PrimMST.js

import { KeyPriorityQueue } from '../Data-Structures/Heap/KeyPriorityQueue'
class GraphWeightedUndirectedAdjacencyList {
  // Weighted Undirected Graph class
  constructor () {
    this.connections = {}
  }

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

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

  PrimMST (start) {
    // Prim's Algorithm to generate a Minimum Spanning Tree (MST) of a graph
    // Details: https://en.wikipedia.org/wiki/Prim%27s_algorithm
    const distance = {}
    const parent = {}
    const priorityQueue = new KeyPriorityQueue()
    // Initialization
    for (const node in this.connections) {
      distance[node] = (node === start.toString() ? 0 : Infinity)
      parent[node] = null
      priorityQueue.push(node, distance[node])
    }
    // Updating 'distance' object
    while (!priorityQueue.isEmpty()) {
      const node = priorityQueue.pop()
      Object.keys(this.connections[node]).forEach(neighbour => {
        if (priorityQueue.contains(neighbour) && distance[node] + this.connections[node][neighbour] < distance[neighbour]) {
          distance[neighbour] = distance[node] + this.connections[node][neighbour]
          parent[neighbour] = node
          priorityQueue.update(neighbour, distance[neighbour])
        }
      })
    }

    // MST Generation from the 'parent' object
    const graph = new GraphWeightedUndirectedAdjacencyList()
    Object.keys(parent).forEach(node => {
      if (node && parent[node]) {
        graph.addEdge(node, parent[node], this.connections[node][parent[node]])
      }
    })
    return graph
  }
}

export { GraphWeightedUndirectedAdjacencyList }