PGX 21.1.1
Documentation

Prim's Algorithm

Category classic graph algorithms

Algorithm ID pgx_builtin_a1_prim

Time Complexity O(E + V log V) with V = number of vertices, E = number of edges

Space Requirement O(2 * E + V) with V = number of vertices, E = number of edges

Javadoc

This implementation of Prim's algorithm works on undirected graphs that are connected and have no multi-edges (i.e. more than one edge connecting the same pair of vertices). The algorithm computes the minimum spanning tree (MST) of the graph using the weights associated to each edge. A minimum spanning tree is a subset of the edges that connects all the vertices in the graph such that it minimizes the total weight assiciated to the edges.

Signature

Input Argument Type Comment
G graph
weight edgeProp edge property holding the weight of each edge in the graph.
Output Argument Type Comment
in_mst edgeProp edge property holding the edges belonging to the minimum spanning tree of the graph (i.e. all the edges with in_mst=true).
Return Value Type Comment
double the total weight associated to the MST.

Code

/*
 * Copyright (C) 2013 - 2020 Oracle and/or its affiliates. All rights reserved.
 */
package oracle.pgx.algorithms;

import oracle.pgx.algorithm.EdgeProperty;
import oracle.pgx.algorithm.annotations.GraphAlgorithm;
import oracle.pgx.algorithm.PgxEdge;
import oracle.pgx.algorithm.PgxGraph;
import oracle.pgx.algorithm.PgxMap;
import oracle.pgx.algorithm.PgxVertex;
import oracle.pgx.algorithm.VertexProperty;
import oracle.pgx.algorithm.annotations.Out;

@GraphAlgorithm
public class Prim {
  public double mst(PgxGraph g, EdgeProperty<Double> weight, @Out EdgeProperty<Boolean> inMst) {
    if (g.getNumVertices() == 0) {
      return 0.0;
    }

    PgxMap<PgxEdge, Double> q = PgxMap.create();
    PgxVertex root = g.getRandomVertex();

    root.getNeighbors().filter(n -> n != root).forSequential(n -> {
      PgxEdge e = n.edge();
      q.set(e, weight.get(e));
    });

    VertexProperty<Boolean> processed = VertexProperty.create();
    processed.setAll(false);
    processed.set(root, true);
    inMst.setAll(false);

    int numNodes = 1;
    double totalWeight = 0.0;

    while (numNodes < g.getNumVertices() && q.size() > 0) {
      PgxEdge newEdge = q.getKeyForMinValue();
      PgxVertex u = newEdge.sourceVertex();
      PgxVertex v = newEdge.destinationVertex();

      if (processed.get(v)) {
        PgxVertex tmp = v;
        v = u;
        u = tmp;
      }

      q.remove(newEdge);

      if (processed.get(v) != processed.get(u)) {
        inMst.set(newEdge, true);
        processed.set(v, true);
        totalWeight += weight.get(newEdge);

        v.getNeighbors().forEach(n -> {
          PgxEdge e = n.edge();
          if (processed.get(n)) {
            q.remove(e);
          } else {
            q.set(e, weight.get(e));
          }
        });
        numNodes++;
      }
    }

    return totalWeight;
  }
}

/*
 * Copyright (C) 2013 - 2020 Oracle and/or its affiliates. All rights reserved.
 */
procedure mst(graph G, edgeProp<double> weight; edgeProp<bool> in_mst) : double {

  if (G.numNodes() == 0) {
    return 0.0;
  }

  map<edge, double> Q;
  node root = G.pickRandom();

  for (n: root.nbrs) (n != root) {
    edge e = n.edge();
    Q[e] = e.weight;
  }

  vertexProp<bool> processed;
  G.processed = false;
  root.processed = true;
  G.in_mst = false;

  int num_nodes = 1;
  double total_weight = 0.0;

  while (num_nodes < G.numNodes() && Q.size() > 0) {
    edge new_edge = Q.getMinKey();
    node u = new_edge.fromNode();
    node v = new_edge.toNode();

    if (v.processed) {
      node tmp = v;
      v = u;
      u = tmp;
    }

    Q.remove(new_edge);

    if (v.processed != u.processed) {
      new_edge.in_mst = true;
      v.processed = true;
      total_weight += new_edge.weight;

      foreach (n: v.nbrs) {
        edge e = n.edge();
        if (n.processed) {
          Q.remove(e);
        } else {
          Q[e] = e.weight;
        }
      }
      num_nodes++;
    }
  }

  return total_weight;
}