/**
* Calculates the minimum description length of this node, i.e., the length of the binary
* encoding that describes the feature and the split value used at this node
*/
public double getMDL() {
int numClasses = m_ilist.getTargetAlphabet().size();
double mdl = getSize() * getGainRatio().getBaseEntropy();
mdl += ((numClasses - 1) * Math.log(getSize() / 2.0)) / (2 * GainRatio.log2);
double piPow = Math.pow(Math.PI, numClasses / 2.0);
double gammaVal = Maths.gamma(numClasses / 2.0);
mdl += Math.log(piPow / gammaVal) / GainRatio.log2;
return mdl;
}
public double computeCostAndPrune() {
double costS = getMDL();
if (isLeaf()) return costS + 1;
double minCost1 = getLeftChild().computeCostAndPrune();
double minCost2 = getRightChild().computeCostAndPrune();
double costSplit = Math.log(m_gainRatio.getNumSplitPointsForBestFeature()) / GainRatio.log2;
double minCostN = Math.min(costS + 1, costSplit + 1 + minCost1 + minCost2);
if (Maths.almostEquals(minCostN, costS + 1)) m_leftChild = m_rightChild = null;
return minCostN;
}