/*
 * 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.
 */

// Based on the following Java implementation:
// https://github.com/apache/commons-statistics/blob/master/commons-statistics-distribution/src/main/java/org/apache/commons/statistics/distribution/ZipfDistribution.java
// https://github.com/apache/commons-rng/blob/master/commons-rng-sampling/src/main/java/org/apache/commons/rng/sampling/distribution/RejectionInversionZipfSampler.java

#ifndef ZIPF_DISTRIBUTION_HPP_
#define ZIPF_DISTRIBUTION_HPP_

#include <stdexcept>
#include <random>

namespace datasketches {

class zipf_distribution {
public:
  zipf_distribution(unsigned num_elements, double exponent);
  unsigned sample();
private:
  static constexpr double TAYLOR_THRESHOLD = 1e-8;
  static constexpr double F_1_2 = 0.5;
  static constexpr double F_1_3 = 1.0 / 3.0;
  static constexpr double F_1_4 = 0.25;

  const unsigned num_elements;
  const double exponent;
  const double h_integral_x1;
  const double h_integral_num_elements;
  const double s;

  std::default_random_engine generator;
  std::uniform_real_distribution<double> distribution;

  double h(double x);
  double h_integral(double x);
  double h_integral_inverse(double x);

  static double helper1(double x);
  static double helper2(double x);
};

}

#endif