/*
 * 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.numbers.fraction;

import java.util.function.Supplier;
import org.apache.commons.numbers.fraction.GeneralizedContinuedFraction.Coefficient;

/**
 * Provides a generic means to evaluate
 * <a href="https://mathworld.wolfram.com/ContinuedFraction.html">continued fractions</a>.
 *
 * <p>The continued fraction uses the following form for the numerator ({@code a}) and
 * denominator ({@code b}) coefficients:
 * <pre>
 *              a1
 * b0 + ------------------
 *      b1 +      a2
 *           -------------
 *           b2 +    a3
 *                --------
 *                b3 + ...
 * </pre>
 *
 * <p>Subclasses must provide the {@link #getA(int,double) a} and {@link #getB(int,double) b}
 * coefficients to evaluate the continued fraction.
 *
 * <p>This class allows evaluation of the fraction for a specified evaluation point {@code x};
 * the point can be used to express the values of the coefficients.
 * Evaluation of a continued fraction from a generator of the coefficients can be performed using
 * {@link GeneralizedContinuedFraction}. This may be preferred if the coefficients can be computed
 * with updates to the previous coefficients.
 */
public abstract class ContinuedFraction {
    /**
     * Defines the <a href="https://mathworld.wolfram.com/ContinuedFraction.html">
     * {@code n}-th "a" coefficient</a> of the continued fraction.
     *
     * @param n Index of the coefficient to retrieve.
     * @param x Evaluation point.
     * @return the coefficient <code>a<sub>n</sub></code>.
     */
    protected abstract double getA(int n, double x);

    /**
     * Defines the <a href="https://mathworld.wolfram.com/ContinuedFraction.html">
     * {@code n}-th "b" coefficient</a> of the continued fraction.
     *
     * @param n Index of the coefficient to retrieve.
     * @param x Evaluation point.
     * @return the coefficient <code>b<sub>n</sub></code>.
     */
    protected abstract double getB(int n, double x);

    /**
     * Evaluates the continued fraction.
     *
     * @param x the evaluation point.
     * @param epsilon Maximum relative error allowed.
     * @return the value of the continued fraction evaluated at {@code x}.
     * @throws ArithmeticException if the algorithm fails to converge.
     * @throws ArithmeticException if the maximal number of iterations is reached
     * before the expected convergence is achieved.
     *
     * @see #evaluate(double,double,int)
     */
    public double evaluate(double x, double epsilon) {
        return evaluate(x, epsilon, GeneralizedContinuedFraction.DEFAULT_ITERATIONS);
    }

    /**
     * Evaluates the continued fraction.
     * <p>
     * The implementation of this method is based on the modified Lentz algorithm as described
     * on page 508 in:
     * </p>
     *
     * <ul>
     *   <li>
     *   I. J. Thompson,  A. R. Barnett (1986).
     *   "Coulomb and Bessel Functions of Complex Arguments and Order."
     *   Journal of Computational Physics 64, 490-509.
     *   <a target="_blank" href="https://www.fresco.org.uk/papers/Thompson-JCP64p490.pdf">
     *   https://www.fresco.org.uk/papers/Thompson-JCP64p490.pdf</a>
     *   </li>
     * </ul>
     *
     * @param x Point at which to evaluate the continued fraction.
     * @param epsilon Maximum relative error allowed.
     * @param maxIterations Maximum number of iterations.
     * @return the value of the continued fraction evaluated at {@code x}.
     * @throws ArithmeticException if the algorithm fails to converge.
     * @throws ArithmeticException if the maximal number of iterations is reached
     * before the expected convergence is achieved.
     */
    public double evaluate(double x, double epsilon, int maxIterations) {
        // Delegate to GeneralizedContinuedFraction

        // Get the first coefficient
        final double b0 = getB(0, x);

        // Generate coefficients from (a1,b1)
        final Supplier<Coefficient> gen = new Supplier<Coefficient>() {
            private int n;
            @Override
            public Coefficient get() {
                n++;
                final double a = getA(n, x);
                final double b = getB(n, x);
                return Coefficient.of(a, b);
            }
        };

        // Invoke appropriate method based on magnitude of first term.

        // If b0 is too small or zero it is set to a non-zero small number to allow
        // magnitude updates. Avoid this by adding b0 at the end if b0 is small.
        //
        // This handles the use case of a negligible initial term. If b1 is also small
        // then the evaluation starting at b0 or b1 may converge poorly.
        // One solution is to manually compute the convergent until it is not small
        // and then evaluate the fraction from the next term:
        // h1 = b0 + a1 / b1
        // h2 = b0 + a1 / (b1 + a2 / b2)
        // ...
        // hn not 'small', start generator at (n+1):
        // value = GeneralizedContinuedFraction.value(hn, gen)
        // This solution is not implemented to avoid recursive complexity.

        if (Math.abs(b0) < GeneralizedContinuedFraction.SMALL) {
            // Updates from initial convergent b1 and computes:
            // b0 + a1 / [  b1 + a2 / (b2 + ... ) ]
            return GeneralizedContinuedFraction.value(b0, gen, epsilon, maxIterations);
        }

        // Use the package-private evaluate method.
        // Calling GeneralizedContinuedFraction.value(gen, epsilon, maxIterations)
        // requires the generator to start from (a0,b0) and repeats computation of b0
        // and wastes computation of a0.

        // Updates from initial convergent b0:
        // b0 + a1 / (b1 + ... )
        return GeneralizedContinuedFraction.evaluate(b0, gen, epsilon, maxIterations);
    }
}
