michael@0: /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
michael@0: /* vim:set ts=2 sw=2 sts=2 et cindent: */
michael@0: /* This Source Code Form is subject to the terms of the Mozilla Public
michael@0:  * License, v. 2.0. If a copy of the MPL was not distributed with this
michael@0:  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
michael@0: 
michael@0: #ifndef NS_TPRIORITY_QUEUE_H_
michael@0: #define NS_TPRIORITY_QUEUE_H_
michael@0: 
michael@0: #include "nsTArray.h"
michael@0: #include "nsDebug.h"
michael@0: 
michael@0: /**
michael@0:  * A templatized priority queue data structure that uses an nsTArray to serve as
michael@0:  * a binary heap. The default comparator causes this to act like a min-heap.
michael@0:  * Only the LessThan method of the comparator is used.
michael@0:  */
michael@0: template<class T, class Compare = nsDefaultComparator<T, T> >
michael@0: class nsTPriorityQueue
michael@0: {
michael@0: public:
michael@0:   typedef typename nsTArray<T>::size_type size_type;
michael@0: 
michael@0:   /**
michael@0:    * Default constructor also creates a comparator object using the default
michael@0:    * constructor for type Compare.
michael@0:    */
michael@0:   nsTPriorityQueue() : mCompare(Compare()) { }
michael@0: 
michael@0:   /**
michael@0:    * Constructor to allow a specific instance of a comparator object to be
michael@0:    * used.
michael@0:    */
michael@0:   nsTPriorityQueue(const Compare& aComp) : mCompare(aComp) { }
michael@0: 
michael@0:   /**
michael@0:    * Copy constructor
michael@0:    */
michael@0:   nsTPriorityQueue(const nsTPriorityQueue& aOther)
michael@0:     : mElements(aOther.mElements),
michael@0:       mCompare(aOther.mCompare)
michael@0:   { }
michael@0: 
michael@0:   /**
michael@0:    * @return True if the queue is empty or false otherwise.
michael@0:    */
michael@0:   bool IsEmpty() const
michael@0:   {
michael@0:     return mElements.IsEmpty();
michael@0:   }
michael@0: 
michael@0:   /**
michael@0:    * @return The number of elements in the queue.
michael@0:    */
michael@0:   size_type Length() const
michael@0:   {
michael@0:     return mElements.Length();
michael@0:   }
michael@0: 
michael@0:   /**
michael@0:    * @return The topmost element in the queue without changing the queue. This
michael@0:    * is the element 'a' such that there is no other element 'b' in the queue for
michael@0:    * which Compare(b, a) returns true. (Since this container does not check
michael@0:    * for duplicate entries there may exist 'b' for which Compare(a, b) returns
michael@0:    * false.)
michael@0:    */
michael@0:   const T& Top() const
michael@0:   {
michael@0:     NS_ABORT_IF_FALSE(!mElements.IsEmpty(), "Empty queue");
michael@0:     return mElements[0];
michael@0:   }
michael@0: 
michael@0:   /**
michael@0:    * Adds an element to the queue
michael@0:    * @param aElement The element to add
michael@0:    * @return true on success, false on out of memory.
michael@0:    */
michael@0:   bool Push(const T& aElement)
michael@0:   {
michael@0:     T* elem = mElements.AppendElement(aElement);
michael@0:     if (!elem)
michael@0:       return false; // Out of memory
michael@0: 
michael@0:     // Sift up
michael@0:     size_type i = mElements.Length() - 1;
michael@0:     while (i) {
michael@0:       size_type parent = (size_type)((i - 1) / 2);
michael@0:       if (mCompare.LessThan(mElements[parent], mElements[i])) {
michael@0:         break;
michael@0:       }
michael@0:       Swap(i, parent);
michael@0:       i = parent;
michael@0:     }
michael@0: 
michael@0:     return true;
michael@0:   }
michael@0: 
michael@0:   /**
michael@0:    * Removes and returns the top-most element from the queue.
michael@0:    * @return The topmost element, that is, the element 'a' such that there is no
michael@0:    * other element 'b' in the queue for which Compare(b, a) returns true.
michael@0:    * @see Top()
michael@0:    */
michael@0:   T Pop()
michael@0:   {
michael@0:     NS_ABORT_IF_FALSE(!mElements.IsEmpty(), "Empty queue");
michael@0:     T pop = mElements[0];
michael@0: 
michael@0:     // Move last to front
michael@0:     mElements[0] = mElements[mElements.Length() - 1];
michael@0:     mElements.TruncateLength(mElements.Length() - 1);
michael@0: 
michael@0:     // Sift down
michael@0:     size_type i = 0;
michael@0:     while (2*i + 1 < mElements.Length()) {
michael@0:       size_type swap = i;
michael@0:       size_type l_child = 2*i + 1;
michael@0:       if (mCompare.LessThan(mElements[l_child], mElements[i])) {
michael@0:         swap = l_child;
michael@0:       }
michael@0:       size_type r_child = l_child + 1;
michael@0:       if (r_child < mElements.Length() &&
michael@0:           mCompare.LessThan(mElements[r_child], mElements[swap])) {
michael@0:         swap = r_child;
michael@0:       }
michael@0:       if (swap == i) {
michael@0:         break;
michael@0:       }
michael@0:       Swap(i, swap);
michael@0:       i = swap;
michael@0:     }
michael@0: 
michael@0:     return pop;
michael@0:   }
michael@0: 
michael@0:   /**
michael@0:    * Removes all elements from the queue.
michael@0:    */
michael@0:   void Clear()
michael@0:   {
michael@0:     mElements.Clear();
michael@0:   }
michael@0: 
michael@0:   /**
michael@0:    * Provides readonly access to the queue elements as an array. Generally this
michael@0:    * should be avoided but may be needed in some situations such as when the
michael@0:    * elements contained in the queue need to be enumerated for cycle-collection.
michael@0:    * @return A pointer to the first element of the array.  If the array is
michael@0:    * empty, then this pointer must not be dereferenced.
michael@0:    */
michael@0:   const T* Elements() const
michael@0:   {
michael@0:     return mElements.Elements();
michael@0:   }
michael@0: 
michael@0: protected:
michael@0:   /**
michael@0:    * Swaps the elements at the specified indices.
michael@0:    */
michael@0:   void Swap(size_type aIndexA, size_type aIndexB)
michael@0:   {
michael@0:     T temp = mElements[aIndexA];
michael@0:     mElements[aIndexA] = mElements[aIndexB];
michael@0:     mElements[aIndexB] = temp;
michael@0:   }
michael@0: 
michael@0:   nsTArray<T> mElements;
michael@0:   Compare mCompare; // Comparator object
michael@0: };
michael@0: 
michael@0: #endif // NS_TPRIORITY_QUEUE_H_