bsb1/object/queue.h

/*! \file
 *	\brief Templated \ref Queue for arbitrary objects.
 */

#pragma once
#include "../arch/core.h"
#include "../debug/assert.h"
#include "../object/outputstream.h"
#include "../types.h"

/*! \brief Templated Queue for arbitrary objects.
 *
 * Queue is implemented by a head-object (Queue<T>) and next-pointers embedded
 * in the queued objects. By passing a different get_link function into the
 * constructor, the member name of the next-pointer can be changed and objects
 * can be contained in different independent queues.
 */
template <class T>
class Queue {
  /// Default get_link implementation returns a pointer to the next-pointer.
  static T** default_get_link(T& obj) { return &obj.queue_link; }
  /// Type definition for the get_link function
  typedef T** (*NextFunc)(T&);

  /// Function pointer to the get_link function, called whenever the
  /// next pointer array is referenced
  const NextFunc get_link;
  /// head points to the first element (the one returned on first dequeue).
  /// Can be nullptr if the queue is empty.
  T* head;
  /// tail points to the last element (the one last added).
  /// Is only valid if head != nullptr
  T* tail;

  // Prevent copies and assignments
  Queue(const Queue&) = delete;
  Queue& operator=(const Queue&) = delete;

 public:
  /*! \brief Minimal forward iterator
   * You can use this iterator to iterate the queue like a normal STL container.
   * It only supports forward iteration, since the queue is single linked.
   */
  class Iterator {
   private:
    Queue<T>& queue;
    T* current;
    friend class Queue<T>;
    Iterator(Queue<T>& queue, T* current) : queue(queue), current(current) {}

   public:
    Iterator operator+(unsigned num) {
      if (current == nullptr) {
        return *this;
      }
      T* temp = current;
      while (num--) {
        temp = queue.next(*temp);
      }
      return Iterator(queue, temp);
    }

    // pre increment
    Iterator& operator++() {
      current = queue.next(*current);
      return *this;
    }

    // post increment
    Iterator operator++(int) {
      auto temp = Iterator(queue, current);
      current = queue.next(*current);
      return temp;
    }

    T* operator*() { return current; }

    bool operator==(const Iterator& other) { return current == other.current; }

    bool operator!=(const Iterator& other) { return !(*this == other); }
  };

  constexpr Queue(Queue&&) = default;

  /*! \brief Constructor
   *	\param[in] get_link A function pointer to the get_link, i.e. a function
   *						which returns a pointer to the
   *next-pointer of an element in the Queue.
   */
  explicit Queue(NextFunc get_link = default_get_link)
      : get_link(get_link), head(nullptr), tail(nullptr) {
    assert(get_link != nullptr &&
           "get_link function pointer must not be nullptr!");
  };

  /*! \brief Enqueues the provided item at the end of the queue. If the element
   *	is already contained in the queue, false will be returned
   * \param[in] item	element to be appended (enqueued).
   * \return false if the element already was enqueued (and nothing was done)
   *		   or not (and it is now enqueued, then true)
   */
  bool enqueue(T& item) {
    T** nextptr = get_link(item);
    if (*nextptr != nullptr || (head != nullptr && tail == &item)) {
      return false;
    }
    *nextptr = nullptr;

    if (head == nullptr) {
      head = tail = &item;
    } else {
      assert(tail != nullptr);
      *get_link(*tail) = &item;
      tail = &item;
    }
    return true;
  }

  /*! \brief insert a new element at the start of the queue
   *  \param[in] item the new item to add
   *  \return true if successful, false if item was already in the queue
   **/
  bool insertFirst(T& item) {
    T** nextptr = get_link(item);
    if (*nextptr != nullptr || (head != nullptr && tail == &item)) {
      return false;
    }

    if (head == nullptr) {
      tail = &item;
    }
    *nextptr = head;
    head = &item;
    return true;
  }

  /*! \brief Insert a new element item into the list after an element after.
   *  Returns false if item is already in the/a list or after is not in this
   *list
   *  \param[in] after the element after which the new one should be inserted
   *  \param[in] item the new element to add
   *  \return true if successful, false if item was in the list or after was not
   **/
  bool insertAfter(T& after, T& item) {
    // if queue is empty there is no after
    // and tail is not valid so we need to check head here
    if (head == nullptr) {
      return false;
    }

    if (&after == tail) {
      return enqueue(item);
    }
    T** nextptr = get_link(item);
    // if item is already in the list return false
    if (*nextptr != nullptr || tail == &item) {
      return false;
    }

    T** pnextptr = get_link(after);
    // if after is NOT in the list, return false
    if (!(pnextptr != nullptr || tail == &after)) {
      return false;
    }

    *nextptr = *pnextptr;
    *pnextptr = &item;
    return true;
  }

  /*! \brief return the next element of a given one or nullptr if the end is
   *reached
   *  \param[in] item the current item
   *  \return the next element or nullptr if the end is reached or the item is
   *not in this list
   **/
  T* next(T& item) {
    T** nextptr = get_link(item);
    if (head == nullptr || (*nextptr == nullptr && tail != &item)) {
      return nullptr;
    }

    return *nextptr;
  }

  /*! \brief Return whether or not the queue is empty
   * \return True if the queue is empty or false otherwise.
   */
  bool is_empty() const { return (head == nullptr); }

  /*! \brief Removes the first element in the queue and returns it.
   *  \note Does not update the tail-pointer
   *	\return Pointer to the removed item or `nullptr` if the queue was empty.
   */
  T* dequeue() {
    T* out = head;
    if (head != nullptr) {
      T** nextptr = get_link(*head);
      head = *nextptr;
      *nextptr = nullptr;
    }
    return out;
  }

  /*! \brief Removes a given element from the queue and returns that element,
   *  or nullptr if it was not present
   *  \return pointer to the removed element, or nullptr if not present
   */
  T* remove(T* that) {
    if (!that) return nullptr;
    T* cur = head;
    T** next_link;

    if (head == that) {
      head = *get_link(*head);

      *get_link(*that) = nullptr;
      return that;
    }
    while (cur) {
      next_link = get_link(*cur);
      if (*next_link == that) {
        *next_link = *get_link(**next_link);

        if (that == tail) {
          tail = cur;
        }

        *get_link(*that) = nullptr;
        return that;
      }
      cur = *next_link;
    }
    return nullptr;
  }

  /// get an iterator to the first element
  Queue<T>::Iterator begin() { return Queue<T>::Iterator(*this, head); }

  /// get an iterator that marks the end of list
  Queue<T>::Iterator end() { return Queue<T>::Iterator(*this, nullptr); }

  /// get the first element of the queue
  T* first() { return head; }

  /// get the last element of the queue
  T* last() { return (head == nullptr ? nullptr : tail); }
};

/*! \brief Overload stream operator for list printing.
 *
 * With this a list can be printed. The elements itself are not printed, just
 * the pointer.
 */
template <class T>
OutputStream& operator<<(OutputStream& os, Queue<T>& queue) {
  os << "{";
  for (typename Queue<T>::Iterator it = queue.begin(); it != queue.end();
       ++it) {
    os << *it;
    if (it + 1 != queue.end()) {
      os << ", ";
    }
  }
  return os << "}";
}