// ******************************************************************************
   //
   // Title:       Force Field X.
   // Description: Force Field X - Software for Molecular Biophysics.
   // Copyright:   Copyright (c) Michael J. Schnieders 2001-2024.
   //
   // This file is part of Force Field X.
   //
   // Force Field X is free software; you can redistribute it and/or modify it
  // under the terms of the GNU General Public License version 3 as published by
  // the Free Software Foundation.
  //
  // Force Field X is distributed in the hope that it will be useful, but WITHOUT
  // ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
  // FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
  // details.
  //
  // You should have received a copy of the GNU General Public License along with
  // Force Field X; if not, write to the Free Software Foundation, Inc., 59 Temple
  // Place, Suite 330, Boston, MA 02111-1307 USA
  //
  // Linking this library statically or dynamically with other modules is making a
  // combined work based on this library. Thus, the terms and conditions of the
  // GNU General Public License cover the whole combination.
  //
  // As a special exception, the copyright holders of this library give you
  // permission to link this library with independent modules to produce an
  // executable, regardless of the license terms of these independent modules, and
  // to copy and distribute the resulting executable under terms of your choice,
  // provided that you also meet, for each linked independent module, the terms
  // and conditions of the license of that module. An independent module is a
  // module which is not derived from or based on this library. If you modify this
  // library, you may extend this exception to your version of the library, but
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  // exception statement from your version.
  //
  // ******************************************************************************
  package ffx.numerics.switching;
  
  import static java.lang.String.format;
  import static org.apache.commons.math3.util.FastMath.pow;
  
  /**
   * A PowerSwitch interpolates between 0 and 1 vi f(x) = (ax)^beta, where x must be between 0 and
   * 1/a.
   *
   * @author Jacob M. Litman
   * @author Michael J. Schnieders
   */
  public class PowerSwitch implements UnivariateSwitchingFunction {
  
    /** The multiplier a. */
    private final double a;
    /** The power of the switch. */
    private final double beta;
    /** The upper bound of the switch. */
    private final double ub;
  
    /** Default Constructor of the PowerSwitch: constructs a linear switch. */
    public PowerSwitch() {
      this(1.0, 1.0);
    }
  
    /**
     * Constructor of the PowerSwitch.
     *
     * @param a The upper bound of the switch is 1.0 / a.
     * @param beta The power of the function f(x) = (ax)^beta,
     */
    public PowerSwitch(double a, double beta) {
      if (a <= 0) {
        throw new IllegalArgumentException(" The constant a must be > 0");
      }
      if (beta == 0) {
        throw new IllegalArgumentException(" The exponent must be > 0 (preferably >= 1)");
      }
      this.a = a;
      this.beta = beta;
      ub = 1.0 / a;
    }
  
    /** {@inheritDoc} */
    @Override
    public boolean constantOutsideBounds() {
      return false;
    }
  
    /** {@inheritDoc} */
    @Override
    public double firstDerivative(double x) throws IllegalArgumentException {
      x *= a;
      return beta * a * pow(x, beta - 1);
    }
  
    /**
     * Gets the value of beta in f(x) = (a*x)^beta
     *
     * @return Exponent of input
     */
   public double getExponent() {
     return beta;
   }
 
   /** {@inheritDoc} */
   @Override
   public int getHighestOrderZeroDerivative() {
     return 0;
   }
 
   /**
    * Gets the value of a in f(x) = (a*x)^beta.
    *
    * @return Multiplier of input
    */
   public double getMultiplier() {
     return a;
   }
 
   /** {@inheritDoc} */
   @Override
   public double getOneBound() {
     return ub;
   }
 
   /** {@inheritDoc} */
   @Override
   public double getZeroBound() {
     return 0;
   }
 
   /**
    * Power switch derivatives can be zero at the zero bound if the exponent is greater than the
    * derivative order.
    *
    * @return the highest order zero derivative at zero bound
    */
   @Override
   public int highestOrderZeroDerivativeAtZeroBound() {
     return beta >= 1 ? ((int) beta) - 1 : 0;
   }
 
   /** {@inheritDoc} */
   @Override
   public double nthDerivative(double x, int order) throws IllegalArgumentException {
     x *= a;
     if (order < 1) {
       throw new IllegalArgumentException("Order must be >= 1");
     }
     switch (order) {
       case 1 -> {
         return firstDerivative(x);
       }
       case 2 -> {
         return secondDerivative(x);
       }
       default -> {
         double orderDiff = order - beta;
         if (orderDiff % 1.0 == 0 && orderDiff >= 1.0) {
           return 0.0;
         } else {
           double val = pow(x, beta - order);
           for (int i = 0; i < order; i++) {
             val *= (beta - i) * a;
           }
           return val;
         }
       }
     }
   }
 
   /** {@inheritDoc} */
   @Override
   public double secondDerivative(double x) throws IllegalArgumentException {
     x *= a;
     return beta == 1.0 ? 0.0 : beta * (beta - 1) * a * a * pow(x, beta - 2);
   }
 
   /** {@inheritDoc} */
   @Override
   public boolean symmetricToUnity() {
     return (beta == 1.0);
   }
 
   /** {@inheritDoc} */
   @Override
   public String toString() {
     return format("Power switching function f(x) = (%8.4g * x)^%8.4g", a, beta);
   }
 
   /** {@inheritDoc} */
   @Override
   public boolean validOutsideBounds() {
     return false;
   }
 
   /** {@inheritDoc} */
   @Override
   public double valueAt(double x) throws IllegalArgumentException {
     x *= a;
     return pow(x, beta);
   }
 }