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import Graph from '../../../data-structures/graph/Graph';

import PriorityQueue from '../../../data-structures/priority-queue/PriorityQueue';

/**

* @param {Graph} graph

* @return {Graph}

*/

export default function prim(graph) {

// It should fire error if graph is directed since the algorithm works only

// for undirected graphs.

if (graph.isDirected) {

throw new Error('Prim\'s algorithms works only for undirected graphs');

}

// Init new graph that will contain minimum spanning tree of original graph.

const minimumSpanningTree = new Graph();

// This priority queue will contain all the edges that are starting from

// visited nodes and they will be ranked by edge weight - so that on each step

// we would always pick the edge with minimal edge weight.

const edgesQueue = new PriorityQueue();

// Set of vertices that has been already visited.

const visitedVertices = {};

// Vertex from which we will start graph traversal.

const startVertex = graph.getAllVertices()[0];

// Add start vertex to the set of visited ones.

visitedVertices[startVertex.getKey()] = startVertex;

// Add all edges of start vertex to the queue.

startVertex.getEdges().forEach((graphEdge) => {

edgesQueue.add(graphEdge, graphEdge.weight);

});

// Now let's explore all queued edges.

while (!edgesQueue.isEmpty()) {

// Fetch next queued edge with minimal weight.

/** @var {GraphEdge} currentEdge */

const currentMinEdge = edgesQueue.poll();

// Find out the next unvisited minimal vertex to traverse.

let nextMinVertex = null;

if (!visitedVertices[currentMinEdge.startVertex.getKey()]) {

nextMinVertex = currentMinEdge.startVertex;

} else if (!visitedVertices[currentMinEdge.endVertex.getKey()]) {

nextMinVertex = currentMinEdge.endVertex;

}

// If all vertices of current edge has been already visited then skip this round.

if (nextMinVertex) {

// Add current min edge to MST.

minimumSpanningTree.addEdge(currentMinEdge);

// Add vertex to the set of visited ones.

visitedVertices[nextMinVertex.getKey()] = nextMinVertex;

// Add all current vertex's edges to the queue.

nextMinVertex.getEdges().forEach((graphEdge) => {

// Add only vertices that link to unvisited nodes.

if (

!visitedVertices[graphEdge.startVertex.getKey()]

|| !visitedVertices[graphEdge.endVertex.getKey()]

) {

edgesQueue.add(graphEdge, graphEdge.weight);

}

});

}

}

return minimumSpanningTree;

}