Priority queue (min/max heap) using raw binary heap.
PriorityQueue is built using raw array for efficient performance.
There are two major reasons what makes this PriorityQueue different from
other binary heap implementations currently available:
1 - Allows data ordering to scores with PartialOrd.
- Every other min-max heap requires total ordering
of scores (e.g. should implement Ord trait). This can be an issue,
for example, when you want to order items based on a float scores,
which doesn't implement Ord trait.
- Because of partial ordering, non-comparable values are thrown in
the end of the queue. One will see non-comparable values only after all
the comparable elements have been pop-ed.
- You can read about Rust's implementation or Ord, PartialOrd and
what's the different here
2 - Separation of score and item you wish to store. - This frees enforcement for associated items to implement any ordering. - Makes easier to evaluate items' order.
3 - Equal scoring items are stored at first available free space. - This gives performance boost for large number of entries.
4 - Easy to use!
You can read more about this crate on my blog
A Min-Max Heap with designated arguments for score and associated item!
A Default implementation is a Min-Heap where the top node (root) is the
lowest scoring element:
10
/ \
58 70
/ \ / \
80 92 97 99
The value of Parent Node is small than Child Node.
Every parent node, including the top (root) node, is less than or equal to equal to the right child.
PriorityQueue allows duplicate score/item values. When you [put]the
item with a similar score that’s already in the queue new entry will be
stored at the first empty location in memory. This gives an incremental
performance boost (instead of resolving by using the associated item as a
secondary tool to priority evaluation). Also, this form of implementation
doesn’t enforce for the element T to have any implemented ordering. This
guarantees that the top node will always be of minimum value.
You can initialize an empty PriorityQueue and later add items:
```rust use priq::PriorityQueue;
// create queue with usize key and String elements
let pq: PriorityQueue
Or you can heapify a Vec and/or a slice:
```rust use priq::PriorityQueue;
let pqfromvec = PriorityQueue::from(vec![(5, 55), (1, 11), (4, 44)]); let pqfromslice = PriorityQueue::from([(5, 55), (1, 11), (4, 44)]); ```
Because priq allows score arguments that only implement PartialOrd,
elements that can't be compared are evaluated and are put in the back of
the queue:
```rust use priq::PriorityQueue;
let mut pq: PriorityQueue
pq.put(1.1, 10); pq.put(f32::NAN, -1); pq.put(2.2, 20); pq.put(3.3, 30); pq.put(f32::NAN, -3); pq.put(4.4, 40);
(1..=4).foreach(|i| asserteq!(i * 10, pq.pop().unwrap().1));
// NAN scores will not have deterministic order // they are just stored after all the comparable scores assert!(0 > pq.pop().unwrap().1); assert!(0 > pq.pop().unwrap().1); ```
The standard usage of this data structure is to [put] an element to the
queue and [pop] to remove the top element and peek to check what’s the
top element in the queue. The stored structure of the elements is a balanced
tree realized using an array with a contiguous memory location. This allows
maintaining a proper parent-child relationship between put-ed items.
Runtime complexity with Big-O Notation:
| method | Time Complexity |
|-----------|-----------------|
| [put] | O(log(n)) |
| [pop] | O(log(n)) |
| [peek] | O(1) |
You can also iterate over elements using for loop but the returned slice
will not be properly order as the heap is re-balanced after each insertion
and deletion. If you want to grab items in a proper priority call [pop]
in a loop until it returns None.
structWhat if you want to custom struct without having a separate and
specific score? You can pass the struct’s clone as a score and as an
associated value, but if in this kind of scenario I’d recommend using
[BinaryHeap] as it better fits the purpose.
If instead of Min-Heap you want to have Max-Heap, where the highest-scoring
element is on top you can pass score using [Reverse] or a custom [Ord]
implementation can be used to have custom prioritization logic.
```rust use priq::PriorityQueue; use std::cmp::Reverse;
let mut pq: PriorityQueue
pq.put(Reverse(26), "Z".tostring()); pq.put(Reverse(1), "A".tostring());
assert_eq!(pq.pop().unwrap().1, "Z"); ```
You can merge another priority queue to this one. Right hand side priority queue will be drained into the left hand side priority queue.
```rust use priq::PriorityQueue;
let mut pq1 = PriorityQueue::from([(5, 55), (6, 66), (3, 33), (2, 22)]); let mut pq2 = PriorityQueue::from([(4, 44), (1, 11)]);
pq1.merge(&mut pq2);
// at this point pq2 is empty
asserteq!(6, pq1.len()); asserteq!(11, pq1.peek().unwrap().1); ```
You can also use + operator to combine two priority queues. Operands will
be intact. New priority queue will be build from cloning and merging them.
```rust use priq::PriorityQueue;
let pq1 = PriorityQueue::from([(5, 55), (1, 11), (4, 44), (2, 22)]); let pq2 = PriorityQueue::from([(8, 44), (1, 22)]);
let res = pq1 + pq2;
asserteq!(6, res.len()); asserteq!(11, res.peek().unwrap().1); ```
This are the benchmark results for priq::PriorityQueue:
| priq benches | median | nanosecs | std.dev |
|-----|-------:|:----------:|:--------|
| pqpop100 | 146 | ns/iter | (+/- 1)
| pqpop100k | 291,818 | ns/iter | (+/- 5,686)
| pqpop10k | 14,129 | ns/iter | (+/- 39)
| pqpop1k | 1,646 | ns/iter | (+/- 32)
| pqpop1mil | 16,517,047 | ns/iter | (+/- 569,128|
| pqput100 | 488 | ns/iter | (+/- 21)
| pqput100k | 758,422 | ns/iter | (+/- 13,961)
| pqput100kwcap| 748,824 | ns/iter | (+/- 7,926)
| pqput10k | 80,668 | ns/iter | (+/- 1,324)
| pqput1k | 8,769 | ns/iter | (+/- 78)
| pqput1mil | 6,728,203 | ns/iter | (+/- 76,416)
| pqput1milwcap| 6,622,341 | ns/iter | (+/- 77,162)
How it compares to std::collections::BinaryHeap:
| BinaryHeap benches | median | nanosecs | std.dev |
|-----|-------:|:----------:|:--------|
| bhpop100 | 272 | ns/iter | (+/- 90)
| bhpop100k | 171,071 | ns/iter | (+/- 6,131)
| bhpop10k | 13,904 | ns/iter | (+/- 130)
| bhpop1k | 1,847 | ns/iter | (+/- 6)
| bhpop1mil | 8,772,066 | ns/iter | (+/- 611,613)
| bhpush100 | 857 | ns/iter | (+/- 50)
| bhpush100k| 943,465 | ns/iter | (+/- 108,698)
| bhpush10k | 92,807 | ns/iter | (+/- 7,930)
| bhpush1k | 8,606 | ns/iter | (+/- 639)
| bhpush1mil| 12,946,815 | ns/iter | (+/- 900,347)
Project is distributed under the MIT license. Please see the LICENSE for more information.