/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You 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 org.apache.commons.math4.legacy.analysis.differentiation;
  
  import java.util.Arrays;
  
  import org.apache.commons.numbers.core.Sum;
  import org.apache.commons.math4.legacy.core.Field;
  import org.apache.commons.math4.legacy.core.FieldElement;
  import org.apache.commons.math4.legacy.core.RealFieldElement;
  import org.apache.commons.math4.legacy.exception.DimensionMismatchException;
  import org.apache.commons.math4.legacy.exception.NumberIsTooLargeException;
  import org.apache.commons.math4.core.jdkmath.JdkMath;
  import org.apache.commons.math4.legacy.core.MathArrays;
  
  /** Class representing both the value and the differentials of a function.
   * <p>This class is the workhorse of the differentiation package.</p>
   * <p>This class is an implementation of the extension to Rall's
   * numbers described in Dan Kalman's paper <a
   * href="http://www1.american.edu/cas/mathstat/People/kalman/pdffiles/mmgautodiff.pdf">Doubly
   * Recursive Multivariate Automatic Differentiation</a>, Mathematics Magazine, vol. 75,
   * no. 3, June 2002. Rall's numbers are an extension to the real numbers used
   * throughout mathematical expressions; they hold the derivative together with the
   * value of a function. Dan Kalman's derivative structures hold all partial derivatives
   * up to any specified order, with respect to any number of free parameters. Rall's
   * numbers therefore can be seen as derivative structures for order one derivative and
   * one free parameter, and real numbers can be seen as derivative structures with zero
   * order derivative and no free parameters.</p>
   * <p>{@link DerivativeStructure} instances can be used directly thanks to
   * the arithmetic operators to the mathematical functions provided as
   * methods by this class (+, -, *, /, %, sin, cos ...).</p>
   * <p>Implementing complex expressions by hand using these classes is
   * a tedious and error-prone task but has the advantage of having no limitation
   * on the derivation order despite not requiring users to compute the derivatives by
   * themselves. Implementing complex expression can also be done by developing computation
   * code using standard primitive double values and to use {@link
   * UnivariateFunctionDifferentiator differentiators} to create the {@link
   * DerivativeStructure}-based instances. This method is simpler but may be limited in
   * the accuracy and derivation orders and may be computationally intensive (this is
   * typically the case for {@link FiniteDifferencesDifferentiator finite differences
   * differentiator}.</p>
   * <p>Instances of this class are guaranteed to be immutable.</p>
   * @see DSCompiler
   * @since 3.1
   */
  public class DerivativeStructure implements RealFieldElement<DerivativeStructure> {
      /** Compiler for the current dimensions. */
      private DSCompiler compiler;
  
      /** Combined array holding all values. */
      private final double[] data;
  
      /** Build an instance with all values and derivatives set to 0.
       * @param compiler compiler to use for computation
       */
      private DerivativeStructure(final DSCompiler compiler) {
          this.compiler = compiler;
          this.data     = new double[compiler.getSize()];
      }
  
      /** Build an instance with all values and derivatives set to 0.
       * @param parameters number of free parameters
       * @param order derivation order
       * @throws NumberIsTooLargeException if order is too large.
       */
      public DerivativeStructure(final int parameters, final int order) {
          this(DSCompiler.getCompiler(parameters, order));
      }
  
      /** Build an instance representing a constant value.
       * @param parameters number of free parameters
       * @param order derivation order
       * @param value value of the constant
       * @throws NumberIsTooLargeException if order is too large.
       * @see #DerivativeStructure(int, int, int, double)
       */
      public DerivativeStructure(final int parameters, final int order, final double value) {
          this(parameters, order);
          this.data[0] = value;
      }
  
      /** Build an instance representing a variable.
       * <p>Instances built using this constructor are considered
       * to be the free variables with respect to which differentials
       * are computed. As such, their differential with respect to
      * themselves is +1.</p>
      * @param parameters number of free parameters
      * @param order derivation order
      * @param index index of the variable (from 0 to {@code parameters - 1})
      * @param value value of the variable
      * @throws NumberIsTooLargeException if {@code index &ge; parameters}.
      * @see #DerivativeStructure(int, int, double)
      */
     public DerivativeStructure(final int parameters, final int order,
                                final int index, final double value) {
         this(parameters, order, value);
 
         if (index >= parameters) {
             throw new NumberIsTooLargeException(index, parameters, false);
         }
 
         if (order > 0) {
             // the derivative of the variable with respect to itself is 1.
             data[DSCompiler.getCompiler(index, order).getSize()] = 1.0;
         }
     }
 
     /** Linear combination constructor.
      * The derivative structure built will be a1 * ds1 + a2 * ds2
      * @param a1 first scale factor
      * @param ds1 first base (unscaled) derivative structure
      * @param a2 second scale factor
      * @param ds2 second base (unscaled) derivative structure
      * @throws org.apache.commons.math4.legacy.exception.DimensionMismatchException
      * if number of free parameters or orders are inconsistent
      */
     public DerivativeStructure(final double a1, final DerivativeStructure ds1,
                                final double a2, final DerivativeStructure ds2) {
         this(ds1.compiler);
         compiler.checkCompatibility(ds2.compiler);
         compiler.linearCombination(a1, ds1.data, 0, a2, ds2.data, 0, data, 0);
     }
 
     /** Linear combination constructor.
      * The derivative structure built will be a1 * ds1 + a2 * ds2 + a3 * ds3
      * @param a1 first scale factor
      * @param ds1 first base (unscaled) derivative structure
      * @param a2 second scale factor
      * @param ds2 second base (unscaled) derivative structure
      * @param a3 third scale factor
      * @param ds3 third base (unscaled) derivative structure
      * @throws org.apache.commons.math4.legacy.exception.DimensionMismatchException
      * if number of free parameters or orders are inconsistent.
      */
     public DerivativeStructure(final double a1, final DerivativeStructure ds1,
                                final double a2, final DerivativeStructure ds2,
                                final double a3, final DerivativeStructure ds3) {
         this(ds1.compiler);
         compiler.checkCompatibility(ds2.compiler);
         compiler.checkCompatibility(ds3.compiler);
         compiler.linearCombination(a1, ds1.data, 0, a2, ds2.data, 0, a3, ds3.data, 0, data, 0);
     }
 
     /** Linear combination constructor.
      * The derivative structure built will be a1 * ds1 + a2 * ds2 + a3 * ds3 + a4 * ds4
      * @param a1 first scale factor
      * @param ds1 first base (unscaled) derivative structure
      * @param a2 second scale factor
      * @param ds2 second base (unscaled) derivative structure
      * @param a3 third scale factor
      * @param ds3 third base (unscaled) derivative structure
      * @param a4 fourth scale factor
      * @param ds4 fourth base (unscaled) derivative structure
      * @throws DimensionMismatchException if number of free parameters or orders are inconsistent.
      */
     public DerivativeStructure(final double a1, final DerivativeStructure ds1,
                                final double a2, final DerivativeStructure ds2,
                                final double a3, final DerivativeStructure ds3,
                                final double a4, final DerivativeStructure ds4) {
         this(ds1.compiler);
         compiler.checkCompatibility(ds2.compiler);
         compiler.checkCompatibility(ds3.compiler);
         compiler.checkCompatibility(ds4.compiler);
         compiler.linearCombination(a1, ds1.data, 0, a2, ds2.data, 0,
                                    a3, ds3.data, 0, a4, ds4.data, 0,
                                    data, 0);
     }
 
     /** Build an instance from all its derivatives.
      * @param parameters number of free parameters
      * @param order derivation order
      * @param derivatives derivatives sorted according to
      * {@link DSCompiler#getPartialDerivativeIndex(int...)}
      * @throws DimensionMismatchException if derivatives array does not match the
      * {@link DSCompiler#getSize() size} expected by the compiler.
      * @throws NumberIsTooLargeException if order is too large.
      * @see #getAllDerivatives()
      */
     public DerivativeStructure(final int parameters, final int order, final double ... derivatives) {
         this(parameters, order);
         if (derivatives.length != data.length) {
             throw new DimensionMismatchException(derivatives.length, data.length);
         }
         System.arraycopy(derivatives, 0, data, 0, data.length);
     }
 
     /** Copy constructor.
      * @param ds instance to copy
      */
     private DerivativeStructure(final DerivativeStructure ds) {
         this.compiler = ds.compiler;
         this.data     = ds.data.clone();
     }
 
     /** Get the number of free parameters.
      * @return number of free parameters
      */
     public int getFreeParameters() {
         return compiler.getFreeParameters();
     }
 
     /** Get the derivation order.
      * @return derivation order
      */
     public int getOrder() {
         return compiler.getOrder();
     }
 
     /** Create a constant compatible with instance order and number of parameters.
      * <p>
      * This method is a convenience factory method, it simply calls
      * {@code new DerivativeStructure(getFreeParameters(), getOrder(), c)}
      * </p>
      * @param c value of the constant
      * @return a constant compatible with instance order and number of parameters
      * @see #DerivativeStructure(int, int, double)
      * @since 3.3
      */
     public DerivativeStructure createConstant(final double c) {
         return new DerivativeStructure(getFreeParameters(), getOrder(), c);
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public double getReal() {
         return data[0];
     }
 
     /** Get the value part of the derivative structure.
      * @return value part of the derivative structure
      * @see #getPartialDerivative(int...)
      */
     public double getValue() {
         return data[0];
     }
 
     /** Get a partial derivative.
      * @param orders derivation orders with respect to each variable (if all orders are 0,
      * the value is returned)
      * @return partial derivative
      * @see #getValue()
      * @throws DimensionMismatchException if the numbers of variables does not
      * match the instance.
      * @throws NumberIsTooLargeException if the sum of derivation orders is larger than
      * the instance limits.
      */
     public double getPartialDerivative(final int ... orders) {
         return data[compiler.getPartialDerivativeIndex(orders)];
     }
 
     /** Get all partial derivatives.
      * @return a fresh copy of partial derivatives, in an array sorted according to
      * {@link DSCompiler#getPartialDerivativeIndex(int...)}
      */
     public double[] getAllDerivatives() {
         return data.clone();
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure add(final double a) {
         final DerivativeStructure ds = new DerivativeStructure(this);
         ds.data[0] += a;
         return ds;
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match.
      */
     @Override
     public DerivativeStructure add(final DerivativeStructure a) {
         compiler.checkCompatibility(a.compiler);
         final DerivativeStructure ds = new DerivativeStructure(this);
         compiler.add(data, 0, a.data, 0, ds.data, 0);
         return ds;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure subtract(final double a) {
         return add(-a);
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      */
     @Override
     public DerivativeStructure subtract(final DerivativeStructure a) {
         compiler.checkCompatibility(a.compiler);
         final DerivativeStructure ds = new DerivativeStructure(this);
         compiler.subtract(data, 0, a.data, 0, ds.data, 0);
         return ds;
     }
 
     /** {@inheritDoc} */
     @Override
     public DerivativeStructure multiply(final int n) {
         return multiply((double) n);
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure multiply(final double a) {
         final DerivativeStructure ds = new DerivativeStructure(this);
         for (int i = 0; i < ds.data.length; ++i) {
             ds.data[i] *= a;
         }
         return ds;
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      */
     @Override
     public DerivativeStructure multiply(final DerivativeStructure a) {
         compiler.checkCompatibility(a.compiler);
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.multiply(data, 0, a.data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure divide(final double a) {
         final DerivativeStructure ds = new DerivativeStructure(this);
         for (int i = 0; i < ds.data.length; ++i) {
             ds.data[i] /= a;
         }
         return ds;
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      */
     @Override
     public DerivativeStructure divide(final DerivativeStructure a) {
         compiler.checkCompatibility(a.compiler);
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.divide(data, 0, a.data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc} */
     @Override
     public DerivativeStructure remainder(final double a) {
         final DerivativeStructure ds = new DerivativeStructure(this);
         ds.data[0] = JdkMath.IEEEremainder(ds.data[0], a);
         return ds;
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      * @since 3.2
      */
     @Override
     public DerivativeStructure remainder(final DerivativeStructure a) {
         compiler.checkCompatibility(a.compiler);
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.remainder(data, 0, a.data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc} */
     @Override
     public DerivativeStructure negate() {
         final DerivativeStructure ds = new DerivativeStructure(compiler);
         for (int i = 0; i < ds.data.length; ++i) {
             ds.data[i] = -data[i];
         }
         return ds;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure abs() {
         if (Double.doubleToLongBits(data[0]) < 0) {
             // we use the bits representation to also handle -0.0
             return negate();
         } else {
             return this;
         }
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure ceil() {
         return new DerivativeStructure(compiler.getFreeParameters(),
                                        compiler.getOrder(),
                                        JdkMath.ceil(data[0]));
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure floor() {
         return new DerivativeStructure(compiler.getFreeParameters(),
                                        compiler.getOrder(),
                                        JdkMath.floor(data[0]));
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure rint() {
         return new DerivativeStructure(compiler.getFreeParameters(),
                                        compiler.getOrder(),
                                        JdkMath.rint(data[0]));
     }
 
     /** {@inheritDoc} */
     @Override
     public long round() {
         return JdkMath.round(data[0]);
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure signum() {
         return new DerivativeStructure(compiler.getFreeParameters(),
                                        compiler.getOrder(),
                                        JdkMath.signum(data[0]));
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure copySign(final DerivativeStructure sign){
         long m = Double.doubleToLongBits(data[0]);
         long s = Double.doubleToLongBits(sign.data[0]);
         if ((m >= 0 && s >= 0) || (m < 0 && s < 0)) { // Sign is currently OK
             return this;
         }
         return negate(); // flip sign
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure copySign(final double sign) {
         long m = Double.doubleToLongBits(data[0]);
         long s = Double.doubleToLongBits(sign);
         if ((m >= 0 && s >= 0) || (m < 0 && s < 0)) { // Sign is currently OK
             return this;
         }
         return negate(); // flip sign
     }
 
     /**
      * Return the exponent of the instance value, removing the bias.
      * <p>
      * For double numbers of the form 2<sup>x</sup>, the unbiased
      * exponent is exactly x.
      * </p>
      * @return exponent for instance in IEEE754 representation, without bias
      */
     public int getExponent() {
         return JdkMath.getExponent(data[0]);
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure scalb(final int n) {
         final DerivativeStructure ds = new DerivativeStructure(compiler);
         for (int i = 0; i < ds.data.length; ++i) {
             ds.data[i] = JdkMath.scalb(data[i], n);
         }
         return ds;
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      * @since 3.2
      */
     @Override
     public DerivativeStructure hypot(final DerivativeStructure y) {
         compiler.checkCompatibility(y.compiler);
 
         if (Double.isInfinite(data[0]) || Double.isInfinite(y.data[0])) {
             return new DerivativeStructure(compiler.getFreeParameters(),
                                            compiler.getFreeParameters(),
                                            Double.POSITIVE_INFINITY);
         } else if (Double.isNaN(data[0]) || Double.isNaN(y.data[0])) {
             return new DerivativeStructure(compiler.getFreeParameters(),
                                            compiler.getFreeParameters(),
                                            Double.NaN);
         } else {
 
             final int expX = getExponent();
             final int expY = y.getExponent();
             if (expX > expY + 27) {
                 // y is neglectible with respect to x
                 return abs();
             } else if (expY > expX + 27) {
                 // x is neglectible with respect to y
                 return y.abs();
             } else {
 
                 // find an intermediate scale to avoid both overflow and underflow
                 final int middleExp = (expX + expY) / 2;
 
                 // scale parameters without losing precision
                 final DerivativeStructure scaledX = scalb(-middleExp);
                 final DerivativeStructure scaledY = y.scalb(-middleExp);
 
                 // compute scaled hypotenuse
                 final DerivativeStructure scaledH =
                         scaledX.multiply(scaledX).add(scaledY.multiply(scaledY)).sqrt();
 
                 // remove scaling
                 return scaledH.scalb(middleExp);
             }
         }
     }
 
     /**
      * Returns the hypotenuse of a triangle with sides {@code x} and {@code y}
      * - sqrt(<i>x</i><sup>2</sup>&nbsp;+<i>y</i><sup>2</sup>)
      * avoiding intermediate overflow or underflow.
      *
      * <ul>
      * <li> If either argument is infinite, then the result is positive infinity.</li>
      * <li> else, if either argument is NaN then the result is NaN.</li>
      * </ul>
      *
      * @param x a value
      * @param y a value
      * @return sqrt(<i>x</i><sup>2</sup>&nbsp;+<i>y</i><sup>2</sup>)
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      * @since 3.2
      */
     public static DerivativeStructure hypot(final DerivativeStructure x, final DerivativeStructure y) {
         return x.hypot(y);
     }
 
     /** Compute composition of the instance by a univariate function.
      * @param f array of value and derivatives of the function at
      * the current point (i.e. [f({@link #getValue()}),
      * f'({@link #getValue()}), f''({@link #getValue()})...]).
      * @return f(this)
      * @throws DimensionMismatchException if the number of derivatives
      * in the array is not equal to {@link #getOrder() order} + 1
      */
     public DerivativeStructure compose(final double ... f) {
         if (f.length != getOrder() + 1) {
             throw new DimensionMismatchException(f.length, getOrder() + 1);
         }
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.compose(data, 0, f, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc} */
     @Override
     public DerivativeStructure reciprocal() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.pow(data, 0, -1, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure sqrt() {
         return rootN(2);
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure cbrt() {
         return rootN(3);
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure rootN(final int n) {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.rootN(data, 0, n, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc} */
     @Override
     public Field<DerivativeStructure> getField() {
         return new Field<DerivativeStructure>() {
 
             /** {@inheritDoc} */
             @Override
             public DerivativeStructure getZero() {
                 return new DerivativeStructure(compiler.getFreeParameters(), compiler.getOrder(), 0.0);
             }
 
             /** {@inheritDoc} */
             @Override
             public DerivativeStructure getOne() {
                 return new DerivativeStructure(compiler.getFreeParameters(), compiler.getOrder(), 1.0);
             }
 
             /** {@inheritDoc} */
             @Override
             public Class<? extends FieldElement<DerivativeStructure>> getRuntimeClass() {
                 return DerivativeStructure.class;
             }
         };
     }
 
     /** Compute a<sup>x</sup> where a is a double and x a {@link DerivativeStructure}.
      * @param a number to exponentiate
      * @param x power to apply
      * @return a<sup>x</sup>
      * @since 3.3
      */
     public static DerivativeStructure pow(final double a, final DerivativeStructure x) {
         final DerivativeStructure result = new DerivativeStructure(x.compiler);
         x.compiler.pow(a, x.data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure pow(final double p) {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.pow(data, 0, p, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure pow(final int n) {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.pow(data, 0, n, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      * @since 3.2
      */
     @Override
     public DerivativeStructure pow(final DerivativeStructure e) {
         compiler.checkCompatibility(e.compiler);
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.pow(data, 0, e.data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure exp() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.exp(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure expm1() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.expm1(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure log() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.log(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure log1p() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.log1p(data, 0, result.data, 0);
         return result;
     }
 
     /** Base 10 logarithm.
      * @return base 10 logarithm of the instance
      */
     @Override
     public DerivativeStructure log10() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.log10(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure cos() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.cos(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure sin() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.sin(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure tan() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.tan(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure acos() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.acos(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure asin() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.asin(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure atan() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.atan(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure atan2(final DerivativeStructure x) {
         compiler.checkCompatibility(x.compiler);
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.atan2(data, 0, x.data, 0, result.data, 0);
         return result;
     }
 
     /** Two arguments arc tangent operation.
      * @param y first argument of the arc tangent
      * @param x second argument of the arc tangent
      * @return atan2(y, x)
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      * @since 3.2
      */
     public static DerivativeStructure atan2(final DerivativeStructure y, final DerivativeStructure x) {
         return y.atan2(x);
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure cosh() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.cosh(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure sinh() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.sinh(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure tanh() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.tanh(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure acosh() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.acosh(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure asinh() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.asinh(data, 0, result.data, 0);
         return result;
     }
 
     /** {@inheritDoc}
      * @since 3.2
      */
     @Override
     public DerivativeStructure atanh() {
         final DerivativeStructure result = new DerivativeStructure(compiler);
         compiler.atanh(data, 0, result.data, 0);
         return result;
     }
 
     /** Convert radians to degrees, with error of less than 0.5 ULP.
      *  @return instance converted into degrees
      */
     public DerivativeStructure toDegrees() {
         final DerivativeStructure ds = new DerivativeStructure(compiler);
         for (int i = 0; i < ds.data.length; ++i) {
             ds.data[i] = JdkMath.toDegrees(data[i]);
         }
         return ds;
     }
 
     /** Convert degrees to radians, with error of less than 0.5 ULP.
      *  @return instance converted into radians
      */
     public DerivativeStructure toRadians() {
         final DerivativeStructure ds = new DerivativeStructure(compiler);
         for (int i = 0; i < ds.data.length; ++i) {
             ds.data[i] = JdkMath.toRadians(data[i]);
         }
         return ds;
     }
 
     /** Evaluate Taylor expansion a derivative structure.
      * @param delta parameters offsets (&Delta;x, &Delta;y, ...)
      * @return value of the Taylor expansion at x + &Delta;x, y + &Delta;y, ...
      * @throws org.apache.commons.math4.legacy.exception.MathArithmeticException
      * if factorials becomes too large
      */
     public double taylor(final double ... delta) {
         return compiler.taylor(data, 0, delta);
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      * @since 3.2
      */
     @Override
     public DerivativeStructure linearCombination(final DerivativeStructure[] a, final DerivativeStructure[] b) {
         // compute an accurate value, taking care of cancellations
         final double[] aDouble = new double[a.length];
         for (int i = 0; i < a.length; ++i) {
             aDouble[i] = a[i].getValue();
         }
         final double[] bDouble = new double[b.length];
         for (int i = 0; i < b.length; ++i) {
             bDouble[i] = b[i].getValue();
         }
         final double accurateValue = Sum.ofProducts(aDouble, bDouble).getAsDouble();
 
         // compute a simple value, with all partial derivatives
         DerivativeStructure simpleValue = a[0].getField().getZero();
         for (int i = 0; i < a.length; ++i) {
             simpleValue = simpleValue.add(a[i].multiply(b[i]));
         }
 
         // create a result with accurate value and all derivatives (not necessarily as accurate as the value)
         final double[] all = simpleValue.getAllDerivatives();
         all[0] = accurateValue;
         return new DerivativeStructure(simpleValue.getFreeParameters(), simpleValue.getOrder(), all);
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      * @since 3.2
      */
     @Override
     public DerivativeStructure linearCombination(final double[] a, final DerivativeStructure[] b) {
         // compute an accurate value, taking care of cancellations
         final double[] bDouble = new double[b.length];
         for (int i = 0; i < b.length; ++i) {
             bDouble[i] = b[i].getValue();
         }
         final double accurateValue = Sum.ofProducts(a, bDouble).getAsDouble();
 
         // compute a simple value, with all partial derivatives
         DerivativeStructure simpleValue = b[0].getField().getZero();
         for (int i = 0; i < a.length; ++i) {
             simpleValue = simpleValue.add(b[i].multiply(a[i]));
         }
 
         // create a result with accurate value and all derivatives (not necessarily as accurate as the value)
         final double[] all = simpleValue.getAllDerivatives();
         all[0] = accurateValue;
         return new DerivativeStructure(simpleValue.getFreeParameters(), simpleValue.getOrder(), all);
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      * @since 3.2
      */
     @Override
     public DerivativeStructure linearCombination(final DerivativeStructure a1, final DerivativeStructure b1,
                                                  final DerivativeStructure a2, final DerivativeStructure b2) {
         // compute an accurate value, taking care of cancellations
         final double accurateValue = Sum.create()
             .addProduct(a1.getValue(), b1.getValue())
             .addProduct(a2.getValue(), b2.getValue()).getAsDouble();
 
         // compute a simple value, with all partial derivatives
         final DerivativeStructure simpleValue = a1.multiply(b1).add(a2.multiply(b2));
 
         // create a result with accurate value and all derivatives (not necessarily as accurate as the value)
         final double[] all = simpleValue.getAllDerivatives();
         all[0] = accurateValue;
         return new DerivativeStructure(getFreeParameters(), getOrder(), all);
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      * @since 3.2
      */
     @Override
     public DerivativeStructure linearCombination(final double a1, final DerivativeStructure b1,
                                                  final double a2, final DerivativeStructure b2) {
         // compute an accurate value, taking care of cancellations
         final double accurateValue = Sum.create()
             .addProduct(a1, b1.getValue())
             .addProduct(a2, b2.getValue()).getAsDouble();
 
         // compute a simple value, with all partial derivatives
         final DerivativeStructure simpleValue = b1.multiply(a1).add(b2.multiply(a2));
 
         // create a result with accurate value and all derivatives (not necessarily as accurate as the value)
         final double[] all = simpleValue.getAllDerivatives();
         all[0] = accurateValue;
         return new DerivativeStructure(getFreeParameters(), getOrder(), all);
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      * @since 3.2
      */
     @Override
     public DerivativeStructure linearCombination(final DerivativeStructure a1, final DerivativeStructure b1,
                                                  final DerivativeStructure a2, final DerivativeStructure b2,
                                                  final DerivativeStructure a3, final DerivativeStructure b3) {
         // compute an accurate value, taking care of cancellations
         final double accurateValue = Sum.create()
             .addProduct(a1.getValue(), b1.getValue())
             .addProduct(a2.getValue(), b2.getValue())
             .addProduct(a3.getValue(), b3.getValue()).getAsDouble();
 
         // compute a simple value, with all partial derivatives
         final DerivativeStructure simpleValue = a1.multiply(b1).add(a2.multiply(b2)).add(a3.multiply(b3));
 
         // create a result with accurate value and all derivatives (not necessarily as accurate as the value)
         final double[] all = simpleValue.getAllDerivatives();
         all[0] = accurateValue;
         return new DerivativeStructure(getFreeParameters(), getOrder(), all);
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      * @since 3.2
      */
     @Override
     public DerivativeStructure linearCombination(final double a1, final DerivativeStructure b1,
                                                  final double a2, final DerivativeStructure b2,
                                                  final double a3, final DerivativeStructure b3) {
         // compute an accurate value, taking care of cancellations
         final double accurateValue = Sum.create()
             .addProduct(a1, b1.getValue())
             .addProduct(a2, b2.getValue())
             .addProduct(a3, b3.getValue()).getAsDouble();
 
         // compute a simple value, with all partial derivatives
         final DerivativeStructure simpleValue = b1.multiply(a1).add(b2.multiply(a2)).add(b3.multiply(a3));
 
         // create a result with accurate value and all derivatives (not necessarily as accurate as the value)
         final double[] all = simpleValue.getAllDerivatives();
         all[0] = accurateValue;
         return new DerivativeStructure(getFreeParameters(), getOrder(), all);
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      * @since 3.2
      */
     @Override
     public DerivativeStructure linearCombination(final DerivativeStructure a1, final DerivativeStructure b1,
                                                  final DerivativeStructure a2, final DerivativeStructure b2,
                                                  final DerivativeStructure a3, final DerivativeStructure b3,
                                                  final DerivativeStructure a4, final DerivativeStructure b4) {
         // compute an accurate value, taking care of cancellations
         final double accurateValue = Sum.create()
             .addProduct(a1.getValue(), b1.getValue())
             .addProduct(a2.getValue(), b2.getValue())
             .addProduct(a3.getValue(), b3.getValue())
             .addProduct(a4.getValue(), b4.getValue()).getAsDouble();
 
         // compute a simple value, with all partial derivatives
         final DerivativeStructure simpleValue = a1.multiply(b1).add(a2.multiply(b2)).add(a3.multiply(b3)).add(a4.multiply(b4));
 
         // create a result with accurate value and all derivatives (not necessarily as accurate as the value)
         final double[] all = simpleValue.getAllDerivatives();
         all[0] = accurateValue;
         return new DerivativeStructure(getFreeParameters(), getOrder(), all);
     }
 
     /** {@inheritDoc}
      * @throws DimensionMismatchException if number of free parameters
      * or orders do not match
      * @since 3.2
      */
     @Override
     public DerivativeStructure linearCombination(final double a1, final DerivativeStructure b1,
                                                  final double a2, final DerivativeStructure b2,
                                                  final double a3, final DerivativeStructure b3,
                                                  final double a4, final DerivativeStructure b4) {
         // compute an accurate value, taking care of cancellations
         final double accurateValue = Sum.create()
             .addProduct(a1, b1.getValue())
             .addProduct(a2, b2.getValue())
             .addProduct(a3, b3.getValue())
             .addProduct(a4, b4.getValue()).getAsDouble();
 
         // compute a simple value, with all partial derivatives
         final DerivativeStructure simpleValue = b1.multiply(a1).add(b2.multiply(a2)).add(b3.multiply(a3)).add(b4.multiply(a4));
 
         // create a result with accurate value and all derivatives (not necessarily as accurate as the value)
         final double[] all = simpleValue.getAllDerivatives();
         all[0] = accurateValue;
         return new DerivativeStructure(getFreeParameters(), getOrder(), all);
     }
 
     /**
      * Test for the equality of two derivative structures.
      * <p>
      * Derivative structures are considered equal if they have the same number
      * of free parameters, the same derivation order, and the same derivatives.
      * </p>
      * @param other Object to test for equality to this
      * @return true if two derivative structures are equal
      * @since 3.2
      */
     @Override
     public boolean equals(Object other) {
 
         if (this == other) {
             return true;
         }
 
         if (other instanceof DerivativeStructure) {
             final DerivativeStructure rhs = (DerivativeStructure)other;
             return getFreeParameters() == rhs.getFreeParameters() &&
                    getOrder() == rhs.getOrder() &&
                    MathArrays.equals(data, rhs.data);
         }
 
         return false;
     }
 
     /**
      * Get a hashCode for the derivative structure.
      * @return a hash code value for this object
      * @since 3.2
      */
     @Override
     public int hashCode() {
         return 227 + 229 * getFreeParameters() + 233 * getOrder() + 239 * Arrays.hashCode(data);
     }
 }