@Override
public int[] cluster(
DataSet dataSet, int lowK, int highK, ExecutorService threadpool, int[] designations) {
if (highK == lowK) return cluster(dataSet, lowK, threadpool, designations);
else if (highK < lowK)
throw new IllegalArgumentException(
"low value of k (" + lowK + ") must be higher than the high value of k(" + highK + ")");
final int N = dataSet.getSampleSize();
final int D = dataSet.getNumNumericalVars();
fKs = new double[highK - 1]; // we HAVE to start from k=2
fKs[0] = 1.0; // see eq(2)
int[] bestCluster = new int[N];
double minFk =
lowK == 1
? 1.0
: Double
.POSITIVE_INFINITY; // If our low k is > 1, force the check later to kick in at the
// first candidate k by making fK appear Inf
if (designations == null || designations.length < N) designations = new int[N];
double alphaKprev = 0, S_k_prev = 0;
// re used every iteration
List<Vec> curMeans = new ArrayList<Vec>(highK);
means = new ArrayList<Vec>(); // the best set of means
// pre-compute cache instead of re-computing every time
List<Double> accelCache = dm.getAccelerationCache(dataSet.getDataVectors(), threadpool);
for (int k = 2; k < highK; k++) {
curMeans.clear();
// kmeans objective function result is the same as S_k
double S_k =
cluster(
dataSet,
accelCache,
k,
curMeans,
designations,
true,
threadpool,
true); // TODO could add a flag to make approximate S_k an option. Though it dosn't
// seem to work great on toy problems, might be fine on more realistic data
double alpha_k;
if (k == 2) alpha_k = 1 - 3.0 / (4 * D); // eq(3a)
else alpha_k = alphaKprev + (1 - alphaKprev) / 6; // eq(3b)
double fK; // eq(2)
if (S_k_prev == 0) fKs[k - 1] = fK = 1;
else fKs[k - 1] = fK = S_k / (alpha_k * S_k_prev);
alphaKprev = alpha_k;
S_k_prev = S_k;
if (k >= lowK && minFk > fK) {
System.arraycopy(designations, 0, bestCluster, 0, N);
minFk = fK;
means.clear();
for (Vec mean : curMeans) means.add(mean.clone());
}
}
// contract is we return designations with the data in it if we can, so copy the values back
System.arraycopy(bestCluster, 0, designations, 0, N);
return designations;
}
