/*
    * 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
    *
    *      https://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.
   */
  
  /*
   * This is not the original file distributed by the Apache Software Foundation
   * It has been modified by the Hipparchus project
   */
  package org.hipparchus.stat.descriptive.summary;
  
  import java.io.Serializable;
  
  import org.hipparchus.exception.MathIllegalArgumentException;
  import org.hipparchus.exception.NullArgumentException;
  import org.hipparchus.stat.descriptive.AbstractStorelessUnivariateStatistic;
  import org.hipparchus.stat.descriptive.AggregatableStatistic;
  import org.hipparchus.stat.descriptive.WeightedEvaluation;
  import org.hipparchus.util.FastMath;
  import org.hipparchus.util.MathArrays;
  import org.hipparchus.util.MathUtils;
  
  /**
   * Returns the product of the available values.
   * <p>
   * If there are no values in the dataset, then 1 is returned.
   * If any of the values are
   * <code>NaN</code>, then <code>NaN</code> is returned.
   * <p>
   * <strong>Note that this implementation is not synchronized.</strong> If
   * multiple threads access an instance of this class concurrently, and at least
   * one of the threads invokes the <code>increment()</code> or
   * <code>clear()</code> method, it must be synchronized externally.
   */
  public class Product extends AbstractStorelessUnivariateStatistic
      implements AggregatableStatistic<Product>, WeightedEvaluation, Serializable {
  
      /** Serializable version identifier */
      private static final long serialVersionUID = 20150412L;
  
      /** The number of values that have been added */
      private long n;
  
      /** The current Running Product */
      private double value;
  
      /**
       * Create a Product instance.
       */
      public Product() {
          n = 0;
          value = 1;
      }
  
      /**
       * Copy constructor, creates a new {@code Product} identical
       * to the {@code original}.
       *
       * @param original the {@code Product} instance to copy
       * @throws NullArgumentException  if original is null
       */
      public Product(Product original) throws NullArgumentException {
          MathUtils.checkNotNull(original);
          this.n     = original.n;
          this.value = original.value;
      }
  
      /** {@inheritDoc} */
      @Override
      public void increment(final double d) {
          value *= d;
          n++;
      }
  
      /** {@inheritDoc} */
      @Override
      public double getResult() {
          return value;
      }
  
      /** {@inheritDoc} */
      @Override
      public long getN() {
          return n;
      }
  
      /** {@inheritDoc} */
     @Override
     public void clear() {
         value = 1;
         n = 0;
     }
 
     /** {@inheritDoc} */
     @Override
     public void aggregate(Product other) {
         MathUtils.checkNotNull(other);
         if (other.n > 0) {
             this.n     += other.n;
             this.value *= other.value;
         }
     }
 
     /**
      * Returns the product of the entries in the specified portion of
      * the input array, or <code>Double.NaN</code> if the designated subarray
      * is empty.
      *
      * @param values the input array
      * @param begin index of the first array element to include
      * @param length the number of elements to include
      * @return the product of the values or 1 if length = 0
      * @throws MathIllegalArgumentException if the array is null or the array index
      *  parameters are not valid
      */
     @Override
     public double evaluate(final double[] values, final int begin, final int length)
         throws MathIllegalArgumentException {
         double product = Double.NaN;
         if (MathArrays.verifyValues(values, begin, length, true)) {
             product = 1.0;
             for (int i = begin; i < begin + length; i++) {
                 product *= values[i];
             }
         }
         return product;
     }
 
     /**
      * Returns the weighted product of the entries in the specified portion of
      * the input array, or <code>Double.NaN</code> if the designated subarray
      * is empty.
      * <p>
      * Throws <code>MathIllegalArgumentException</code> if any of the following are true:
      * <ul><li>the values array is null</li>
      *     <li>the weights array is null</li>
      *     <li>the weights array does not have the same length as the values array</li>
      *     <li>the weights array contains one or more infinite values</li>
      *     <li>the weights array contains one or more NaN values</li>
      *     <li>the weights array contains negative values</li>
      *     <li>the start and length arguments do not determine a valid array</li>
      * </ul>
      * <p>
      * Uses the formula \(\mathrm{weighted product} = \prod \mathrm{values}_i^{\mathrm{weights}_i}\)
      * <p>
      * that is, the weights are applied as exponents when computing the weighted product.
      *
      * @param values the input array
      * @param weights the weights array
      * @param begin index of the first array element to include
      * @param length the number of elements to include
      * @return the product of the values or 1 if length = 0
      * @throws MathIllegalArgumentException if the parameters are not valid
      */
     @Override
     public double evaluate(final double[] values, final double[] weights,
                            final int begin, final int length) throws MathIllegalArgumentException {
         double product = Double.NaN;
         if (MathArrays.verifyValues(values, weights, begin, length, true)) {
             product = 1.0;
             for (int i = begin; i < begin + length; i++) {
                 product *= FastMath.pow(values[i], weights[i]);
             }
         }
         return product;
     }
 
     /** {@inheritDoc} */
     @Override
     public Product copy() {
         return new Product(this);
     }
 
 }