kth_max_element.hxx

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/*===========================================================================================================
 *
 * SHA - Simple Hybesis Algorithms
 *
 * Copyright (c) Michael Jeulin-Lagarrigue
 *
 *  Licensed under the MIT License, you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *         https://github.com/michael-jeulinl/Simple-Hybesis-Algorithms/blob/master/LICENSE
 *
 * 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.
 *
 * The above copyright notice and this permission notice shall be included in all copies or
 * substantial portions of the Software.
 *
 *=========================================================================================================*/
#ifndef MODULE_SEARCH_MAX_KTH_ELEMENT_HXX
#define MODULE_SEARCH_MAX_KTH_ELEMENT_HXX

#include <Sort/partition.hxx>

// STD includes
#include <iterator>

namespace SHA_Search
{
  template <typename IT, typename Compare = std::less_equal<typename std::iterator_traits<IT>::value_type>>
  IT MaxKthElement(const IT& begin, const IT& end, unsigned int k)
  {
    const auto kSize = static_cast<const int>(std::distance(begin, end));
    if (k >= static_cast<unsigned int>(kSize))
      return end;

    auto pivot = begin + (rand() % kSize);                               // Take random pivot
    auto newPivot = SHA_Sort::Partition<IT, Compare>(begin, pivot, end); // Partition

    // Get the index of the pivot (i'th smallest/biggest value)
    const auto kPivotIndex = std::distance(begin, newPivot);

    // Return if at the kth position
    if (kPivotIndex == k)
      return newPivot;

    // Recurse search on left part if there is more than k elements within the left sequence
    // Recurse search on right otherwise
    return (kPivotIndex > k) ? MaxKthElement<IT, Compare>(begin, newPivot, k)
                             : MaxKthElement<IT, Compare>(newPivot, end, k - kPivotIndex);

  }
}

#endif // MODULE_SEARCH_MAX_KTH_ELEMENT_HXX