Add a value type version of CircularBuffer

src/OpenTelemetry/Internal/CircularBuffer.cs

@@ -174,7 +174,7 @@ public T Read()
             var index = (int)(this.tail % this.Capacity);
             while (true)
             {
-                T value = this.trait[index];
+                var value = this.trait[index];
                 if (value == null)
                 {
                     // If we got here it means a writer isn't done.

src/OpenTelemetry/Internal/CircularBufferStruct.cs

@@ -0,0 +1,190 @@
+// <copyright file="CircularBufferStruct.cs" company="OpenTelemetry Authors">
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// </copyright>
+
+using System;
+using System.Runtime.CompilerServices;
+using System.Threading;
+
+namespace OpenTelemetry.Internal
+{
+    /// <summary>
+    /// Lock-free implementation of single-reader multi-writer circular buffer.
+    /// </summary>
+    /// <typeparam name="T">The type of the underlying value.</typeparam>
+    internal class CircularBufferStruct<T>
+        where T : struct
+    {
+        private readonly Trait[] trait;
+        private long head;
+        private long tail;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="CircularBufferStruct{T}"/> class.
+        /// </summary>
+        /// <param name="capacity">The capacity of the circular buffer, must be a positive integer.</param>
+        public CircularBufferStruct(int capacity)
+        {
+            if (capacity <= 0)
+            {
+                throw new ArgumentOutOfRangeException(nameof(capacity));
+            }
+
+            this.Capacity = capacity;
+            this.trait = new Trait[capacity];
+        }
+
+        /// <summary>
+        /// Gets the capacity of the <see cref="CircularBufferStruct{T}"/>.
+        /// </summary>
+        public int Capacity { get; }
+
+        /// <summary>
+        /// Gets the number of items contained in the <see cref="CircularBufferStruct{T}"/>.
+        /// </summary>
+        public int Count
+        {
+            get
+            {
+                var tailSnapshot = this.tail;
+                return (int)(this.head - tailSnapshot);
+            }
+        }
+
+        /// <summary>
+        /// Gets the number of items added to the <see cref="CircularBufferStruct{T}"/>.
+        /// </summary>
+        public long AddedCount => this.head;
+
+        /// <summary>
+        /// Gets the number of items removed from the <see cref="CircularBufferStruct{T}"/>.
+        /// </summary>
+        public long RemovedCount => this.tail;
+
+        /// <summary>
+        /// Adds the specified item to the buffer.
+        /// </summary>
+        /// <param name="value">The value to add.</param>
+        /// <returns>
+        /// Returns <c>true</c> if the item was added to the buffer successfully;
+        /// <c>false</c> if the buffer is full.
+        /// </returns>
+        public bool Add(T value)
+        {
+            while (true)
+            {
+                var tailSnapshot = this.tail;
+                var headSnapshot = this.head;
+
+                if (headSnapshot - tailSnapshot >= this.Capacity)
+                {
+                    return false; // buffer is full
+                }
+
+                var head = Interlocked.CompareExchange(ref this.head, headSnapshot + 1, headSnapshot);
+                if (head != headSnapshot)
+                {
+                    continue;
+                }
+
+                var index = (int)(head % this.Capacity);
+                this.trait[index].Value = value;
+                this.trait[index].IsReady = true;
+                return true;
+            }
+        }
+
+        /// <summary>
+        /// Attempts to add the specified item to the buffer.
+        /// </summary>
+        /// <param name="value">The value to add.</param>
+        /// <param name="maxSpinCount">The maximum allowed spin count, when set to a negative number or zero, will spin indefinitely.</param>
+        /// <returns>
+        /// Returns <c>true</c> if the item was added to the buffer successfully;
+        /// <c>false</c> if the buffer is full or the spin count exceeded <paramref name="maxSpinCount"/>.
+        /// </returns>
+        public bool TryAdd(T value, int maxSpinCount)
+        {
+            if (maxSpinCount <= 0)
+            {
+                return this.Add(value);
+            }
+
+            var spinCountDown = maxSpinCount;
+
+            while (true)
+            {
+                var tailSnapshot = this.tail;
+                var headSnapshot = this.head;
+
+                if (headSnapshot - tailSnapshot >= this.Capacity)
+                {
+                    return false; // buffer is full
+                }
+
+                var head = Interlocked.CompareExchange(ref this.head, headSnapshot + 1, headSnapshot);
+                if (head != headSnapshot)
+                {
+                    if (spinCountDown-- == 0)
+                    {
+                        return false; // exceeded maximum spin count
+                    }
+
+                    continue;
+                }
+
+                var index = (int)(head % this.Capacity);
+                this.trait[index].Value = value;
+                this.trait[index].IsReady = true;
+                return true;
+            }
+        }
+
+        /// <summary>
+        /// Reads an item from the <see cref="CircularBufferStruct{T}"/>.
+        /// </summary>
+        /// <remarks>
+        /// This function is not reentrant-safe, only one reader is allowed at any given time.
+        /// Warning: There is no bounds check in this method. Do not call unless you have verified Count > 0.
+        /// </remarks>
+        /// <returns>Item read.</returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public T Read()
+        {
+            var index = (int)(this.tail % this.Capacity);
+            while (true)
+            {
+                if (!this.trait[index].IsReady)
+                {
+                    // If we got here it means a writer isn't done.
+                    continue;
+                }
+
+                // TODO: we are doing an extra copy from the buffer, this can be optimized if Read() could take a callback
+                var value = this.trait[index].Value;
+                this.trait[index].IsReady = false;
+                this.trait[index].Value = default(T);
+                this.tail++;
+                return value;
+            }
+        }
+
+        private struct Trait
+        {
+            internal bool IsReady;
+            internal T Value;
+        }
+    }
+}