SwiftPriorityQueue is an open-source Swift library for priority queues. It supports A* search by keeping frontier elements ordered so the item with the lowest f(n) is popped first. A binary-heap implementation provides logarithmic pushes and pops. section 2.2
Use the library by satisfying its ordering contract:
- Define searchable elements such as Node as Comparable. section 2.2
- Compare nodes using their f(n) value, calculated as cost plus heuristic. section 2.2
- Create an ascending PriorityQueue and initialize it with the starting node. section 2.2
- Pop frontier nodes while searching the maze, allowing A* to prioritize promising states. section 2.2
Using the queue for A*
An A* implementation initializes PriorityQueue<Node<StateType>> with ascending: true, stores the start node with its heuristic, and repeatedly pops the next node. A Manhattan-distance heuristic supplies the node ordering for a four-direction maze search. section 2.2
class Node<T>: Comparable, Hashable {
let state: T
let parent: Node?
let cost: Float
let heuristic: Float
init(state: T, parent: Node?, cost: Float = 0.0, heuristic: Float = 0.0) {
self.state = state
self.parent = parent
self.cost = cost
self.heuristic = heuristic
}
var hashValue: Int { return Int(cost + heuristic) }
}
func < <T>(lhs: Node<T>, rhs: Node<T>) -> Bool {
return (lhs.cost + lhs.heuristic) < (rhs.cost + rhs.heuristic)
}
func == <T>(lhs: Node<T>, rhs: Node<T>) -> Bool {
return lhs === rhs
}
The Node definition shows the Comparable implementation that lets SwiftPriorityQueue order search states by cost plus heuristic. section 2.2
