// Prim's algorithm without a priority queue
//
// Author: Rahul Simha
// Date: Oct, 2001.

import edu.gwu.algtest.*;
import edu.gwu.util.*;
import edu.gwu.debug.*;

import java.util.*;
import java.io.*;

/**
 * Class <code>Prim</code> implements Prim's algorithm but without
 * the customary priority-queue for the sake of illustration.
 *
 * @see SpanningTreeAlgorithm
 */

public class Prim implements SpanningTreeAlgorithm {

  // Debugging for internal use.
  private static final boolean debug = true;
  
  int algID;                         // Given by test-environment.
  PropertyExtractor props;           // Properties file.

  int numVertices;                   // Number of vertices (when initialized).

  boolean useMatrix;                 // Adjacency matrix or list?
  
  double[][] adjMatrix;              // The adjacency matrix (given as input).
  GraphVertex[] adjList;             // The adjacency list (given as input).

  int[] predecessor;                 // Store predecessors to build MST.
  double[] predecessorEdgeWeight;    // Store edge weights as well.

  double[][] treeMatrix;             // The MST, if required in matrix form.
  GraphVertex[] treeList;            // The MST, if required in list form.

  double treeWeight;                 // Total weight of tree.


  //---------------- Algorithm interface ------------------------------------
  
  public String getName ()
  {
    return "Prim";
  }


  public void setPropertyExtractor (int algID, PropertyExtractor props)
  {
    this.algID = algID;
    this.props = props;
  }


  //---------------- SpanningTreeAlgorithm interface -------------------------

  public void initialize (int numVertices)
  {
    this.numVertices = numVertices;
    predecessor = new int [numVertices];
    predecessorEdgeWeight = new double [numVertices];
  }



  public double[][] minimumSpanningTree (double[][] adjMatrix)
  {
    // Consistency check:
    if (numVertices != adjMatrix.length) {
      System.out.println ("ERROR: adjMatrix length not equal to numVertices");
      return null;
    }

    // We are using an adjacency list.
    useMatrix = true;

    // Maintain two sets: one set contains vertices in the current MST.
    // The other set contains those outside. Prim's algorithm always
    // adds an edge (the best one) between the two sets. Here, 
    // inSet[i]=true if vertex is in the MST.

    boolean[] inSet = new boolean [numVertices];

    // vertexKey[i] = priority of vertex i.
    double[] vertexKeys = new double [numVertices];
    for (int i=0; i<numVertices; i++)
      vertexKeys[i] = Double.MAX_VALUE;   // Infinity.

    // Set the priority of vertex 0 to 0.
    vertexKeys[0] = 0;

    // We know we're done when we've added all the vertices. 
    int numVerticesAdded = 1;
    

    // Process vertices one by one.

    while (numVerticesAdded < numVertices) {

      // Find the best vertex not in the inSet using simple
      // unsorted array of vertex keys.
      double min = Double.MAX_VALUE;
      int v = -1;
      for (int i=0; i<numVertices; i++) {
        if ( (! inSet[i]) && (vertexKeys[i] < min) ) {
          min = vertexKeys[i];
          v = i;
        }
      }

      // Put v in the MST.
      inSet[v] = true;

      numVerticesAdded ++;
      

      // Now scan neighbors in adjMatrix and adjust priorities if needed.

      for (int i=0; i<numVertices; i++) {

        // Look at row adjMatrix[v][i].
        if ( (v != i) && (adjMatrix[v][i] > 0) ) {

          // We have found a neighbor: explore the edge (v,i).
          double w = adjMatrix[v][i];

          // If the current priority is higher, lower it via edge (v,i).
          if (vertexKeys[i] > w) {

            // decreaseKey operation.
            vertexKeys[i] = w;

            // Record predecessor and weight for building MST later.
            predecessor[i] = v;
            predecessorEdgeWeight[i] = w;

          }

        }

      } //endfor
      
    } // endwhile
    

    // Create space for tree.
    treeMatrix = new double [numVertices][numVertices];
    treeWeight = 0;

    // Now build tree using pred edges. Note: we start from "1"
    // since "0" is its own predecessor (the root of the tree).

    // ... code not shown (see in-class exercise) 

    return treeMatrix;
  }

  public GraphVertex[] minimumSpanningTree (GraphVertex[] adjList)
  {
    // ... (not shown) ...
    return null;
  }

  public double getTreeWeight ()
  {
    return treeWeight;
  }
  

}