free_list.h

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/* Copyright (c) 2015, 2023, Oracle and/or its affiliates.
 
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#ifndef DD_CACHE__FREE_LIST_INCLUDED
#define DD_CACHE__FREE_LIST_INCLUDED
 
#include <assert.h>
#include <vector>  // vector
 
#include "sql/malloc_allocator.h"  // Malloc_allocator.
#include "sql/psi_memory_key.h"    // key_memory_DD_cache_infrastructure
 
namespace dd {
namespace cache {
 
/**
  Template for management of a free list based on a std::vector.
 
  The free list template defines functions mostly wrapping the std::vector
  functions, but additionally doing some asserts to ensure correct usage.
 
  The first element in the free list is the least recently used element.
  When a new element becomes unused, it is added to the end of the free list.
  An element may also be removed from within the middle of the free list
  when the element is being acquired. This use case means iterating through
  the vector to find the correct element.
 
  @tparam  E  Element type (a Cache_element wrapping some dictionary
              object type).
*/
 
template <typename E>
class Free_list {
 private:
  typedef std::vector<E *, Malloc_allocator<E *>> List_type;
  List_type m_list;  // The actual list.
 
 public:
  Free_list()
      : m_list(Malloc_allocator<E *>(key_memory_DD_cache_infrastructure)) {}
 
  // Return the actual free list length.
  size_t length() const { return m_list.size(); }
 
  /**
    Add an element to the end of the free list.
 
    @param   element Element to add to the end of the list.
  */
 
  void add_last(E *element) {
    assert(element != nullptr && element->usage() == 0);
    m_list.push_back(element);
  }
 
  /**
    Remove an element from the free list.
 
    @param   element Element to be removed from the list.
  */
 
  void remove(E *element) {
    assert(element != nullptr && element->usage() == 0);
    assert(!m_list.empty());
 
    for (typename List_type::iterator it = m_list.begin(); it != m_list.end();
         ++it)
      if (*it == element) {
        m_list.erase(it);
        return;
      }
 
    assert(false); /* purecov: deadcode */
  }
 
  /**
    Get the least recently used element in the list, i.e., the first element.
 
    @return  The least recently used element in the list.
  */
 
  E *get_lru() const {
    assert(!m_list.empty());
    if (!m_list.empty()) return m_list.front();
    return nullptr;
  }
 
  /**
    Debug dump of the free list to stderr.
  */
  /* purecov: begin inspected */
  void dump() const {
#ifndef NDEBUG
    if (m_list.empty()) {
      fprintf(stderr, "    lru-> NULL\n");
      return;
    }
    fprintf(stderr, "    lru-> ");
    for (typename List_type::const_iterator it = m_list.begin();
         it != m_list.end(); ++it)
      fprintf(stderr, "%llu ", (*it)->object()->id());
    fprintf(stderr, "\n");
#endif
  }
  /* purecov: end */
};
 
}  // namespace cache
}  // namespace dd
 
#endif  // DD_CACHE__FREE_LIST_INCLUDED