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I have this priority queue in C++ that is stable. Stability here means that if the queue contains different elements with equal priority, they will be popped in the insertion order.

#ifndef NET_CODERODDE_UTIL_STABLE_PRIORITY_QUEUE
#define NET_CODERODDE_UTIL_STABLE_PRIORITY_QUEUE

#include <map>
#include <stdexcept>
#include <unordered_map>

namespace net {
namespace coderodde {
namespace util {

    template<typename T, typename Priority>
    class StablePriorityQueue {
    private:

        struct ElementList;

        struct ElementListNode {
            T m_element;
            ElementListNode* m_next;
            ElementListNode* m_prev;
            ElementList* m_owner_list;

            ElementListNode(T element, ElementList* owner_list)
            :
            m_element{element},
            m_owner_list{owner_list}
            {}
        };

        struct ElementList {
            Priority m_priority;
            ElementListNode* m_head;
            ElementListNode* m_tail;

            ElementList(Priority priority)
            :
            m_priority{priority},
            m_head{nullptr},
            m_tail{nullptr}
            {}

            void appendElementListNode(ElementListNode* element_list_node) {
                if (m_head == nullptr) {
                    m_head = element_list_node;
                    m_tail = element_list_node;
                    element_list_node->m_prev = nullptr;
                    element_list_node->m_next = nullptr;
                } else {
                    m_tail->m_next = element_list_node;
                    element_list_node->m_prev = m_tail;
                    element_list_node->m_next = nullptr;
                    m_tail = element_list_node;
                }
            }

            T removeHead() {
                T ret = m_head->m_element;
                ElementListNode* old_node = m_head;
                m_head = m_head->m_next;
                delete old_node;

                if (m_head != nullptr) {
                    m_head->m_prev = nullptr;
                }

                return ret;
            }

            bool isEmpty() {
                return m_head == nullptr;
            }
        };

    public:

        ~StablePriorityQueue() {
            for (std::pair<Priority, ElementList*> p : m_priority_map) {
                delete p.second;
            }

            for (std::pair<T, ElementListNode*> p : m_element_map) {
                delete p.second;
            }
        }

        void add(T element, Priority priority) {
            if (m_element_map.find(element) == m_element_map.cend()) {
                addNewEntry(element, priority);
            } else {
                updateEntry(m_element_map[element], priority);
            }
        }

        T top() {
            checkQueueNotEmpty();
            Priority priority = (*m_priority_map.cbegin()).first;
            return m_priority_map[priority]->m_head->m_element;
        }

        T extractMinimum()  {
            checkQueueNotEmpty();
            Priority priority = (*m_priority_map.cbegin()).first;
            ElementList* element_list = m_priority_map[priority];
            T result = element_list->removeHead();

            if (element_list->isEmpty()) {
                m_priority_map.erase(element_list->m_priority);
                delete element_list;
            }

            m_element_map.erase(result);
            return result;
        }

        bool empty() {
            return m_element_map.empty();
        }

        size_t size() {
            return m_element_map.size();
        }

    private:
        std::map<Priority, ElementList*> m_priority_map;
        std::unordered_map<T, ElementListNode*> m_element_map;

    private:

        void addNewEntry(T element, Priority priority) {
            ElementList* element_list = nullptr;

            if (m_priority_map.find(priority) != m_priority_map.cend()) {
                element_list = m_priority_map[priority];
            }

            if (element_list == nullptr) {
                element_list = new ElementList(priority);
                m_priority_map[priority] = element_list;
            }

            ElementListNode* element_list_node = new ElementListNode(
                                                            element,
                                                            element_list);

            element_list->appendElementListNode(element_list_node);
            m_element_map[element] = element_list_node;
        }

        void updateEntry(ElementListNode* element_list_node,
                         Priority new_priority) {
            unlink(element_list_node);
            relink(element_list_node, new_priority);
        }

        void unlink(ElementListNode* element_list_node) {
            ElementList* element_list = element_list_node->m_owner_list;

            if (element_list_node->m_prev != nullptr) {
                element_list_node->m_prev->m_next = element_list_node->m_next;
            } else {
                element_list->m_head = element_list->m_head->m_next;

                if (element_list->m_head != nullptr) {
                    element_list->m_head->m_prev = nullptr;
                }
            }

            if (element_list_node->m_next != nullptr) {
                element_list_node->m_next->m_prev = element_list_node->m_prev;
            } else {
                element_list->m_tail = element_list->m_tail->m_prev;

                if (element_list->m_tail != nullptr) {
                    element_list->m_tail->m_next = nullptr;
                }
            }

            if (element_list->isEmpty()) {
                m_priority_map.erase(element_list->m_priority);
                delete element_list;
            }
        }

        void relink(ElementListNode* element_list_node, Priority priority) {
            ElementList* element_list = nullptr;

            if (m_priority_map.find(priority) != m_priority_map.cend()) {
                element_list = m_priority_map[priority];
            }

            if (element_list == nullptr) {
                element_list = new ElementList(priority);
                m_priority_map[priority] = element_list;
            }

            element_list->appendElementListNode(element_list_node);
        }

        void checkQueueNotEmpty() {
            if (empty()) {
                throw std::underflow_error{"The priority queue is empty."};
            }
        }
    };

} // End of namespace net::coderodde::util.
} // End of namespace net::coderodde.
} // End of namespace net.

#endif // NET_CODERODDE_UTIL_STABLE_PRIORITY_QUEUE

Critique request

Please tell me anything that comes to mind. Also, valgrind says I leak memory like a sieve. Can you spot the leaks?

(Unit tests may be found here.)

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new and delete...

Manual use of new and delete very often leads to leaks. In this case you have a destructor but not any other of the rule of 5 requirements so every List and Node will get double deleted once the queue has been copied once.

If instead if you had m_priority_map and m_element_map hold their values by value there wouldn't be any leak possibility and you could remove the destructor. You can do that because the values of map and unordered_map have referential consistency (the locations remain fixed once they are in the map regardless of what happens to the map).

You aren't very consistent with m_owner_list, if you ever update priority of an element you don't update it.

Every T has 2 copies being stored. once in the node and once as value in the m_element_map one should be a pointer to the other. Using a unordered_map with T as key requires that it has a sane std::hash and operator==. It's primary purpose seems to be to avoid duplicates in the queue, however it's not uncommon to need duplicate entries in the queue so its purpose is questionable.

addNewEntry should take the element by T&& and add should std::move it into the call. you can also reuse the relink function in there:

void addNewEntry(T&& element, Priority priority) {
    ElementListNode* element_list_node = new ElementListNode(
                                                    element);//not storing the list at this point.

    relink(element_list_node, priority); //should set the owning list
}

Having said that, std::list lets you slice out the internal nodes and splice them into another std::list. So implementing that functionality yourself is not needed and using std::list will simplify a lot of your code.

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  • \$\begingroup\$ Thank you for your time! However, I am still confused: could you provide an alternative implementation? \$\endgroup\$ – coderodde Oct 19 '17 at 13:26
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I think you're over-complicating this problem. The easiest way to make a stable priority queue is to ensure that no two items can have the same priority. You can do that by creating a monotonically increasing counter and setting the priority of each item equal to the combination of its natural priority and the counter value. In other words, you break ties by considering which item was inserted first.

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  • \$\begingroup\$ Yes. On the other hand, it runs both the pop and push operation in \$\Theta(\log P)\$, where \$P\$ is the number of distinct priority keys. If the priority key space is small, this might be faster in asymptotic sense. \$\endgroup\$ – coderodde Oct 19 '17 at 14:58

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