SpacedQueue is a Rust no_std (alloc) library that implements a spatially distancing queue, which ensures that its items are equitably interleaved. This data structure enforces round-robin scheduling semantics, guaranteeing that items associated with the same key are spaced apart maximally during processing.

The crate spacedqueue provides a generic key-value API with constraint on keys implementing total ordering (Ord). Presented algorithm leverages BTreeMap for key management and VecDeque for reduced overhead during queue operations.

use spacedqueue::SpacedQueue;

fn main() {
    let mut queue = SpacedQueue::new();

    queue.insert(&1, &"A");
    queue.insert(&1, &"B");
    queue.insert(&2, &"C");

    assert_eq!(queue.pop(), Some(&"A"));
    assert_eq!(queue.pop(), Some(&"C"));
    assert_eq!(queue.pop(), Some(&"B"));
    assert_eq!(queue.pop(), None);
}

Empirical benchmark demonstrates that SpacedQueue outperforms multi-deque fair queuing implementation based on HashMap with interleaving, which is theoretically expected to have lesser O complexity due to use of hash-based lookups. The following results illustrate performance for processing 50,000 items:

Fair Queue Comparison/SpacedQueue/50000
                        time:   [946.91 µs 954.83 µs 963.05 µs]

Fair Queue Comparison/HashMapFairQueue/50000
                        time:   [1.6140 ms 1.6226 ms 1.6319 ms]

• new() -> SpacedQueue - Constructs a new, empty instance of the queue.

• insert(&mut self, key: &K, value: &V) - Adds a value to the queue under the specified key, dynamically updating group management.

• pop(&mut self) -> Option<&V> - Removes and returns the next item in the queue, adhering to round-robin scheduling.

• peek(&self) -> Option<&&V> - Returns a reference to the next item in the queue without removing it.

SpacedQueue is distributed under the BSD Zero Clause License.