# Crate keyed_priority_queue[−][src]

Expand description

This is priority queue that supports elements priority modification and early removal.

It uses HashMap and own implementation of binary heap to achieve this.

Each entry has associated key and priority. Keys must be unique, and hashable; priorities must implement Ord trait.

Popping returns element with biggest priority. Pushing adds element to queue. Also it is possible to change priority or remove item by key.

Pop, push, change priority, remove by key have O(log n) time complexity; peek, lookup by key are O(1).

## Examples

This is implementation of A* algorithm for 2D grid. Each cell in grid has the cost. This algorithm finds shortest path to target using heuristics.

Let open set be the set of position where algorithm can move in next step. Sometimes better path for node in open set is found so the priority of it needs to be updated with new value.

This example shows how to change priority in `KeyedPriorityQueue` when needed.

``````use keyed_priority_queue::{KeyedPriorityQueue, Entry};
use std::cmp::Reverse;
use std::collections::HashSet;
use std::ops::Index;

struct Field {
rows: usize,
columns: usize,
costs: Box<[u32]>,
}

#[derive(Eq, PartialEq, Debug, Hash, Copy, Clone)]
struct Position {
row: usize,
column: usize,
}

impl Index<Position> for Field {
type Output = u32;

fn index(&self, index: Position) -> &Self::Output {
&self.costs[self.columns * index.row + index.column]
}
}

// From cell we can move upper, right, bottom and left
fn get_neighbors(pos: Position, field: &Field) -> Vec<Position> {
let mut items = Vec::with_capacity(4);
if pos.row > 0 {
items.push(Position { row: pos.row - 1, column: pos.column });
}
if pos.row + 1 < field.rows {
items.push(Position { row: pos.row + 1, column: pos.column });
}
if pos.column > 0 {
items.push(Position { row: pos.row, column: pos.column - 1 });
}
if pos.column + 1 < field.columns {
items.push(Position { row: pos.row, column: pos.column + 1 });
}
items
}

fn find_path(start: Position, target: Position, field: &Field) -> Option<u32> {
if start == target {
return Some(field[start]);
}
let calc_heuristic = |pos: Position| -> u32 {
((target.row as isize - pos.row as isize).abs()
+ (target.column as isize - pos.column as isize).abs()) as u32
};

let mut closed_set: HashSet<Position> = HashSet::new();
// Positions sortered by total cost and real cost.
// We prefer items with lower real cost if total ones are same.
#[derive(Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]
struct Cost {
total: u32,
real: u32,
}
// Queue that contains all nodes that available for next step
// Min-queue required so Reverse struct used as priority.
let mut available = KeyedPriorityQueue::<Position, Reverse<Cost>>::new();
available.push(
start,
Reverse(Cost {
total: calc_heuristic(start),
real: 0,
}),
);
while let Some((current_pos, Reverse(current_cost))) = available.pop() {
// We have reached target
if current_pos == target {
return Some(current_cost.real);
}

closed_set.insert(current_pos);

for next in get_neighbors(current_pos, &field).into_iter()
.filter(|x| !closed_set.contains(x))
{
let real = field[next] + current_cost.real;
let total = current_cost.real + calc_heuristic(next);
let cost = Cost { total, real };
// Entire this interaction will make only one hash lookup
match available.entry(next) {
Entry::Vacant(entry) => {
// Add new position to queue
entry.set_priority(Reverse(cost));
}
Entry::Occupied(entry) if *entry.get_priority() < Reverse(cost) => {
// Have found better path to node in queue
entry.set_priority(Reverse(cost));
}
_ => { /* Have found worse path. */ }
};
}
}
None
}

let field = Field {
rows: 4,
columns: 4,
costs: vec![
1, 3, 3, 6, //
4, 4, 3, 8, //
3, 1, 2, 4, //
4, 8, 9, 4, //
].into_boxed_slice(),
};

let start = Position { row: 0, column: 0 };
let end = Position { row: 3, column: 3 };
assert_eq!(find_path(start, end, &field), Some(18));``````

## Structs

A priority queue that support lookup by key.

This is unordered borrowing iterator over queue.

This is consuming iterator that returns elements in decreasing order

A view into an occupied entry in a `KeyedPriorityQueue`. It is part of the `Entry` enum.

This is error type for `set_priority` method of `KeyedPriorityQueue`. It means that queue doesn’t contain such key.

A view into a vacant entry in a `KeyedPriorityQueue`. It is part of the `Entry` enum.

## Enums

A view into a single entry in a queue, which may either be vacant or occupied.