/*******************************************************************************
 * Copyright (c) 2010 Haifeng Li
 *   
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *  
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *******************************************************************************/

package smile.interpolation.variogram;

/**
 * Exponential variogram.
 * 

 * v(r) = c + b * (1 - e-3r/a)
 * 

 * where a is the range parameter and b is sill paramter. The distance of two
 * pairs increase, the variogram of those two pairs also increase. Eventually,
 * the increase of the distance can not cause the variogram increase. The
 * distance which cause the variogram reach plateau is called range. The sill
 * parameter b is the maximum variogram value (by default, we set c = 0).
 * 

 * The parameter c is the so-called nugget effect. Though the value of the
 * variogram for h = 0 is strictly 0, several factors, such as sampling error
 * and short scale variability, may cause sample values separated by extremely
 * small distances to be quite dissimilar. This causes a discontinuity at the
 * origin of the variogram. The vertical jump from the value of 0 at the origin
 * to the value of the variogram at extremely small separation distances is
 * called the nugget effect.
 * 
 * @author Haifeng Li
 */
public class ExponentialVariogram implements Variogram {
    private double a;
    private double b;
    private double c;

    /**
     * Constructor. No nugget effect.
     * @param a the range parameter.
     * @param b the sill parameter.
     */
    public ExponentialVariogram(double a, double b) {
        this(a, b, 0.0);
    }

    /**
     * Constructor.
     * @param a the range parameter.
     * @param b the sill parameter.
     * @param c the nugget effect parameter.
     */
    public ExponentialVariogram(double a, double b, double c) {
        if (a <= 0)
            throw new IllegalArgumentException("Invalid parameter a = " + a);

        if (b <= 0)
            throw new IllegalArgumentException("Invalid parameter b = " + b);

        if (c < 0)
            throw new IllegalArgumentException("Invalid parameter c = " + c);

        this.a = a;
        this.b = b;
        this.c = c;
    }

    @Override
    public double f(double r) {
        return c + b * (1 - Math.exp(-3.0*r/a));
    }

    @Override
    public String toString() {
        return String.format("Exponential Variogram (range = %.4f, sill = %.4f, nugget effect = %.4f)", a, b, c);
    }
}