Merge pull request #41 from 5sw/graphs

Depth-first and breadth-first search in graphs

Author: carloshmartins

DIRECTORY.md

@@ -55,6 +55,7 @@
     * [Union Find](https://github.com/TheAlgorithms/Swift/blob/master/data_structures/union_find/union_find.swift)
 
 ## Graph
+  * [Graph](https://github.com/TheAlgorithms/Swift/blob/master/graph/Graph.swift)
   * Bfs
     * [Bfs](https://github.com/TheAlgorithms/Swift/blob/master/graph/BFS/BFS.swift)
   * Dfs

graph/Graph.swift

@@ -0,0 +1,109 @@
+struct GraphEdge<G: Graph> {
+    var from: G.Node
+    var to: G.Node
+    var value: G.EdgeValue
+}
+
+protocol Graph {
+    typealias Edge = GraphEdge<Self>
+
+    associatedtype Node: Equatable
+    associatedtype EdgeValue
+
+    func edges(from: Node) -> [Edge]
+}
+
+struct AdjacencyList<Node: Hashable, EdgeValue>: Graph {
+    typealias EdgeValue = EdgeValue
+    typealias Node = Node
+
+    var graph: [Node: [Edge]] = [:]
+
+    func edges(from node: Node) -> [Edge] {
+        graph[node, default: []]
+    }
+
+    mutating func insert(from: Node, to: Node, value: EdgeValue) {
+        graph[from, default: []].append(Edge(from: from, to: to, value: value))
+    }
+
+    var allEdges: [Edge] {
+        graph.values.flatMap { $0 }
+    }
+}
+
+extension AdjacencyList where EdgeValue == () {
+    mutating func insert(from: Node, to: Node) {
+        insert(from: from, to: to, value: ())
+    }
+}
+
+extension Graph {
+    func depthFirstSearch(start: Node, destination: Node) -> [Edge]? {
+        if start == destination {
+            return []
+        }
+
+        for edge in edges(from: start) {
+            if let path = depthFirstSearch(start: edge.to, destination: destination) {
+                return [edge] + path
+            }
+        }
+
+        return nil
+    }
+}
+
+extension Graph where Node: Hashable {
+    func breadthFirstSearch(start: Node, destination: Node) -> [Edge]? {
+        var queue: [(Node, [Edge])] = [(start, [])]
+        var visited: Set<Node> = [start]
+
+        while !queue.isEmpty {
+            let (current, path) = queue.removeFirst()
+            if current == destination {
+                return path
+            }
+
+            for edge in edges(from: current) where visited.insert(edge.to).inserted {
+                queue.append((edge.to, path + [edge]))
+            }
+        }
+
+        return nil
+    }
+}
+
+extension GraphEdge: CustomDebugStringConvertible {
+    var debugDescription: String {
+        if type(of: value) == Void.self {
+            return "\(from) -- \(to)"
+        } else {
+            return "\(from) -\(value)- \(to)"
+        }
+    }
+}
+
+var graph = AdjacencyList<String, Void>()
+graph.insert(from: "a", to: "b")
+graph.insert(from: "a", to: "d")
+graph.insert(from: "b", to: "c")
+graph.insert(from: "c", to: "d")
+
+func test<G: Graph>(_ message: String, _ list: [GraphEdge<G>]?) {
+    print(message)
+    if let list = list {
+        for edge in list {
+            print(edge)
+        }
+    } else {
+        print("Not found")
+    }
+    print("")
+}
+
+test("Depth-first a -> d", graph.depthFirstSearch(start: "a", destination: "d"))
+test("Depth-first a -> e", graph.depthFirstSearch(start: "a", destination: "e"))
+
+test("Breadth-first a -> d", graph.breadthFirstSearch(start: "a", destination: "d"))
+test("Breadth-first a -> e", graph.breadthFirstSearch(start: "a", destination: "e"))