From 06f8423b5ac4661189cd9877889b9c4c7a403477 Mon Sep 17 00:00:00 2001
From: Aaron Stannard <aaron@petabridge.com>
Date: Fri, 30 Aug 2024 16:39:49 -0500
Subject: [PATCH] Mqtt311Decoder: use ImmutableList.Builder

Fewer object allocations during packet parsing this way
---
 src/TurboMqtt/Protocol/Mqtt311Decoder.cs | 41 +++++++++++++++---------
 1 file changed, 25 insertions(+), 16 deletions(-)

diff --git a/src/TurboMqtt/Protocol/Mqtt311Decoder.cs b/src/TurboMqtt/Protocol/Mqtt311Decoder.cs
index 27ed79a7..24c8f38d 100644
--- a/src/TurboMqtt/Protocol/Mqtt311Decoder.cs
+++ b/src/TurboMqtt/Protocol/Mqtt311Decoder.cs
@@ -19,7 +19,7 @@ public class Mqtt311Decoder
     private ReadOnlyMemory<byte> _remainder = ReadOnlyMemory<byte>.Empty;
 
     public bool TryDecode(in ReadOnlyMemory<byte> additionalData, out ImmutableList<MqttPacket> packets)
-    {
+    { 
         packets = ImmutableList<MqttPacket>.Empty;
         var rValue = false;
 
@@ -43,6 +43,8 @@ public bool TryDecode(in ReadOnlyMemory<byte> additionalData, out ImmutableList<
             _remainder = workingBuffer;
             return false;
         }
+        
+        var packetBuilder = ImmutableList.CreateBuilder<MqttPacket>();
 
         while (workingBuffer.Length > 0)
         {
@@ -59,13 +61,18 @@ public bool TryDecode(in ReadOnlyMemory<byte> additionalData, out ImmutableList<
 
             // extract packet size (the packet span will automatically advance past the size header)
             if (!TryGetPacketLength(ref currentPacket, out packetSize))
-                return rValue; // we need more data to decode the packet size
+            {
+                packets = packetBuilder.ToImmutable();
+                return rValue;
+            }
+                
 
             // check to see if we have enough data to decode the packet
             if (currentPacket.Length < packetSize)
             {
                 // save the remainder
                 _remainder = workingBuffer;
+                packets = packetBuilder.ToImmutable();
                 return rValue;
             }
 
@@ -89,67 +96,67 @@ public bool TryDecode(in ReadOnlyMemory<byte> additionalData, out ImmutableList<
                 {
                     case MqttPacketType.Publish:
                     {
-                        packets = packets.Add(DecodePublish(ref bufferForMsg, packetSize, headerLength));
+                        packetBuilder.Add(DecodePublish(ref bufferForMsg, packetSize, headerLength));
                         break;
                     }
                     case MqttPacketType.PubAck:
                     {
-                        packets = packets.Add(DecodePubAck(ref bufferForMsg, packetSize, headerLength));
+                        packetBuilder.Add(DecodePubAck(ref bufferForMsg, packetSize, headerLength));
                         break;
                     }
                     case MqttPacketType.PubRec:
                     {
-                        packets = packets.Add(DecodePubRec(ref bufferForMsg, packetSize, headerLength));
+                        packetBuilder.Add(DecodePubRec(ref bufferForMsg, packetSize, headerLength));
                         break;
                     }
                     case MqttPacketType.PubRel:
                     {
-                        packets = packets.Add(DecodePubRel(ref bufferForMsg, packetSize, headerLength));
+                        packetBuilder.Add(DecodePubRel(ref bufferForMsg, packetSize, headerLength));
                         break;
                     }
                     case MqttPacketType.PubComp:
                     {
-                        packets = packets.Add(DecodePubComp(ref bufferForMsg, packetSize, headerLength));
+                        packetBuilder.Add(DecodePubComp(ref bufferForMsg, packetSize, headerLength));
                         break;
                     }
                     case MqttPacketType.PingReq:
-                        packets = packets.Add(PingReqPacket.Instance);
+                        packetBuilder.Add(PingReqPacket.Instance);
                         break;
                     case MqttPacketType.PingResp:
-                        packets = packets.Add(PingRespPacket.Instance);
+                        packetBuilder.Add(PingRespPacket.Instance);
                         break;
                     case MqttPacketType.Connect:
                     {
-                        packets = packets.Add(DecodeConnect(ref bufferForMsg, packetSize, headerLength));
+                        packetBuilder.Add(DecodeConnect(ref bufferForMsg, packetSize, headerLength));
                         break;
                     }
                     case MqttPacketType.ConnAck:
                     {
-                        packets = packets.Add(DecodeConnAck(ref bufferForMsg, packetSize, headerLength));
+                        packetBuilder.Add(DecodeConnAck(ref bufferForMsg, packetSize, headerLength));
                         break;
                     }
                     case MqttPacketType.SubAck:
                     {
-                        packets = packets.Add(DecodeSubAck(ref bufferForMsg, packetSize, headerLength));
+                        packetBuilder.Add(DecodeSubAck(ref bufferForMsg, packetSize, headerLength));
                         break;
                     }
                     case MqttPacketType.Subscribe:
                     {
-                        packets = packets.Add(DecodeSubscribe(ref bufferForMsg, packetSize, headerLength));
+                        packetBuilder.Add(DecodeSubscribe(ref bufferForMsg, packetSize, headerLength));
                         break;
                     }
                     case MqttPacketType.Unsubscribe:
                     {
-                        packets = packets.Add(DecodeUnsubscribe(ref bufferForMsg, packetSize, headerLength));
+                        packetBuilder.Add(DecodeUnsubscribe(ref bufferForMsg, packetSize, headerLength));
                         break;
                     }
                     case MqttPacketType.UnsubAck:
                     {
-                        packets = packets.Add(DecodeUnsubAck(ref bufferForMsg, packetSize, headerLength));
+                        packetBuilder.Add(DecodeUnsubAck(ref bufferForMsg, packetSize, headerLength));
                         break;
                     }
                     case MqttPacketType.Disconnect:
-                        packets = packets.Add(DecodeDisconnect(ref bufferForMsg, packetSize, headerLength));
+                        packetBuilder.Add(DecodeDisconnect(ref bufferForMsg, packetSize, headerLength));
                         break;
                     case MqttPacketType.Auth: // MQTT 5.0 only - should throw an exception if we see this
                         throw new NotSupportedException("MQTT 5.0 packets are not supported.");
@@ -170,6 +177,8 @@ public bool TryDecode(in ReadOnlyMemory<byte> additionalData, out ImmutableList<
                 throw new MqttDecoderException($"Error decoding packet of predicted size [{headerLength + packetSize}]", ex, MqttProtocolVersion.V3_1_1, packetType);
             }
         }
+        
+        packets = packetBuilder.ToImmutable();
 
         return rValue;
     }