Create BinaryHeap.cs

ZeroLevel.HNSW/Services/BinaryHeap.cs

@@ -0,0 +1,145 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace ZeroLevel.HNSW.Services
+{
+    /// <summary>
+    /// Binary heap wrapper around the <see cref="IList{T}"/>
+    /// It's a max-heap implementation i.e. the maximum element is always on top.
+    /// But the order of elements can be customized by providing <see cref="IComparer{T}"/> instance.
+    /// </summary>
+    /// <typeparam name="T">The type of the items in the source list.</typeparam>
+    public class BinaryHeap<T>
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="BinaryHeap{T}"/> class.
+        /// </summary>
+        /// <param name="buffer">The buffer to store heap items.</param>
+        public BinaryHeap(IList<T> buffer)
+            : this(buffer, Comparer<T>.Default)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="BinaryHeap{T}"/> class.
+        /// </summary>
+        /// <param name="buffer">The buffer to store heap items.</param>
+        /// <param name="comparer">The comparer which defines order of items.</param>
+        public BinaryHeap(IList<T> buffer, IComparer<T> comparer)
+        {
+            if (buffer == null)
+            {
+                throw new ArgumentNullException(nameof(buffer));
+            }
+
+            this.Buffer = buffer;
+            this.Comparer = comparer;
+            for (int i = 1; i < this.Buffer.Count; ++i)
+            {
+                this.SiftUp(i);
+            }
+        }
+
+        /// <summary>
+        /// Gets the heap comparer.
+        /// </summary>
+        public IComparer<T> Comparer { get; private set; }
+
+        /// <summary>
+        /// Gets the buffer of the heap.
+        /// </summary>
+        public IList<T> Buffer { get; private set; }
+
+        /// <summary>
+        /// Pushes item to the heap.
+        /// </summary>
+        /// <param name="item">The item to push.</param>
+        public void Push(T item)
+        {
+            this.Buffer.Add(item);
+            this.SiftUp(this.Buffer.Count - 1);
+        }
+
+        /// <summary>
+        /// Pops the item from the heap.
+        /// </summary>
+        /// <returns>The popped item.</returns>
+        public T Pop()
+        {
+            if (this.Buffer.Any())
+            {
+                var result = this.Buffer.First();
+
+                this.Buffer[0] = this.Buffer.Last();
+                this.Buffer.RemoveAt(this.Buffer.Count - 1);
+                this.SiftDown(0);
+
+                return result;
+            }
+
+            throw new InvalidOperationException("Heap is empty");
+        }
+
+        /// <summary>
+        /// Restores the heap property starting from i'th position down to the bottom
+        /// given that the downstream items fulfill the rule.
+        /// </summary>
+        /// <param name="i">The position of item where heap property is violated.</param>
+        private void SiftDown(int i)
+        {
+            while (i < this.Buffer.Count)
+            {
+                int l = (2 * i) + 1;
+                int r = l + 1;
+                if (l >= this.Buffer.Count)
+                {
+                    break;
+                }
+
+                int m = r < this.Buffer.Count && this.Comparer.Compare(this.Buffer[l], this.Buffer[r]) < 0 ? r : l;
+                if (this.Comparer.Compare(this.Buffer[m], this.Buffer[i]) <= 0)
+                {
+                    break;
+                }
+
+                this.Swap(i, m);
+                i = m;
+            }
+        }
+
+        /// <summary>
+        /// Restores the heap property starting from i'th position up to the head
+        /// given that the upstream items fulfill the rule.
+        /// </summary>
+        /// <param name="i">The position of item where heap property is violated.</param>
+        private void SiftUp(int i)
+        {
+            while (i > 0)
+            {
+                int p = (i - 1) / 2;
+                if (this.Comparer.Compare(this.Buffer[i], this.Buffer[p]) <= 0)
+                {
+                    break;
+                }
+
+                this.Swap(i, p);
+                i = p;
+            }
+        }
+
+        /// <summary>
+        /// Swaps items with the specified indicies.
+        /// </summary>
+        /// <param name="i">The first index.</param>
+        /// <param name="j">The second index.</param>
+        private void Swap(int i, int j)
+        {
+            var temp = this.Buffer[i];
+            this.Buffer[i] = this.Buffer[j];
+            this.Buffer[j] = temp;
+        }
+    }
+}