Optimize the delay delivery routing key calculation for throughput an…

…d fewer allocations (#1411)

* Use BitVector32 instead of BitArray to avoid array allocation and array copying for BitArray

* Use string.create instead

* Avoid fixed by using Unsafe.AsPointer

* More var usage

* Align constant naming because I can't unsee it

* Going back to make sure things are not getting moved

* Comments

* Tweaks

---------

Co-authored-by: danielmarbach <[email protected]>
Co-authored-by: Brandon Ording <[email protected]>

src/NServiceBus.Transport.RabbitMQ.CommandLine/NServiceBus.Transport.RabbitMQ.CommandLine.csproj

@@ -11,6 +11,7 @@
     <Nullable>enable</Nullable>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\NServiceBus.snk</AssemblyOriginatorKeyFile>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
 
   <ItemGroup>

src/NServiceBus.Transport.RabbitMQ/DelayedDelivery/DelayInfrastructure.cs

@@ -1,17 +1,17 @@
 namespace NServiceBus.Transport.RabbitMQ
 {
     using System;
-    using System.Collections;
     using System.Collections.Generic;
-    using System.Text;
+    using System.Collections.Specialized;
+    using System.Runtime.CompilerServices;
     using global::RabbitMQ.Client;
 
     static class DelayInfrastructure
     {
-        const int maxNumberOfBitsToUse = 28;
+        const int MaxNumberOfBitsToUse = 28;
 
-        public const int MaxLevel = maxNumberOfBitsToUse - 1;
-        public const int MaxDelayInSeconds = (1 << maxNumberOfBitsToUse) - 1;
+        public const int MaxLevel = MaxNumberOfBitsToUse - 1;
+        public const int MaxDelayInSeconds = (1 << MaxNumberOfBitsToUse) - 1;
         public const string DelayHeader = "NServiceBus.Transport.RabbitMQ.DelayInSeconds";
         public const string XDeathHeader = "x-death";
         public const string XFirstDeathExchangeHeader = "x-first-death-exchange";
@@ -78,30 +78,53 @@ public static void TearDown(IModel channel)
             }
         }
 
-        public static string CalculateRoutingKey(int delayInSeconds, string address, out int startingDelayLevel)
+        public static unsafe string CalculateRoutingKey(int delayInSeconds, string address, out int startingDelayLevel)
         {
             if (delayInSeconds < 0)
             {
                 delayInSeconds = 0;
             }
 
-            var bitArray = new BitArray(new[] { delayInSeconds });
-            var sb = new StringBuilder();
             startingDelayLevel = 0;
-
-            for (var level = MaxLevel; level >= 0; level--)
+            // Pinning the address of the startingDelayLevel so that we can safely write back to the address of the local.
+            // This trickery is done because when string.Create returns, there is no way to pass state outside of the lambda
+            // without doing a closure over a local variable, which would create DisplayClass allocations for every call.
+            fixed (int* pinnedStartingDelayLevel = &startingDelayLevel)
             {
-                if (startingDelayLevel == 0 && bitArray[level])
+                var startingDelayLevelPtr = (nint)pinnedStartingDelayLevel;
+
+                // The length of the string is determined by the max level, taking into account the number of dots, the address length
+                // and additional space since we are zero based.
+                return string.Create((2 * MaxLevel) + 2 + address.Length, (delayInSeconds, address, startingDelayLevelPtr), Action);
+
+                static void Action(Span<char> span, (int, string, nint) state)
                 {
-                    startingDelayLevel = level;
-                }
+                    var (delayInSeconds, address, startingDelayLevelPtr) = state;
 
-                sb.Append(bitArray[level] ? "1." : "0.");
-            }
+                    var startingDelayLevel = 0;
+                    var mask = BitVector32.CreateMask();
 
-            sb.Append(address);
+                    var bitVector = new BitVector32(delayInSeconds);
 
-            return sb.ToString();
+                    var index = 0;
+                    for (var level = MaxLevel; level >= 0; level--)
+                    {
+                        var flag = bitVector[mask << level];
+                        if (startingDelayLevel == 0 && flag)
+                        {
+                            startingDelayLevel = level;
+                        }
+
+                        span[index++] = flag ? '1' : '0';
+                        span[index++] = '.';
+                    }
+
+                    address.AsSpan().CopyTo(span[index..]);
+
+                    // Write back the startingDelayLevel to the address of the local
+                    Unsafe.Write(startingDelayLevelPtr.ToPointer(), startingDelayLevel);
+                }
+            }
         }
     }
 }

src/NServiceBus.Transport.RabbitMQ/NServiceBus.Transport.RabbitMQ.csproj

@@ -4,6 +4,7 @@
     <TargetFramework>net8.0</TargetFramework>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\NServiceBus.snk</AssemblyOriginatorKeyFile>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
 
   <ItemGroup>