bounded_queue.h

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/* Copyright (c) 2010, 2025, Oracle and/or its affiliates.
 
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   it under the terms of the GNU General Public License, version 2.0,
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   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */
 
#ifndef BOUNDED_QUEUE_INCLUDED
#define BOUNDED_QUEUE_INCLUDED
 
#include "my_base.h"
#include "my_sys.h"
#include "mysys_err.h"
#include "priority_queue.h"
#include "sql/malloc_allocator.h"
 
/**
  A priority queue with a fixed, limited size.
 
  This is a wrapper on top of Priority_queue.
  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 Key_compare.
 
  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,
  sort the array and access it sequentially.
 */
template <typename Element_type, typename Key_type, typename Key_generator,
          typename Key_compare = std::less<Key_type>>
class Bounded_queue {
 public:
  typedef Priority_queue<
      Key_type, std::vector<Key_type, Malloc_allocator<Key_type>>, Key_compare>
      Queue_type;
 
  typedef typename Queue_type::allocator_type allocator_type;
 
  Bounded_queue(
      size_t element_size = sizeof(Element_type),
      const allocator_type &alloc = allocator_type(PSI_NOT_INSTRUMENTED))
      : m_queue(Key_compare(), alloc),
        m_sort_keys(nullptr),
        m_sort_param(nullptr),
        m_element_size(element_size) {}
 
  /**
    Initialize the queue.
 
    @param max_elements   The size of the queue.
    @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 false OK, true Could not allocate memory.
 
    We do *not* take ownership of any of the input pointer arguments.
   */
  bool init(ha_rows max_elements, Key_generator *sort_param,
            Key_type *sort_keys) {
    m_sort_keys = sort_keys;
    m_sort_param = sort_param;
    DBUG_EXECUTE_IF("bounded_queue_init_fail",
                    my_error(EE_OUTOFMEMORY, MYF(ME_FATALERROR), 42);
                    return true;);
 
    // We allocate space for one extra element, for replace when queue is full.
    if (m_queue.reserve(max_elements + 1)) return true;
    // We cannot have packed keys in the queue.
    m_queue.m_compare_length = sort_param->max_compare_length();
    // We can have variable length keys though.
    if (sort_param->using_varlen_keys()) m_queue.m_param = sort_param;
    return false;
  }
 
  /**
    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 opaque    Parameter to send on to make_sortkey().
 
    @retval false OK, true error.
   */
  template <class Opaque>
  [[nodiscard]] bool push(const Opaque &opaque) {
    /*
      Add one extra byte to each key, so that sort-key generating functions
      won't be returning out-of-space. Since we know there's always room
      given a "m_element_size"-sized buffer even in the worst case (by
      definition), we could in principle make a special mode flag in
      Sort_param::make_sortkey() instead for the case of fixed-length records,
      but this is much simpler.
     */
    assert(m_element_size < 0xFFFFFFFF);
    const uint element_size = m_element_size + 1;
 
    if (m_queue.size() == m_queue.capacity()) {
      const Key_type &pq_top = m_queue.top();
      const uint rec_sz =
          m_sort_param->make_sortkey(pq_top, element_size, opaque);
      // UINT_MAX means error, but we do not want to add a dependency
      // on class THD here, as in current_thd->is_error().
      if (rec_sz == UINT_MAX) return true;
      assert(rec_sz <= m_element_size);
      m_queue.update_top();
      return false;
    } else {
      const uint rec_sz = m_sort_param->make_sortkey(
          m_sort_keys[m_queue.size()], element_size, opaque);
      // UINT_MAX means error, but we do not want to add a dependency
      // on class THD here, as in current_thd->is_error().
      if (rec_sz == UINT_MAX) return true;
      assert(rec_sz <= m_element_size);
      return m_queue.push(m_sort_keys[m_queue.size()]);
    }
  }
 
  /**
    The number of elements in the queue.
   */
  size_t num_elements() const { return m_queue.size(); }
 
 private:
  Queue_type m_queue;
  Key_type *m_sort_keys;
  Key_generator *m_sort_param;
  size_t m_element_size;
};
 
#endif  // BOUNDED_QUEUE_INCLUDED