Indexed Priority Queue (C# / Unity)
A Generic priority Queue with random access to its elements. I’ve used this frequently for A* pathfinding implementations, since it works well as a data structure for the “open list”. It allows for finding the next best visit-able node with the ease-of-use of typical priority queues, yet provides random access to update estimations at specific indexes.
Check out the Indexed Priority Queue from my open-source Unity utility library, Atlas, here. You can also see it in context in the A* implementation from Atlas, here.
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using System; using UnityEngine.Assertions; namespace Atlas { /// <summary> /// Generic priority queue data structure, providing random access to its elements /// </summary> /// <typeparam name="T">Type of contained elements</typeparam> public sealed class IndexedPriorityQueue<T> where T : IComparable<T> { #region public /// <summary> /// Current number of elements in the queue /// </summary> public int Count { get; private set; } /// <summary> /// Accesses the element at the given index /// </summary> /// <param name="index">Index of the element to access</param> /// <returns>The value at the given index</returns> public T this[int index] { get { Assert.IsTrue( index < m_objects.Length && index >= 0, string.Format( "IndexedPriorityQueue.[]: Index '{0}' out of range", index ) ); return m_objects[index]; } set { Assert.IsTrue( index < m_objects.Length && index >= 0, string.Format( "IndexedPriorityQueue.[]: Index '{0}' out of range", index ) ); Set( index, value ); } } /// <summary> /// Constructor /// </summary> public IndexedPriorityQueue() { Resize( 1 ); } /// <summary> /// Constructor /// </summary> /// <param name="maxSize">Max number of elements the queue can contain</param> public IndexedPriorityQueue( int maxSize ) { Resize( maxSize ); } /// <summary> /// Inserts a new value with the given index /// </summary> /// <param name="index">index to insert at</param> /// <param name="value">value to insert</param> public void Insert( int index, T value ) { Assert.IsTrue( index < m_objects.Length && index >= 0, string.Format( "IndexedPriorityQueue.Insert: Index '{0}' out of range", index ) ); ++Count; // add object m_objects[index] = value; // add to heap m_heapInverse[index] = Count; m_heap[Count] = index; // update heap SortHeapUpward( Count ); } /// <summary> /// Gets the top element of the queue /// </summary> /// <returns>The top element</returns> public T Top() { // top of heap [first element is 1, not 0] return m_objects[m_heap[1]]; } /// <summary> /// Removes the top element from the queue /// </summary> /// <returns>The removed element</returns> public T Pop() { Assert.IsTrue( Count > 0, "IndexedPriorityQueue.Pop: Queue is empty" ); if ( Count == 0 ) { return default( T ); } // swap front to back for removal Swap( 1, Count-- ); // re-sort heap SortHeapDownward( 1 ); // return popped object return m_objects[m_heap[Count + 1]]; } /// <summary> /// Updates the value at the given index. Note that this function is not /// as efficient as the DecreaseIndex/IncreaseIndex methods, but is /// best when the value at the index is not known /// </summary> /// <param name="index">Index of the value to set</param> /// <param name="newValue">New value</param> /// <remarks>This will cause either an upward or downard sort of the internal heap</remarks> public void Set( int index, T newValue ) { if ( newValue.CompareTo( m_objects[index] ) <= 0 ) { DecreaseValueAtIndex( index, newValue ); } else { IncreaseValueAtIndex( index, newValue ); } } /// <summary> /// Decreases the value at the current index to the given value /// </summary> /// <param name="index">Index to decrease value of</param> /// <param name="decreasedValue">New value</param> /// <remarks>This will cause an upward sort of the internal heap</remarks> public void DecreaseValueAtIndex( int index, T decreasedValue ) { Assert.IsTrue( index < m_objects.Length && index >= 0, string.Format( "IndexedPriorityQueue.DecreaseIndex: Index '{0}' out of range", index ) ); Assert.IsTrue( decreasedValue.CompareTo( m_objects[index] ) <= 0, string.Format( "IndexedPriorityQueue.DecreaseIndex: object '{0}' isn't less than current value '{1}'", decreasedValue, m_objects[index] ) ); m_objects[index] = decreasedValue; SortUpward( index ); } /// <summary> /// Increases the value at the current index to the given value /// </summary> /// <param name="index">Index to increase value of</param> /// <param name="increasedValue">New value</param> /// <remarks>This will cause a downward sort of the internal heap</remarks> public void IncreaseValueAtIndex( int index, T increasedValue ) { Assert.IsTrue( index < m_objects.Length && index >= 0, string.Format( "IndexedPriorityQueue.DecreaseIndex: Index '{0}' out of range", index ) ); Assert.IsTrue( increasedValue.CompareTo( m_objects[index] ) >= 0, string.Format( "IndexedPriorityQueue.DecreaseIndex: object '{0}' isn't greater than current value '{1}'", increasedValue, m_objects[index] ) ); m_objects[index] = increasedValue; SortDownward( index ); } /// <summary> /// Removes all elements from the queue /// </summary> public void Clear() { Count = 0; } /// <summary> /// Set the maximum capacity of the queue /// </summary> /// <param name="maxSize">The desired maximum capacity</param> public void Resize( int maxSize ) { Assert.IsTrue( maxSize >= 0, string.Format( "IndexedPriorityQueue.Resize: Invalid size '{0}'", maxSize ) ); m_objects = new T[maxSize]; m_heap = new int[maxSize + 1]; m_heapInverse = new int[maxSize]; Count = 0; } #endregion // public #region private private T[] m_objects; private int[] m_heap; private int[] m_heapInverse; private void SortUpward( int index ) { SortHeapUpward( m_heapInverse[index] ); } private void SortDownward( int index ) { SortHeapDownward( m_heapInverse[index] ); } private void SortHeapUpward( int heapIndex ) { // move toward top if better than parent while ( heapIndex > 1 && m_objects[m_heap[heapIndex]].CompareTo( m_objects[m_heap[Parent( heapIndex )]] ) < 0 ) { // swap this node with its parent Swap( heapIndex, Parent( heapIndex ) ); // reset iterator to be at parents old position // (child's new position) heapIndex = Parent( heapIndex ); } } private void SortHeapDownward( int heapIndex ) { // move node downward if less than children while ( FirstChild( heapIndex ) <= Count ) { int child = FirstChild( heapIndex ); // find smallest of two children (if 2 exist) if ( child < Count && m_objects[m_heap[child + 1]].CompareTo( m_objects[m_heap[child]] ) < 0 ) { ++child; } // swap with child if less if ( m_objects[m_heap[child]].CompareTo( m_objects[m_heap[heapIndex]] ) < 0 ) { Swap( child, heapIndex ); heapIndex = child; } // no swap necessary else { break; } } } private void Swap( int i, int j ) { // swap elements in heap int temp = m_heap[i]; m_heap[i] = m_heap[j]; m_heap[j] = temp; // reset inverses m_heapInverse[m_heap[i]] = i; m_heapInverse[m_heap[j]] = j; } private int Parent( int heapIndex ) { return ( heapIndex / 2 ); } private int FirstChild( int heapIndex ) { return ( heapIndex * 2 ); } private int SecondChild( int heapIndex ) { return ( heapIndex * 2 + 1 ); } #endregion // private } } |