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A library of composable components enabling simpler Computational Intelligence

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/**           __  __
 *    _____ _/ /_/ /_    Computational Intelligence Library (CIlib)
 *   / ___/ / / / __ \   (c) CIRG @ UP
 *  / /__/ / / / /_/ /   http://cilib.net
 *  \___/_/_/_/_.___/
 */
package net.sourceforge.cilib.functions.continuous.unconstrained;

import net.sourceforge.cilib.functions.ContinuousFunction;
import net.sourceforge.cilib.type.types.container.Vector;

import com.google.common.base.Preconditions;

/**
 * Bird Function
 *
 * Reference: S.K. Mishra, Some New Test Functions
 * for Global Optimization and Performance of Repulsive Particle Swarm Methods
 * North-Eastern Hill University, India, 2002
 *
 * Minimum:
 * 

 *  ƒ(x*) = -106.764537 (approx) 
 *  x* = (??,??)
 *  for x₁, x₂ in [-2*π, 2*π]
 * 
 * 
 *
 * Characteristics:
 * 

 * Only defined for 2 dimensions
 * Unimodal
 * Seperable
 * Regular
 * 
 * 
 *
 * R(-6.285714286,6.285714286)^2
 *
 */
public class Bird extends ContinuousFunction {

    private static final long serialVersionUID = -7803711986955989075L;

    /**
     * {@inheritDoc}
     */
    @Override
    public Double f(Vector input) {
        Preconditions.checkArgument(input.size() == 2, "Bird function is only defined for 2 dimensions");

        double x1 = input.doubleValueOf(0);
        double x2 = input.doubleValueOf(1);

        return Math.sin(x1) * Math.exp((1 - Math.cos(x2)) * (1 - Math.cos(x2))) + Math.cos(x2) * Math.exp((1 - Math.sin(x1)) * (1 - Math.sin(x1))) + (x1 - x2) * (x1 - x2);
    }
}