Logistic.java

package mikera.vectorz.ops;

import java.io.ObjectStreamException;

import mikera.vectorz.AVector;

public final class Logistic extends ABoundedOp {
	
	public static final Logistic INSTANCE=new Logistic();
	
	public static double logisticFunction(double a) {
		double ea=Math.exp(-a);
		double df=(1/(1.0f+ea));
		if (Double.isNaN(df)) return (a>0)?1:0;
		return df;
	}
	
	private static double inverseLogistic (double a) {
		if (a>=1) return 800;
		if (a<=0) return -800;
		double ea=a/(1.0-a);
		return Math.log(ea);
	}
	
	@Override
	public double apply(double x) {
		return logisticFunction(x);
	}
	
	@Override
	public double applyInverse(double y) {
		return inverseLogistic(y);
	}
	
	@Override
	public void applyTo(double[] data, int start,int length) {
		for (int i=0; i<length; i++) {
			data[i+start]=logisticFunction(data[i+start]);
		}	
	}
	
	@Override
	public void applyTo(AVector v) {
		v.logistic();
	}
	
	@Override
	public boolean hasDerivative() {
		return true;
	}
	
	@Override
	public double derivativeForOutput(double y) {
		return y*(1.0-y);
	}
	
	@Override
	public double derivative(double x) {
		double y=logisticFunction(x);
		return y*(1.0-y);
	}

	@Override
	public double minValue() {
		return 0.0;
	}

	@Override
	public double maxValue() {
		return 1.0;
	}
	
	@Override
	public double averageValue() {
		return 0.5;
	}
	
	private Object readResolve() throws ObjectStreamException {
		return INSTANCE; 
	}

}