighbor make sure that it is of a large enough capacity. Return all the neighbors that have large enough capacity.

What should be the return value for neighbors_with_send_capacity? We could choose something like Vec, obtaining this implementation:

fn neighbors_with_send_capacity_vec(&self, a: N, capacity: u128) -> Vec<&N> {
    let a_map = match self.nodes.get(&a) {
        Some(a_map) => a_map,
        None => return vec![],
    };

    a_map
        .keys()
        .filter(move |b| self.get_send_capacity(&a,b) >= capacity)
        .collect::<Vec<&N>>()
}

This is a working solution, however, note that we allocate a vector every time we call this function, and where is the fun in using Rust if we don't have zero cost abstractions?

Another important detail is that every usage of neighbors_with_send_capacity_vec() in our code uses it to iterate over the resulting neighbors. It would have been nice if we could somehow return an Iterator from the function.

Returning Option of impl Iterator

Using the impl trait feature we can do the following:

fn neighbors_with_send_capacity_option(&self, a: N, capacity: u128) -> Option<impl Iterator<Item=&N>> {
    let a_map = match self.nodes.get(&a) {
        Some(a_map) => a_map,
        None => return None,
    };
    let iter = a_map.keys().filter(move |b| self.get_send_capacity(&a,b) >= capacity);
    Some(iter)
}

However, this solution is not very ergonomic, as it requires the user of the function to first check if the returned value is Some(iterator) and only then iterate over the iterator. The user code will look like this:

if let Some(neighbors) = self.neighbors_with_send_capacity_option(a, capacity) {
    for neighbor in neighbors {
        // Do something here
    }
}

std::iter::empty() attempt

After some searching in Rust's std documentation we can find std::iter::empty(). This is a function that creates an empty iterator. A naive attempt to use it will look like this:

fn neighbors_with_send_capacity_iter_empty(&self, a: N, capacity: u128) -> impl Iterator<Item=&N> {
    let a_map = match self.nodes.get(&a) {
        Some(a_map) => a_map,
        None => return std::iter::empty(),
    };
    let iter = a_map.keys().filter(move |b| self.get_send_capacity(&a,b) >= capacity);
    iter
}

This function will not compile. The compiler throws the following error:

error[E0308]: mismatched types
  --> components/index_server/src/graph/simple_capacity_graph.rs:87:9
   |
87 |         iter
   |         ^^^^ expected struct `std::iter::Empty`, found struct `std::iter::Filter`
   |
   = note: expected type `std::iter::Empty<&N>`
              found type `std::iter::Filter<std::collections::hash_map::Keys<'_, N, (u128, u128)>, [closure@components/index_server/src/graph/simple_capacity_graph.rs:86:40: 86:89 self:_, a:_, capacity:_]>`

The compiler points out that std::iter::Empty<&N> and our other returned filter are not the same. The compiler has to choose a single return value for the function, and therefore the function could not be compiled.

Boxing

To make this function compile we can try to box the the return values:

fn neighbors_with_send_capacity_iter_empty_box(&self, a: N, capacity: u128) -> Box<dyn Iterator<Item=&N>> {
    let a_map = match self.nodes.get(&a) {
        Some(a_map) => a_map,
        None => return Box::new(std::iter::empty()),
    };
    let iter = a_map.keys().filter(move |b| self.get_send_capacity(&a,b) >= capacity);
    Box::new(iter)
}

This function will not compile. The compilation error:

error: unsatisfied lifetime constraints
  --> components/index_server/src/graph/simple_capacity_graph.rs:98:9
   |
92 |     fn neighbors_with_send_capacity_iter_empty_box(&self, a: N, capacity: u128) -> Box<dyn Iterator<Item=&N>> {
   |                                                    - let's call the lifetime of this reference `'1`
...
98 |         Box::new(iter)
   |         ^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'static`

But with some anonymous lifetime annotations magic this function will compile:

fn neighbors_with_send_capacity_iter_empty_box(&self, a: N, capacity: u128) -> Box<dyn Iterator<Item=&N> + '_> {
    let a_map = match self.nodes.get(&a) {
        Some(a_map) => a_map,
        None => return Box::new(std::iter::empty()),
    };
    let iter = a_map.keys().filter(move |b| self.get_send_capacity(&a,b) >= capacity);
    Box::new(iter)
}

OptionIterator

The Boxing solution still feels like some kind of a compromise.

To solve this problem, let's introduce a helper type called OptionIterator. OptionIterator will be able to wrap an Option<I>, where I is some Iterator. If the Option is None, the wrapper iterator will finish immediately. Otherwise, if the Option is Some(iterator), the wrapping iterator will do exactly what the internal iterator is doing.

pub struct OptionIterator<I> {
    opt_iterator: Option<I>,
}

impl<I,T> Iterator for OptionIterator<I> 
where
    I: Iterator<Item=T>,
{
    type Item = T;
    fn next(&mut self) -> Option<T> {
        match &mut self.opt_iterator {
            Some(iterator) => iterator.next(),
            None => None,
        }
    }
}

impl<I> OptionIterator<I> {
    pub fn new(opt_iterator: Option<I>) -> OptionIterator<I> {
        OptionIterator {
            opt_iterator,
        }
    }
}

As can be seen, the only field of OptionIterator is opt_iterator, which either contains an iterator or nothing. Next, let's look at the next() method of the Iterator implementation for OptionIterator. If there exists an internal iterator, the internal iterator's next() method is called. Otherwise, None is returned, which means that the iterator has finished.

In other words, for some iterator, OptionIterator::new(Some(iterator)) will behave exactly like iterator. However, OptionIterator::new(None) will behave like an empty iterator.

The advantage of using the OptionIterator wrapper is that both the existing internal iterator and the empty internal iterator cases produce the same type: OptionIterator. Recall our non compiling attempt of neighbors_with_send_capacity_iter_empty() above. We can now rewrite this function as follows:

fn neighbors_with_send_capacity(&self, a: N, capacity: u128) -> OptionIterator<impl Iterator<Item=&N>> {
    let a_map = match self.nodes.get(&a) {
        Some(a_map) => a_map,
        None => return OptionIterator::new(None),
    };
    let iter = a_map.keys().filter(move |b| self.get_send_capacity(&a,b) >= capacity);
    OptionIterator::new(Some(iter))
}

Zero cost abstractions, as promised (:

This is the actual implementation currently chosen for the code in offst. If you are interested, you can view the full code of SimpleCapacityGraph implementation on github.

Further reading

  • Discussion on reddit. Contains some very interesting advice for other possible solutions. For example, using either::Either or res.into_iter().flat_map(|iter| iter).

  • One idea we did not explore here is the possiblity of creating a generator and somehow converting it to an iterator. See: generator_to_iterator.

  • "How to create an empty iterator for a certain collection type?" on Stack Overflow