Reimplement RepeatedField<T> using an array as the backing store.

This is effectively reimplementing List<T>, but with a few advantages:
- We know that an empty repeated field is common, so don't allocate an array until we need to
- With direct access to the array, we can easily convert enum values to int without boxing
- We can relax the restrictions over what happens if the repeated field is modified while iterating, avoiding so much checking

This is somewhat risky, in that reimplementing a building block like this is *always* risky, but hey...
(The performance benefits are significant...)

1 files changed, 14 insertions, 9 deletions

diff --git a/csharp/src/ProtocolBuffers/CodedOutputStream.cs b/csharp/src/ProtocolBuffers/CodedOutputStream.cs
index dfcaf8a2..bc3ed7d7 100644
--- a/csharp/src/ProtocolBuffers/CodedOutputStream.cs
+++ b/csharp/src/ProtocolBuffers/CodedOutputStream.cs
@@ -743,10 +743,11 @@ namespace Google.Protobuf
             {

                 return;

             }

-            // TODO(jonskeet): Avoid the Cast call here. Work out a better mass "T to int" conversion.

-            foreach (int value in list.Cast<int>())

+            // Bit of a hack, to access the values as ints

+            var iterator = list.GetInt32Enumerator();

+            while (iterator.MoveNext())

             {

-                WriteEnum(fieldNumber, value);

+                WriteEnum(fieldNumber, iterator.Current);

             }

         }

 

@@ -956,15 +957,19 @@ namespace Google.Protobuf
             {

                 return;

             }

-            // Obviously, we'll want to get rid of this hack...

-            var temporaryHack = new RepeatedField<int>();

-            temporaryHack.Add(list.Cast<int>());

-            uint size = temporaryHack.CalculateSize(ComputeEnumSizeNoTag);

+            // Bit of a hack, to access the values as ints

+            var iterator = list.GetInt32Enumerator();

+            uint size = 0;

+            while (iterator.MoveNext())

+            {

+                size += (uint) ComputeEnumSizeNoTag(iterator.Current);

+            }

+            iterator.Reset();

             WriteTag(fieldNumber, WireFormat.WireType.LengthDelimited);

             WriteRawVarint32(size);

-            foreach (int value in temporaryHack)

+            while (iterator.MoveNext())

             {

-                WriteEnumNoTag(value);

+                WriteEnumNoTag(iterator.Current);

             }

         }