Swift Uniform Cost Search

graph.swift

#!/usr/bin/env quips
/*
    Requires quips - github.com/donn/quips

    yeet    

    Swift
    --
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    software under copyright law.
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*/
@quipgh SwiftPriorityQueue:davecom/SwiftPriorityQueue:1.3.0

struct Edge {
    let from: String
    let to: String
    let cost: Int
}

struct Node: Comparable {
    let name: String
    let cost: Int
    let path: [String]

    static func <(lhs: Node, rhs: Node) -> Bool {
        return lhs.cost < rhs.cost
    }

    static func ==(lhs: Node, rhs: Node) -> Bool {
        return lhs.cost == rhs.cost
    }
}

struct Graph {
    let edgesByNode: [String: [Edge]]

    init(_ edges: [Edge]) {
        var edgesByNode: [String: [Edge]] = [:]
        for edge in edges {
            edgesByNode[edge.from] = edgesByNode[edge.from] ?? []
            edgesByNode[edge.from]!.append(edge)
        }
        self.edgesByNode = edgesByNode
    }

    func uniformCostSearch(from start: String, to goal: String) -> (path: [String], cost: Int)? {       
        var frontier: PriorityQueue<Node> = PriorityQueue(ascending: true)
        var explored: Set<String> = []

        frontier.push(Node(name: start, cost: 0, path: []))

        while true {
            guard let current = frontier.pop() else {
                return nil
            }
            
            print("Arrived at \(current.name)")
            explored.insert(current.name)

            let currentPath = current.path + [current.name]
            let currentCost = current.cost

            if current.name == goal {
                print("\(current.name) is the goal. Leaving...")
                return (path: currentPath, cost: currentCost)
            } else {
                print("\(current.name) is not the goal. Continuing...")
            }

            let edges = edgesByNode[current.name] ?? []
            for edge in edges {
                let totalCost = currentCost + edge.cost

                print("Found an edge from \(edge.from) to \(edge.to) with cost \(edge.cost) (Total: \(currentCost) + \(edge.cost) = \(totalCost)).")
                if explored.contains(edge.to) {
                    print("Node \(edge.to) already explored. Continuing...")
                    continue
                }
                    
                var foundStop: Node? = nil

                for stop in frontier {
                    if stop.name == edge.to {
                        foundStop = stop
                    }
                }

                if let stop = foundStop {
                    if totalCost < stop.cost {
                        print("Found a cheaper way to \(edge.to) (\(totalCost) vs \(stop.cost)). The more expensive one will be replaced.")
                        frontier.remove(stop)
                    } else {
                        print("There is already a better way to \(edge.to) costing \(stop.cost) on the frontier.")
                        continue
                    }
                } else {
                    print("The node \(edge.to) was not explored or in the frontier, so it will be added.") 
                }
                               
                frontier.push(Node(name: edge.to, cost: totalCost, path: currentPath))

            }
        }
    }
}

let simple = false
if simple {
    let graph = Graph([
        Edge(from: "S", to: "B", cost: 5),
        Edge(from: "S", to: "D", cost: 3),
        Edge(from: "B", to: "C", cost: 1),
        Edge(from: "C", to: "E", cost: 6),
        Edge(from: "C", to: "G", cost: 8),
        Edge(from: "D", to: "E", cost: 2),
        Edge(from: "D", to: "F", cost: 2),
        Edge(from: "E", to: "B", cost: 4),
        Edge(from: "F", to: "G", cost: 3),
        Edge(from: "G", to: "E", cost: 4),
    ]) // Expected solution: SDFG, 8

    print(graph.uniformCostSearch(from: "S", to: "G") ?? "No solution.")
} else {
    let graph = Graph([
        Edge(from: "S", to: "d", cost: 3),
        Edge(from: "S", to: "e", cost: 9),
        Edge(from: "S", to: "p", cost: 1),
        Edge(from: "b", to: "a", cost: 2),
        Edge(from: "c", to: "a", cost: 2),
        Edge(from: "d", to: "b", cost: 1),
        Edge(from: "d", to: "c", cost: 8),
        Edge(from: "d", to: "e", cost: 2),
        Edge(from: "e", to: "h", cost: 1),
        Edge(from: "e", to: "r", cost: 9),
        Edge(from: "h", to: "q", cost: 4),
        Edge(from: "h", to: "p", cost: 4),
        Edge(from: "f", to: "G", cost: 5),
        Edge(from: "p", to: "q", cost: 15),
        Edge(from: "q", to: "r", cost: 3),
        Edge(from: "r", to: "f", cost: 5)
    ]) // Expected Solution: SdehqrfG, 23

    print(graph.uniformCostSearch(from: "S", to: "G") ?? "No solution.")
}