/*
    * 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.solvers;
  
  import org.apache.commons.math4.legacy.analysis.UnivariateFunction;
  import org.apache.commons.math4.legacy.exception.NoBracketingException;
  import org.apache.commons.math4.legacy.exception.NotStrictlyPositiveException;
  import org.apache.commons.math4.legacy.exception.NullArgumentException;
  import org.apache.commons.math4.legacy.exception.NumberIsTooLargeException;
  import org.apache.commons.math4.legacy.exception.util.LocalizedFormats;
  import org.apache.commons.math4.core.jdkmath.JdkMath;
  
  /**
   * Utility routines for {@link UnivariateSolver} objects.
   *
   */
  public final class UnivariateSolverUtils {
      /**
       * Class contains only static methods.
       */
      private UnivariateSolverUtils() {}
  
      /**
       * Convenience method to find a zero of a univariate real function.  A default
       * solver is used.
       *
       * @param function Function.
       * @param x0 Lower bound for the interval.
       * @param x1 Upper bound for the interval.
       * @return a value where the function is zero.
       * @throws NoBracketingException if the function has the same sign at the
       * endpoints.
       * @throws NullArgumentException if {@code function} is {@code null}.
       */
      public static double solve(UnivariateFunction function, double x0, double x1)
          throws NullArgumentException,
                 NoBracketingException {
          if (function == null) {
              throw new NullArgumentException(LocalizedFormats.FUNCTION);
          }
          final UnivariateSolver solver = new BrentSolver();
          return solver.solve(Integer.MAX_VALUE, function, x0, x1);
      }
  
      /**
       * Convenience method to find a zero of a univariate real function.  A default
       * solver is used.
       *
       * @param function Function.
       * @param x0 Lower bound for the interval.
       * @param x1 Upper bound for the interval.
       * @param absoluteAccuracy Accuracy to be used by the solver.
       * @return a value where the function is zero.
       * @throws NoBracketingException if the function has the same sign at the
       * endpoints.
       * @throws NullArgumentException if {@code function} is {@code null}.
       */
      public static double solve(UnivariateFunction function,
                                 double x0, double x1,
                                 double absoluteAccuracy)
          throws NullArgumentException,
                 NoBracketingException {
          if (function == null) {
              throw new NullArgumentException(LocalizedFormats.FUNCTION);
          }
          final UnivariateSolver solver = new BrentSolver(absoluteAccuracy);
          return solver.solve(Integer.MAX_VALUE, function, x0, x1);
      }
  
      /**
       * Force a root found by a non-bracketing solver to lie on a specified side,
       * as if the solver were a bracketing one.
       *
       * @param maxEval maximal number of new evaluations of the function
       * (evaluations already done for finding the root should have already been subtracted
       * from this number)
       * @param f function to solve
       * @param bracketing bracketing solver to use for shifting the root
       * @param baseRoot original root found by a previous non-bracketing solver
       * @param min minimal bound of the search interval
       * @param max maximal bound of the search interval
       * @param allowedSolution the kind of solutions that the root-finding algorithm may
       * accept as solutions.
       * @return a root approximation, on the specified side of the exact root
       * @throws NoBracketingException if the function has the same sign at the
      * endpoints.
      */
     public static double forceSide(final int maxEval, final UnivariateFunction f,
                                    final BracketedUnivariateSolver<UnivariateFunction> bracketing,
                                    final double baseRoot, final double min, final double max,
                                    final AllowedSolution allowedSolution)
         throws NoBracketingException {
 
         if (allowedSolution == AllowedSolution.ANY_SIDE) {
             // no further bracketing required
             return baseRoot;
         }
 
         // find a very small interval bracketing the root
         final double step = JdkMath.max(bracketing.getAbsoluteAccuracy(),
                                          JdkMath.abs(baseRoot * bracketing.getRelativeAccuracy()));
         double xLo        = JdkMath.max(min, baseRoot - step);
         double fLo        = f.value(xLo);
         double xHi        = JdkMath.min(max, baseRoot + step);
         double fHi        = f.value(xHi);
         int remainingEval = maxEval - 2;
         while (remainingEval > 0) {
 
             if ((fLo >= 0 && fHi <= 0) || (fLo <= 0 && fHi >= 0)) {
                 // compute the root on the selected side
                 return bracketing.solve(remainingEval, f, xLo, xHi, baseRoot, allowedSolution);
             }
 
             // try increasing the interval
             boolean changeLo = false;
             boolean changeHi = false;
             if (fLo < fHi) {
                 // increasing function
                 if (fLo >= 0) {
                     changeLo = true;
                 } else {
                     changeHi = true;
                 }
             } else if (fLo > fHi) {
                 // decreasing function
                 if (fLo <= 0) {
                     changeLo = true;
                 } else {
                     changeHi = true;
                 }
             } else {
                 // unknown variation
                 changeLo = true;
                 changeHi = true;
             }
 
             // update the lower bound
             if (changeLo) {
                 xLo = JdkMath.max(min, xLo - step);
                 fLo  = f.value(xLo);
                 remainingEval--;
             }
 
             // update the higher bound
             if (changeHi) {
                 xHi = JdkMath.min(max, xHi + step);
                 fHi  = f.value(xHi);
                 remainingEval--;
             }
         }
 
         throw new NoBracketingException(LocalizedFormats.FAILED_BRACKETING,
                                         xLo, xHi, fLo, fHi,
                                         maxEval - remainingEval, maxEval, baseRoot,
                                         min, max);
     }
 
     /**
      * This method simply calls {@link #bracket(UnivariateFunction, double, double, double,
      * double, double, int) bracket(function, initial, lowerBound, upperBound, q, r, maximumIterations)}
      * with {@code q} and {@code r} set to 1.0 and {@code maximumIterations} set to {@code Integer.MAX_VALUE}.
      * <p>
      * <strong>Note: </strong> this method can take {@code Integer.MAX_VALUE}
      * iterations to throw a {@code ConvergenceException.}  Unless you are
      * confident that there is a root between {@code lowerBound} and
      * {@code upperBound} near {@code initial}, it is better to use
      * {@link #bracket(UnivariateFunction, double, double, double, double,double, int)
      * bracket(function, initial, lowerBound, upperBound, q, r, maximumIterations)},
      * explicitly specifying the maximum number of iterations.</p>
      *
      * @param function Function.
      * @param initial Initial midpoint of interval being expanded to
      * bracket a root.
      * @param lowerBound Lower bound (a is never lower than this value)
      * @param upperBound Upper bound (b never is greater than this
      * value).
      * @return a two-element array holding a and b.
      * @throws NoBracketingException if a root cannot be bracketted.
      * @throws NotStrictlyPositiveException if {@code maximumIterations <= 0}.
      * @throws NullArgumentException if {@code function} is {@code null}.
      */
     public static double[] bracket(UnivariateFunction function,
                                    double initial,
                                    double lowerBound, double upperBound)
         throws NullArgumentException,
                NotStrictlyPositiveException,
                NoBracketingException {
         return bracket(function, initial, lowerBound, upperBound, 1.0, 1.0, Integer.MAX_VALUE);
     }
 
      /**
      * This method simply calls {@link #bracket(UnivariateFunction, double, double, double,
      * double, double, int) bracket(function, initial, lowerBound, upperBound, q, r, maximumIterations)}
      * with {@code q} and {@code r} set to 1.0.
      * @param function Function.
      * @param initial Initial midpoint of interval being expanded to
      * bracket a root.
      * @param lowerBound Lower bound (a is never lower than this value).
      * @param upperBound Upper bound (b never is greater than this
      * value).
      * @param maximumIterations Maximum number of iterations to perform
      * @return a two element array holding a and b.
      * @throws NoBracketingException if the algorithm fails to find a and b
      * satisfying the desired conditions.
      * @throws NotStrictlyPositiveException if {@code maximumIterations <= 0}.
      * @throws NullArgumentException if {@code function} is {@code null}.
      */
     public static double[] bracket(UnivariateFunction function,
                                    double initial,
                                    double lowerBound, double upperBound,
                                    int maximumIterations)
         throws NullArgumentException,
                NotStrictlyPositiveException,
                NoBracketingException {
         return bracket(function, initial, lowerBound, upperBound, 1.0, 1.0, maximumIterations);
     }
 
     /**
      * This method attempts to find two values a and b satisfying <ul>
      * <li> {@code lowerBound <= a < initial < b <= upperBound} </li>
      * <li> {@code f(a) * f(b) <= 0} </li>
      * </ul>
      * If {@code f} is continuous on {@code [a,b]}, this means that {@code a}
      * and {@code b} bracket a root of {@code f}.
      * <p>
      * The algorithm checks the sign of \( f(l_k) \) and \( f(u_k) \) for increasing
      * values of k, where \( l_k = max(lower, initial - \delta_k) \),
      * \( u_k = min(upper, initial + \delta_k) \), using recurrence
      * \( \delta_{k+1} = r \delta_k + q, \delta_0 = 0\) and starting search with \( k=1 \).
      * The algorithm stops when one of the following happens: <ul>
      * <li> at least one positive and one negative value have been found --  success!</li>
      * <li> both endpoints have reached their respective limits -- NoBracketingException </li>
      * <li> {@code maximumIterations} iterations elapse -- NoBracketingException </li></ul>
      * <p>
      * If different signs are found at first iteration ({@code k=1}), then the returned
      * interval will be \( [a, b] = [l_1, u_1] \). If different signs are found at a later
      * iteration {@code k>1}, then the returned interval will be either
      * \( [a, b] = [l_{k+1}, l_{k}] \) or \( [a, b] = [u_{k}, u_{k+1}] \). A root solver called
      * with these parameters will therefore start with the smallest bracketing interval known
      * at this step.
      * </p>
      * <p>
      * Interval expansion rate is tuned by changing the recurrence parameters {@code r} and
      * {@code q}. When the multiplicative factor {@code r} is set to 1, the sequence is a
      * simple arithmetic sequence with linear increase. When the multiplicative factor {@code r}
      * is larger than 1, the sequence has an asymptotically exponential rate. Note than the
      * additive parameter {@code q} should never be set to zero, otherwise the interval would
      * degenerate to the single initial point for all values of {@code k}.
      * </p>
      * <p>
      * As a rule of thumb, when the location of the root is expected to be approximately known
      * within some error margin, {@code r} should be set to 1 and {@code q} should be set to the
      * order of magnitude of the error margin. When the location of the root is really a wild guess,
      * then {@code r} should be set to a value larger than 1 (typically 2 to double the interval
      * length at each iteration) and {@code q} should be set according to half the initial
      * search interval length.
      * </p>
      * <p>
      * As an example, if we consider the trivial function {@code f(x) = 1 - x} and use
      * {@code initial = 4}, {@code r = 1}, {@code q = 2}, the algorithm will compute
      * {@code f(4-2) = f(2) = -1} and {@code f(4+2) = f(6) = -5} for {@code k = 1}, then
      * {@code f(4-4) = f(0) = +1} and {@code f(4+4) = f(8) = -7} for {@code k = 2}. Then it will
      * return the interval {@code [0, 2]} as the smallest one known to be bracketing the root.
      * As shown by this example, the initial value (here {@code 4}) may lie outside of the returned
      * bracketing interval.
      * </p>
      * @param function function to check
      * @param initial Initial midpoint of interval being expanded to
      * bracket a root.
      * @param lowerBound Lower bound (a is never lower than this value).
      * @param upperBound Upper bound (b never is greater than this
      * value).
      * @param q additive offset used to compute bounds sequence (must be strictly positive)
      * @param r multiplicative factor used to compute bounds sequence
      * @param maximumIterations Maximum number of iterations to perform
      * @return a two element array holding the bracketing values.
      * @exception NoBracketingException if function cannot be bracketed in the search interval
      */
     public static double[] bracket(final UnivariateFunction function, final double initial,
                                    final double lowerBound, final double upperBound,
                                    final double q, final double r, final int maximumIterations)
         throws NoBracketingException {
 
         if (function == null) {
             throw new NullArgumentException(LocalizedFormats.FUNCTION);
         }
         if (q <= 0)  {
             throw new NotStrictlyPositiveException(q);
         }
         if (maximumIterations <= 0)  {
             throw new NotStrictlyPositiveException(LocalizedFormats.INVALID_MAX_ITERATIONS, maximumIterations);
         }
         verifySequence(lowerBound, initial, upperBound);
 
         // initialize the recurrence
         double a     = initial;
         double b     = initial;
         double fa    = Double.NaN;
         double fb    = Double.NaN;
         double delta = 0;
 
         for (int numIterations = 0;
              numIterations < maximumIterations && (a > lowerBound || b < upperBound);
              ++numIterations) {
 
             final double previousA  = a;
             final double previousFa = fa;
             final double previousB  = b;
             final double previousFb = fb;
 
             delta = r * delta + q;
             a     = JdkMath.max(initial - delta, lowerBound);
             b     = JdkMath.min(initial + delta, upperBound);
             fa    = function.value(a);
             fb    = function.value(b);
 
             if (numIterations == 0) {
                 // at first iteration, we don't have a previous interval
                 // we simply compare both sides of the initial interval
                 if (fa * fb <= 0) {
                     // the first interval already brackets a root
                     return new double[] { a, b };
                 }
             } else {
                 // we have a previous interval with constant sign and expand it,
                 // we expect sign changes to occur at boundaries
                 if (fa * previousFa <= 0) {
                     // sign change detected at near lower bound
                     return new double[] { a, previousA };
                 } else if (fb * previousFb <= 0) {
                     // sign change detected at near upper bound
                     return new double[] { previousB, b };
                 }
             }
         }
 
         // no bracketing found
         throw new NoBracketingException(a, b, fa, fb);
     }
 
     /**
      * Compute the midpoint of two values.
      *
      * @param a first value.
      * @param b second value.
      * @return the midpoint.
      */
     public static double midpoint(double a, double b) {
         return (a + b) * 0.5;
     }
 
     /**
      * Check whether the interval bounds bracket a root. That is, if the
      * values at the endpoints are not equal to zero, then the function takes
      * opposite signs at the endpoints.
      *
      * @param function Function.
      * @param lower Lower endpoint.
      * @param upper Upper endpoint.
      * @return {@code true} if the function values have opposite signs at the
      * given points.
      * @throws NullArgumentException if {@code function} is {@code null}.
      */
     public static boolean isBracketing(UnivariateFunction function,
                                        final double lower,
                                        final double upper)
         throws NullArgumentException {
         if (function == null) {
             throw new NullArgumentException(LocalizedFormats.FUNCTION);
         }
         final double fLo = function.value(lower);
         final double fHi = function.value(upper);
         return (fLo >= 0 && fHi <= 0) || (fLo <= 0 && fHi >= 0);
     }
 
     /**
      * Check whether the arguments form a (strictly) increasing sequence.
      *
      * @param start First number.
      * @param mid Second number.
      * @param end Third number.
      * @return {@code true} if the arguments form an increasing sequence.
      */
     public static boolean isSequence(final double start,
                                      final double mid,
                                      final double end) {
         return start < mid && mid < end;
     }
 
     /**
      * Check that the endpoints specify an interval.
      *
      * @param lower Lower endpoint.
      * @param upper Upper endpoint.
      * @throws NumberIsTooLargeException if {@code lower >= upper}.
      */
     public static void verifyInterval(final double lower,
                                       final double upper)
         throws NumberIsTooLargeException {
         if (lower >= upper) {
             throw new NumberIsTooLargeException(LocalizedFormats.ENDPOINTS_NOT_AN_INTERVAL,
                                                 lower, upper, false);
         }
     }
 
     /**
      * Check that {@code lower < initial < upper}.
      *
      * @param lower Lower endpoint.
      * @param initial Initial value.
      * @param upper Upper endpoint.
      * @throws NumberIsTooLargeException if {@code lower >= initial} or
      * {@code initial >= upper}.
      */
     public static void verifySequence(final double lower,
                                       final double initial,
                                       final double upper)
         throws NumberIsTooLargeException {
         verifyInterval(lower, initial);
         verifyInterval(initial, upper);
     }
 
     /**
      * Check that the endpoints specify an interval and the end points
      * bracket a root.
      *
      * @param function Function.
      * @param lower Lower endpoint.
      * @param upper Upper endpoint.
      * @throws NoBracketingException if the function has the same sign at the
      * endpoints.
      * @throws NullArgumentException if {@code function} is {@code null}.
      */
     public static void verifyBracketing(UnivariateFunction function,
                                         final double lower,
                                         final double upper)
         throws NullArgumentException,
                NoBracketingException {
         if (function == null) {
             throw new NullArgumentException(LocalizedFormats.FUNCTION);
         }
         verifyInterval(lower, upper);
         if (!isBracketing(function, lower, upper)) {
             throw new NoBracketingException(lower, upper,
                                             function.value(lower),
                                             function.value(upper));
         }
     }
 }