/* Copyright (c) 2010, 2023, Oracle and/or its affiliates.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License, version 2.0,
   as published by the Free Software Foundation.

   This program is also distributed with certain software (including
   but not limited to OpenSSL) that is licensed under separate terms,
   as designated in a particular file or component or in included license
   documentation.  The authors of MySQL hereby grant you an additional
   permission to link the program and your derivative works with the
   separately licensed software that they have included with MySQL.

   This program 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 2.0, for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA */

#ifndef BOUNDED_QUEUE_C_INCLUDED
#define BOUNDED_QUEUE_C_INCLUDED

#include <string.h>
#include "my_global.h"
#include "my_base.h"
#include "my_sys.h"
#include "queues.h"
#include "test_utils.h"

/**
  A priority queue with a fixed, limited size.

  This is a wrapper on top of QUEUE and the queue_xxx() functions.
  It keeps the top-N elements which are inserted.

  Elements of type Element_type are pushed into the queue.
  For each element, we call a user-supplied Key_generator::make_sortkey(),
  to generate a key of type Key_type for the element.
  Instances of Key_type are compared with the user-supplied compare_function.

  The underlying QUEUE implementation needs one extra element for replacing
  the lowest/highest element when pushing into a full queue.

  Pointers to the top-N elements are stored in the sort_keys array given
  to the init() function below. To access elements in sorted order, use
  repeated calls to pop(), or, simply sort the array and access it sequentially.
  The pop() interface is for unit testing only.
 */
template<typename Element_type, typename Key_type, typename Key_generator>
class Bounded_QUEUE
{
public:
  Bounded_QUEUE()
  {
    memset(&m_queue, 0, sizeof(m_queue));
  }

  ~Bounded_QUEUE()
  {
    delete_queue(&m_queue);
  }

  /**
     Function for comparing two keys.
     @param  n Pointer to number of bytes to compare.
     @param  a First key.
     @param  b Second key.
     @retval -1, 0, or 1 depending on whether the left argument is 
             less than, equal to, or greater than the right argument.
   */
  typedef int (*compare_function)(size_t *n, Key_type *a, Key_type *b);

  /**
    Initialize the queue.

    @param max_elements   The size of the queue.
    @param max_at_top     Set to true if you want biggest element on top.
           false: We keep the n largest elements.
                  pop() will return the smallest key in the result set.
           true:  We keep the n smallest elements.
                  pop() will return the largest key in the result set.
    @param compare        Compare function for elements, takes 3 arguments.
           If NULL, we use get_ptr_compare(sort_param->compare_length()).
    @param sort_param     Sort parameters. We call sort_param->make_sortkey()
                          to generate keys for elements.
    @param[in,out] sort_keys Array of keys to sort.
                             Must be initialized by caller.
                             Will be filled with pointers to the top-N elements.

    @retval 0 OK, 1 Could not allocate memory.

    We do *not* take ownership of any of the input pointer arguments.
   */
  int init(ha_rows max_elements, bool max_at_top,
           compare_function compare,
           Key_generator *sort_param,
           Key_type *sort_keys);

  /**
    Pushes an element on the queue.
    If the queue is already full, we discard one element.
    Calls m_sort_param::make_sortkey() to generate a key for the element.

    @param element        The element to be pushed.
   */
  void push(Element_type element);

  /**
    Removes the top element from the queue.

    @retval Pointer to the (key of the) removed element.

    @note This function is for unit testing, where we push elements into to the
          queue, and test that the appropriate keys are retained.
          Interleaving of push() and pop() operations has not been tested.
   */
  Key_type *pop()
  {
    // Don't return the extra element to the client code.
    if (queue_is_full((&m_queue)))
      queue_remove(&m_queue, 0);
    assert(m_queue.elements > 0);
    if (m_queue.elements == 0)
      return NULL;
    return reinterpret_cast<Key_type*>(queue_remove(&m_queue, 0));
  }

  /**
    The number of elements in the queue.
   */
  uint num_elements() const { return m_queue.elements; }

  /**
    Is the queue initialized?
   */
  bool is_initialized() const { return m_queue.max_elements > 0; }

private:
  Key_type          *m_sort_keys;
  size_t             m_compare_length;
  Key_generator     *m_sort_param;
  st_queue           m_queue;
};


template<typename Element_type, typename Key_type, typename Key_generator>
int Bounded_QUEUE<Element_type, Key_type, Key_generator>
  ::init(ha_rows max_elements,
         bool max_at_top,
         compare_function compare,
         Key_generator *sort_param,
         Key_type *sort_keys)
{
  assert(sort_keys != NULL);

  m_sort_keys=      sort_keys;
  m_compare_length= sort_param->compare_length();
  m_sort_param=     sort_param;
  // init_queue() takes an uint, and also does (max_elements + 1)
  if (max_elements >= (UINT_MAX - 1))
    return 1;
  if (compare == NULL)
    compare=
      reinterpret_cast<compare_function>
        (my_testing::get_ptr_compare(m_compare_length));

  DBUG_EXECUTE_IF("bounded_queue_init_fail",
                  DBUG_SET("+d,simulate_out_of_memory"););

  // We allocate space for one extra element, for replace when queue is full.
  return init_queue(&m_queue, (uint) max_elements + 1,
                    0, max_at_top,
                    reinterpret_cast<queue_compare>(compare),
                    &m_compare_length);
}


template<typename Element_type, typename Key_type, typename Key_generator>
void Bounded_QUEUE<Element_type, Key_type, Key_generator>
  ::push(Element_type element)
{
  assert(is_initialized());
  if (queue_is_full((&m_queue)))
  {
    // Replace top element with new key, and re-order the queue.
    Key_type *pq_top= reinterpret_cast<Key_type *>(queue_top(&m_queue));
    m_sort_param->make_sortkey(*pq_top, element);
    queue_replaced(&m_queue);
  }
  else
  {
    // Insert new key into the queue.
    m_sort_param->make_sortkey(m_sort_keys[m_queue.elements], element);
    queue_insert(&m_queue,
                 reinterpret_cast<uchar*>(&m_sort_keys[m_queue.elements]));
  }
}


#endif  // BOUNDED_QUEUE_C_INCLUDED