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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

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 * published by the Free Software Foundation.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

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 */

#ifndef SHARE_UTILITIES_QUICKSORT_HPP

#define SHARE_UTILITIES_QUICKSORT_HPP

#include "memory/allocation.hpp"

#include "runtime/globals.hpp"

#include "utilities/debug.hpp"

class QuickSort : AllStatic {

 private:

  template<class T>

  static void swap(T* array, size_t x, size_t y) {

    T tmp = array[x];

    array[x] = array[y];

    array[y] = tmp;

  }

  // As pivot we use the median of the first, last and middle elements.

  // We swap in these three values at the right place in the array. This

  // means that this method not only returns the index of the pivot

  // element. It also alters the array so that:

  //     array[first] <= array[middle] <= array[last]

  // A side effect of this is that arrays of length <= 3 are sorted.

  template<class T, class C>

  static size_t find_pivot(T* array, size_t length, C comparator) {

    assert(length > 1, "length of array must be > 0");

    size_t middle_index = length / 2;

    size_t last_index = length - 1;

    if (comparator(array[0], array[middle_index]) > 0) {

      swap(array, 0, middle_index);

    }

    if (comparator(array[0], array[last_index]) > 0) {

      swap(array, 0, last_index);

    }

    if (comparator(array[middle_index], array[last_index]) > 0) {

      swap(array, middle_index, last_index);

    }

    // Now the value in the middle of the array is the median

    // of the fist, last and middle values. Use this as pivot.

    return middle_index;

  }

  template<bool idempotent, class T, class C>

  static size_t partition(T* array, size_t pivot, size_t length, C comparator) {

    size_t left_index = 0;

    size_t right_index = length - 1;

    T pivot_val = array[pivot];

    for ( ; true; ++left_index, --right_index) {

      for ( ; comparator(array[left_index], pivot_val) < 0; ++left_index) {

        assert(left_index < length, "reached end of partition");

      }

      for ( ; comparator(array[right_index], pivot_val) > 0; --right_index) {

        assert(right_index > 0, "reached start of partition");

      }

      if (left_index < right_index) {

        if (!idempotent || comparator(array[left_index], array[right_index]) != 0) {

          swap(array, left_index, right_index);

        }

      } else {

        return right_index;

      }

    }

    ShouldNotReachHere();

    return 0;

  }

  template<bool idempotent, class T, class C>

  static void inner_sort(T* array, size_t length, C comparator) {

    if (length < 2) {

      return;

    }

    size_t pivot = find_pivot(array, length, comparator);

    if (length < 4) {

      // arrays up to length 3 will be sorted after finding the pivot

      return;

    }

    size_t split = partition<idempotent>(array, pivot, length, comparator);

    size_t first_part_length = split + 1;

    inner_sort<idempotent>(array, first_part_length, comparator);

    inner_sort<idempotent>(&array[first_part_length], length - first_part_length, comparator);

  }

 public:

  // The idempotent parameter prevents the sort from

  // reordering a previous valid sort by not swapping

  // fields that compare as equal. This requires extra

  // calls to the comparator, so the performance

  // impact depends on the comparator.

  template<class T, class C>

  static void sort(T* array, size_t length, C comparator, bool idempotent) {

    // Switch "idempotent" from function paramter to template parameter

    if (idempotent) {

      inner_sort<true>(array, length, comparator);

    } else {

      inner_sort<false>(array, length, comparator);

    }

  }

};

#endif // SHARE_UTILITIES_QUICKSORT_HPP