text: // Copyright (c) 2018, The Mask Project
  //
  // All rights reserved.
  //
  // Redistribution and use in source and binary forms, with or without modification, are
  // permitted provided that the following conditions are met:
  //
  // 1. Redistributions of source code must retain the above copyright notice, this list of
  //    conditions and the following disclaimer.
  //
  // 2. Redistributions in binary form must reproduce the above copyright notice, this list
  //    of conditions and the following disclaimer in the documentation and/or other
  //    materials provided with the distribution.
  //
  // 3. Neither the name of the copyright holder nor the names of its contributors may be
  //    used to endorse or promote products derived from this software without specific
  //    prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
  // EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
  // MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
  // THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  // PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  // INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
  // STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
  // THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  //
  // Parts of this file are originally copyright (c) 2012-2013 The Cryptonote developers
  
  #pragma once
  
  #include <iostream>
  #include <vector>
  #include <stdexcept>
  
  namespace tools {
  
  uint64_t combinations_count(uint32_t k, uint32_t n);
  
  template<typename T>
  class Combinator {
  public:
    Combinator(const std::vector<T>& v) : origin(v) { }
  
    std::vector<std::vector<T>> combine(size_t k);
  
  private:
    void doCombine(size_t from, size_t k);
  
    std::vector<T> origin;
    std::vector<std::vector<T>> combinations;
    std::vector<size_t> current;
  };
  
  template<typename T>
  std::vector<std::vector<T>> Combinator<T>::combine(size_t k)
  {
    if (k > origin.size())
    {
      throw std::runtime_error("k must be smaller than elements number");
    }
  
    if (k == 0)
    {
      throw std::runtime_error("k must be greater than zero");
    }
  
    combinations.clear();
    doCombine(0, k);
    return combinations;
  }
  
  template<typename T>
  void Combinator<T>::doCombine(size_t from, size_t k)
  {
    current.push_back(0);
  
    for (size_t i = from; i <= origin.size() - k; ++i)
    {
      current.back() = i;
  
      if (k > 1) {
        doCombine(i + 1, k - 1);
      } else {
        std::vector<T> comb;
        for (auto ind: current) {
          comb.push_back(origin[ind]);
        }
        combinations.push_back(comb);
      }
    }
  
    current.pop_back();
  }
  
  } //namespace tools
  