/**
* Splits recursively the points of the graph while the value of the best cut found is less of a
* specified limit (the alpha star factor).
*
* @param W the weight matrix of the graph
* @param alpha_star the alpha star factor
* @return an array of sets of points (partitions)
*/
protected int[][] partition(DoubleMatrix2D W, double alpha_star) {
numPartitions++;
// System.out.println("!");
// If the graph contains only one point
if (W.columns() == 1) {
int[][] p = new int[1][1];
p[0][0] = 0;
return p;
// Otherwise
} else {
// Computes the best cut
int[][] cut = bestCut(W);
// Computes the value of the found cut
double cutVal = Ncut(W, cut[0], cut[1], null);
// System.out.println("cutVal = "+cutVal +"\tnumPartitions = "+numPartitions);
// If the value is less than alpha star
if (cutVal < alpha_star && numPartitions < 2) {
// Recursively partitions the first one found ...
DoubleMatrix2D W0 = W.viewSelection(cut[0], cut[0]);
int[][] p0 = partition(W0, alpha_star);
// ... and the second one
DoubleMatrix2D W1 = W.viewSelection(cut[1], cut[1]);
int[][] p1 = partition(W1, alpha_star);
// Merges the partitions found in the previous recursive steps
int[][] p = new int[p0.length + p1.length][];
for (int i = 0; i < p0.length; i++) {
p[i] = new int[p0[i].length];
for (int j = 0; j < p0[i].length; j++) p[i][j] = cut[0][p0[i][j]];
}
for (int i = 0; i < p1.length; i++) {
p[i + p0.length] = new int[p1[i].length];
for (int j = 0; j < p1[i].length; j++) p[i + p0.length][j] = cut[1][p1[i][j]];
}
return p;
} else {
// Otherwise returns the partitions found in current step
// w/o recursive invocation
int[][] p = new int[1][W.columns()];
for (int i = 0; i < p[0].length; i++) p[0][i] = i;
return p;
}
}
}
/**
* Computes the association degree between two partitions of a graph.<br>
* The association degree is defined as the sum of the weights of all the edges between points of
* the two partitions.
*
* @param W the weight matrix of the graph
* @param a the points of the first partition
* @param b the points of the second partition
* @return the association degree
*/
protected static double asso(DoubleMatrix2D W, int[] a, int[] b) {
return W.viewSelection(a, b).zSum();
}