/*
 * (c) Copyright Christian P. Fries, Germany. All rights reserved. Contact: email@christian-fries.de.
 *
 * Created on 08.08.2005
 */
package net.finmath.montecarlo.interestrate.modelplugins;

import net.finmath.montecarlo.RandomVariable;
import net.finmath.stochastic.RandomVariableInterface;
import net.finmath.time.TimeDiscretizationInterface;

/**
 * Implements the volatility model
 * \[
 * 	\sigma_{i}(t_{j}) = ( a + b (T_{i}-t_{j}) ) exp(-c (T_{i}-t_{j})) + d \text{.}
 * \]
 * 
 * The parameters here have some interpretation:
 * 

 * 
The parameter a: an initial volatility level.
 * 
The parameter b: the slope at the short end (shortly before maturity).
 * 
The parameter c: exponential decay of the volatility in time-to-maturity.
 * 
The parameter d: if c > 0 this is the very long term volatility level.
 * 
 *
 * Note that this model results in a terminal (Black 76) volatility which is given
 * by
 * \[
 * 	\left( \sigma^{\text{Black}}_{i}(t_{k}) \right)^2 = \frac{1}{t_{k}} \sum_{j=0}^{k-1} \left( ( a + b (T_{i}-t_{j}) ) exp(-c (T_{i}-t_{j})) + d \right)^{2} (t_{j+1}-t_{j})
 * \]
 * i.e., the instantaneous volatility is given by the picewise constant approximation of the function
 * \[
 * 	\sigma_{i}(t) = ( a + b (T_{i}-t) ) exp(-c (T_{i}-t)) + d
 * \]
 * on the time discretization \( \{ t_{j} \} \). For the exact integration of this formula see {@link LIBORVolatilityModelFourParameterExponentialFormIntegrated}.
 * 
 * @author Christian Fries
 */
public class LIBORVolatilityModelFourParameterExponentialForm extends LIBORVolatilityModel {

    private double a;
    private double b;
    private double c;
    private double d;
    
    private boolean isCalibrateable = false;

    /**
     * Creates the volatility model σi(tj) = ( a + b * (Ti-tj) ) * exp(-c (Ti-tj)) + d
     * 
     * @param timeDiscretization The simulation time discretization tj.
     * @param liborPeriodDiscretization The period time discretization Ti.
     * @param a The parameter a: an initial volatility level.
     * @param b The parameter b: the slope at the short end (shortly before maturity).
     * @param c The parameter c: exponential decay of the volatility in time-to-maturity.
     * @param d The parameter d: if c > 0 this is the very long term volatility level.
     * @param isCalibrateable Set this to true, if the parameters are available for calibration.
     */
    public LIBORVolatilityModelFourParameterExponentialForm(TimeDiscretizationInterface timeDiscretization, TimeDiscretizationInterface liborPeriodDiscretization, double a, double b, double c, double d, boolean isCalibrateable) {
        super(timeDiscretization, liborPeriodDiscretization);
        this.a = a;
        this.b = b;
        this.c = c;
        this.d = d;
        this.isCalibrateable = isCalibrateable;
    }


	@Override
	public double[] getParameter() {
		if(!isCalibrateable) return null;

		double[] parameter = new double[4];
		parameter[0] = a;
		parameter[1] = b;
		parameter[2] = c;
		parameter[3] = d;

		return parameter;
	}

	@Override
	public void setParameter(double[] parameter) {
		if(!isCalibrateable) return;

		this.a = parameter[0];
        this.b = parameter[1];
        this.c = parameter[2];
        this.d = parameter[3];
	}

    /* (non-Javadoc)
     * @see net.finmath.montecarlo.interestrate.modelplugins.LIBORVolatilityModel#getVolatility(int, int)
     */
    @Override
    public RandomVariableInterface getVolatility(int timeIndex, int liborIndex) {
        // Create a very simple volatility model here
        double time             = getTimeDiscretization().getTime(timeIndex);
        double maturity         = getLiborPeriodDiscretization().getTime(liborIndex);
        double timeToMaturity   = maturity-time;

        double volatilityInstanteaneous; 
        if(timeToMaturity <= 0)
        {
            volatilityInstanteaneous = 0.0;   // This forward rate is already fixed, no volatility
        }
        else
        {
            volatilityInstanteaneous = (a + b * timeToMaturity) * Math.exp(-c * timeToMaturity) + d;
        }
        if(volatilityInstanteaneous < 0.0) volatilityInstanteaneous = Math.max(volatilityInstanteaneous,0.0);

        return new RandomVariable(getTimeDiscretization().getTime(timeIndex),volatilityInstanteaneous);
    }

	@Override
	public Object clone() {
		return new LIBORVolatilityModelFourParameterExponentialForm(
				super.getTimeDiscretization(),
				super.getLiborPeriodDiscretization(),
				a,
				b,
				c,
				d,
				isCalibrateable
				);
	}
}