Performance tweek for reading entire unpacked array

src/ProtocolBuffers/CodedInputStream.cs

@@ -67,6 +67,9 @@ namespace Google.ProtocolBuffers
         private readonly Stream input;
         private uint lastTag = 0;
 
+        private uint nextTag = 0;
+        private bool hasNextTag = false;
+
         internal const int DefaultRecursionLimit = 64;
         internal const int DefaultSizeLimit = 64 << 20; // 64MB
         public const int BufferSize = 4096;
@@ -163,6 +166,26 @@ namespace Google.ProtocolBuffers
 
         #region Reading of tags etc
         
+        /// <summary>
+        /// Attempt to peek at the next field tag.
+        /// </summary>
+        [CLSCompliant(false)]
+        public bool PeekNextTag(out uint fieldTag, out string fieldName)
+        {
+            if (hasNextTag)
+            {
+                fieldName = null;
+                fieldTag = nextTag;
+                return true;
+            }
+
+            uint savedLast = lastTag;
+            hasNextTag = ReadTag(out nextTag, out fieldName);
+            lastTag = savedLast;
+            fieldTag = nextTag;
+            return hasNextTag;
+        }
+
         /// <summary>
         /// Attempt to read a field tag, returning false if we have reached the end
         /// of the input data.
@@ -183,6 +206,13 @@ namespace Google.ProtocolBuffers
         {
             fieldName = null;
 
+            if (hasNextTag)
+            {
+                lastTag = fieldTag = nextTag;
+                hasNextTag = false;
+                return true;
+            }
+
             if (IsAtEnd)
             {
                 lastTag = fieldTag = 0;
@@ -483,6 +513,24 @@ namespace Google.ProtocolBuffers
             return true;
         }
 
+        /// <summary>
+        /// Returns true if the next tag is also part of the same unpacked array
+        /// </summary>
+        private bool ContinueArray(uint currentTag)
+        {
+            string ignore;
+            uint next;
+            if (PeekNextTag(out next, out ignore))
+            {
+                if (next == currentTag)
+                {
+                    hasNextTag = false;
+                    return true;
+                }
+            }
+            return false;
+        }
+
         [CLSCompliant(false)]
         public void ReadPrimitiveArray<T>(FieldType fieldType, uint fieldTag, string fieldName, ICollection<T> list)
         {
@@ -505,9 +553,13 @@ namespace Google.ProtocolBuffers
             else
             {
                 Object value = null;
+                do
+                {
                     if (ReadPrimitiveField(fieldType, ref value))
                         list.Add((T)value);
                 }
+                while (ContinueArray(fieldTag));
+            }
         }
 
         [CLSCompliant(false)]
@@ -537,11 +589,19 @@ namespace Google.ProtocolBuffers
                 PopLimit(limit);
             }
             else
+            {
+                do
                 {
                     if (ReadEnum(ref value, out unkval, mapping))
                         list.Add(value);
                     else
-                    unknown = new object[] { unkval };
+                    {
+                        if (unknown == null)
+                            unknown = new List<object>();
+                        unknown.Add(unkval);
+                    }
+                }
+                while (ContinueArray(fieldTag));
             }
         }
 
@@ -573,29 +633,45 @@ namespace Google.ProtocolBuffers
                 PopLimit(limit);
             }
             else
+            {
+                do
                 {
                     if (ReadEnum(ref value, out unkval))
                         list.Add(value);
                     else
-                    unknown = new object[] { unkval };
+                    {
+                        if (unknown == null)
+                            unknown = new List<object>();
+                        unknown.Add(unkval);
+                    }
+                }
+                while (ContinueArray(fieldTag));
             }
         }
 
         [CLSCompliant(false)]
         public void ReadMessageArray<T>(uint fieldTag, string fieldName, ICollection<T> list, T messageType, ExtensionRegistry registry) where T : IMessageLite
+        {
+            do
             {
                 IBuilderLite builder = messageType.WeakCreateBuilderForType();
                 ReadMessage(builder, registry);
                 list.Add((T)builder.WeakBuildPartial());
             }
+            while (ContinueArray(fieldTag));
+        }
 
         [CLSCompliant(false)]
         public void ReadGroupArray<T>(uint fieldTag, string fieldName, ICollection<T> list, T messageType, ExtensionRegistry registry) where T : IMessageLite
+        {
+            do
             {
                 IBuilderLite builder = messageType.WeakCreateBuilderForType();
                 ReadGroup(WireFormat.GetTagFieldNumber(fieldTag), builder, registry);
                 list.Add((T)builder.WeakBuildPartial());
             }
+            while (ContinueArray(fieldTag));
+        }
 
         /// <summary>
         /// Reads a field of any primitive type. Enums, groups and embedded