From 90c370f0213a176676a1347b20e51f4190e81e08 Mon Sep 17 00:00:00 2001
From: Reiley Yang <reyang@microsoft.com>
Date: Sat, 15 Aug 2020 22:13:40 -0700
Subject: [PATCH] Refactor exporter - step 4 (#1085)

* add a circular buffer

* add memory barrier

* ++

* clean up

Co-authored-by: Mikel Blanchard <mblanchard@macrosssoftware.com>
---
 src/OpenTelemetry/Internal/CircularBuffer.cs | 135 +++++++++++++++++++
 1 file changed, 135 insertions(+)
 create mode 100644 src/OpenTelemetry/Internal/CircularBuffer.cs

diff --git a/src/OpenTelemetry/Internal/CircularBuffer.cs b/src/OpenTelemetry/Internal/CircularBuffer.cs
new file mode 100644
index 00000000000..c4f16f504f3
--- /dev/null
+++ b/src/OpenTelemetry/Internal/CircularBuffer.cs
@@ -0,0 +1,135 @@
+// <copyright file="CircularBuffer.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.Collections.Generic;
+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 CircularBuffer<T>
+        where T : class
+    {
+        private readonly int capacity;
+        private readonly T[] trait;
+        private long head = 0;
+        private long tail = 0;
+
+        public CircularBuffer(int capacity)
+        {
+            if (capacity <= 0)
+            {
+                throw new ArgumentOutOfRangeException(nameof(capacity));
+            }
+
+            this.capacity = capacity;
+            this.trait = new T[capacity];
+        }
+
+        public int Capacity
+        {
+            get
+            {
+                return this.capacity;
+            }
+        }
+
+        /// <summary>
+        /// Gets the number of items contained in the <see cref="CircularBuffer{T}"/>.
+        /// </summary>
+        public int Count
+        {
+            get
+            {
+                var tailSnapshot = this.tail;
+                return (int)(this.head - tailSnapshot);
+            }
+        }
+
+        /// <summary>
+        /// Attempts to add the specified item to the buffer.
+        /// </summary>
+        /// <param name="value">The value to add.</param>
+        /// <returns>Returns true if the item was added to the buffer successfully; false if the buffer is full.</returns>
+        public bool TryAdd(T value)
+        {
+            if (value == null)
+            {
+                throw new ArgumentNullException(nameof(value));
+            }
+
+            while (true)
+            {
+                var tailSnapshot = this.tail;
+                var headSnapshot = this.head;
+
+                if (headSnapshot - tailSnapshot >= this.capacity)
+                {
+                    return false; // buffer is full
+                }
+
+                var index = (int)(headSnapshot % this.capacity);
+
+                if (this.SwapIfNull(index, value))
+                {
+                    if (Interlocked.CompareExchange(ref this.head, headSnapshot + 1, headSnapshot) == headSnapshot)
+                    {
+                        return true;
+                    }
+
+                    this.trait[index] = null;
+                }
+            }
+        }
+
+        public IEnumerable<T> Consume(int count)
+        {
+            if (count <= 0)
+            {
+                yield break;
+            }
+
+            count = Math.Min(count, this.Count);
+
+            for (int i = 0; i < count; i++)
+            {
+                var index = (int)(this.tail % this.capacity);
+                var value = this.trait[index];
+                this.trait[index] = null;
+                this.tail++;
+                yield return value;
+            }
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private bool CompareAndSwap(int index, T value, T comparand)
+        {
+            var result = Interlocked.CompareExchange(ref this.trait[index], value, comparand);
+            return object.ReferenceEquals(result, comparand);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private bool SwapIfNull(int index, T value)
+        {
+            return this.CompareAndSwap(index, value, null);
+        }
+    }
+}