/*
    * 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.Locale;
  import org.junit.jupiter.api.Assertions;
  import org.junit.jupiter.api.Test;
  import org.junit.jupiter.params.ParameterizedTest;
  import org.junit.jupiter.params.provider.ValueSource;
  
  /**
   * Tests for {@link ContinuedFraction}.
   */
  class ContinuedFractionTest {
      /** Golden ratio constant. */
      private static final double GOLDEN_RATIO = 1.618033988749894848204586834365638117720309;
  
      /**
       * Evaluates the golden ratio.
       *
       * <pre>
       * 1 +        1
       *     ----------------
       *     1 +      1
       *         ------------
       *         1 +     1
       *              -------
       *              1 + ...
       * </pre>
       *
       * <p>This is used to test various conditions for the {@link ContinuedFraction}.
       *
       * @see <a href="https://mathworld.wolfram.com/GoldenRatio.html">MathWorld Golden
       * Ratio equation 17</a>
       */
      private static final class GoldenRatio extends ContinuedFraction {
          private static final GoldenRatio INSTANCE = new GoldenRatio();
  
          /**
           * @return single instance of GoldenRatio
           */
          static GoldenRatio getInstance() {
              return INSTANCE;
          }
  
          /** {@inheritDoc} */
          @Override
          public double getA(int n, double x) {
              return 1;
          }
  
          /** {@inheritDoc} */
          @Override
          public double getB(int n, double x) {
              return 1;
          }
      }
  
      @Test
      void testGoldenRatio() {
          final double eps = 1e-8;
          final double gr = GoldenRatio.getInstance().evaluate(0, eps);
          Assertions.assertEquals(GOLDEN_RATIO, gr, GOLDEN_RATIO * eps);
      }
  
      /**
       * Test a continued fraction where the leading term is zero.
       * Evaluates the reciprocal of the golden ratio.
       */
      @Test
      void testGoldenRatioReciprocal() {
          final double eps = 1e-8;
          final ContinuedFraction cf = new ContinuedFraction() {
              @Override
              public double getA(int n, double x) {
                  // Check this is not called with n=0
                  Assertions.assertNotEquals(0, n, "a0 should never require evaluation");
                  return 1;
              }
  
              @Override
              public double getB(int n, double x) {
                  // b0 = 0
                  return n == 0 ? 0 : 1;
              }
         };
         final double gr = cf.evaluate(0, eps);
         Assertions.assertEquals(1 / GOLDEN_RATIO, gr, eps / GOLDEN_RATIO);
     }
 
     /**
      * Test an invalid epsilon (zero, negative or NaN).
      * See NUMBERS-173.
      *
      * @param epsilon Epsilon
      */
     @ParameterizedTest
     @ValueSource(doubles = {0, -1, Double.NaN})
     void testGoldenRatioEpsilonZero(double epsilon) {
         // An invalid epsilon is set to the minimum epsilon.
         // It should converge to 1 ULP.
         final double tolerance = Math.ulp(GOLDEN_RATIO);
         Assertions.assertEquals(GOLDEN_RATIO, GoldenRatio.getInstance().evaluate(0, epsilon), tolerance);
     }
 
     @Test
     void test415Over93() {
         // https://en.wikipedia.org/wiki/Continued_fraction
         // 415             1
         // ---  = 4 + ---------
         //  93        2 +   1
         //                -----
         //                6 + 1
         //                    -
         //                    7
         //      = [4; 2, 6, 7]
 
         final ContinuedFraction cf = new ContinuedFraction() {
             @Override
             public double getA(int n, double x) {
                 return n <= 3 ? 1 : 0;
             }
 
             @Override
             public double getB(int n, double x) {
                 switch (n) {
                 case 0:
                     return 4;
                 case 1:
                     return 2;
                 case 2:
                     return 6;
                 case 3:
                     return 7;
                 default:
                     return 1;
                 }
             }
         };
 
         final double eps = 1e-8;
         final double gr = cf.evaluate(0, eps, 5);
         Assertions.assertEquals(415.0 / 93.0, gr, eps);
     }
 
     @Test
     void testMaxIterationsThrows() {
         final ContinuedFraction cf = GoldenRatio.getInstance();
 
         final double eps = 1e-8;
         final int maxIterations = 3;
         final Throwable t = Assertions.assertThrows(FractionException.class, () -> cf.evaluate(0, eps, maxIterations));
         assertExceptionMessageContains(t, "max");
     }
 
     @Test
     void testNaNThrows() {
         // Create a NaN during the iteration
         final ContinuedFraction cf = new ContinuedFraction() {
             @Override
             public double getA(int n, double x) {
                 return 1;
             }
 
             @Override
             public double getB(int n, double x) {
                 return n == 0 ? 1 : Double.NaN;
             }
         };
 
         final double eps = 1e-8;
         final Throwable t = Assertions.assertThrows(FractionException.class, () -> cf.evaluate(0, eps, 5));
         assertExceptionMessageContains(t, "nan");
     }
 
     @Test
     void testInfThrows() {
         // Create an infinity during the iteration:
         // a / cPrev => a_1 / b_0 => Double.MAX_VALUE / 0.5
         final ContinuedFraction cf = new ContinuedFraction() {
             @Override
             public double getA(int n, double x) {
                 return n == 0 ? 1 : Double.MAX_VALUE;
             }
 
             @Override
             public double getB(int n, double x) {
                 return 0.5;
             }
         };
 
         final double eps = 1e-8;
         final Throwable t = Assertions.assertThrows(FractionException.class, () -> cf.evaluate(0, eps, 5));
         assertExceptionMessageContains(t, "infinity");
     }
 
     private static void assertExceptionMessageContains(Throwable t, String text) {
         Assertions.assertTrue(t.getMessage().toLowerCase(Locale.ROOT).contains(text),
             () -> "Missing '" + text + "' from exception message: " + t.getMessage());
     }
 
     // NUMBERS-46
     @Test
     void testOneIteration() {
         final double eps = 0.5;
         final double gr = GoldenRatio.getInstance().evaluate(0, eps, 1);
         // Expected: 1 + 1 / 1
         Assertions.assertEquals(2.0, gr);
     }
 
     // NUMBERS-46
     @Test
     void testTwoIterations() {
         final double eps = 0.25;
         final double gr = GoldenRatio.getInstance().evaluate(0, eps, 2);
         // Expected: 1 + 1 / (1 + 1 / 1)
         Assertions.assertEquals(1.5, gr);
     }
 }