Kosaraju.js

/**
 * Author: Adrito Mukherjee
 * Kosaraju's Algorithm implementation in Javascript
 * Kosaraju's Algorithm finds all the connected components in a Directed Acyclic Graph (DAG)
 * It uses Stack data structure to store the Topological Sorted Order of vertices and also Graph data structure
 *
 * Wikipedia: https://en.wikipedia.org/wiki/Kosaraju%27s_algorithm
 *
 */

class Kosaraju {
 constructor(graph) {
 this.connections = {}
 this.reverseConnections = {}
 this.stronglyConnectedComponents = []
 for (const [i, j] of graph) {
 this.addEdge(i, j)
 }
 this.topoSort()
 return this.kosaraju()
 }

 addNode(node) {
 // Function to add a node to the graph (connection represented by set)
 this.connections[node] = new Set()
 this.reverseConnections[node] = new Set()
 this.topoSorted = []
 }

 addEdge(node1, node2) {
 // Function to add an edge (adds the node too if they are not present in the graph)
 if (!(node1 in this.connections) || !(node1 in this.reverseConnections)) {
 this.addNode(node1)
 }
 if (!(node2 in this.connections) || !(node2 in this.reverseConnections)) {
 this.addNode(node2)
 }
 this.connections[node1].add(node2)
 this.reverseConnections[node2].add(node1)
 }

 dfsTopoSort(node, visited) {
 visited.add(node)
 for (const child of this.connections[node]) {
 if (!visited.has(child)) this.dfsTopoSort(child, visited)
 }
 this.topoSorted.push(node)
 }

 topoSort() {
 // Function to perform topological sorting
 const visited = new Set()
 const nodes = Object.keys(this.connections).map((key) => Number(key))
 for (const node of nodes) {
 if (!visited.has(node)) this.dfsTopoSort(node, visited)
 }
 }

 dfsKosaraju(node, visited) {
 visited.add(node)
 this.stronglyConnectedComponents[
 this.stronglyConnectedComponents.length - 1
 ].push(node)
 for (const child of this.reverseConnections[node]) {
 if (!visited.has(child)) this.dfsKosaraju(child, visited)
 }
 }

 kosaraju() {
 // Function to perform Kosaraju Algorithm
 const visited = new Set()
 while (this.topoSorted.length > 0) {
 const node = this.topoSorted.pop()
 if (!visited.has(node)) {
 this.stronglyConnectedComponents.push([])
 this.dfsKosaraju(node, visited)
 }
 }
 return this.stronglyConnectedComponents
 }
}

function kosaraju(graph) {
 const stronglyConnectedComponents = new Kosaraju(graph)
 return stronglyConnectedComponents
}

export { kosaraju }

// kosaraju([
// [1, 2],
// [2, 3],
// [3, 1],
// [2, 4],
// [4, 5],
// [5, 6],
// [6, 4],
// ])

// [ [ 1, 3, 2 ], [ 4, 6, 5 ] ]