Add short-cut path taking when the paremter type is equal to own type

Change-Id: I0edceb6630e0312ce5139d3e453a183a0cf06fd5

@@ -87,7 +87,13 @@ final class EncodableDiscreteResources implements DiscreteResources {
 
     @Override
     public DiscreteResources difference(DiscreteResources other) {
-        return of(parent, Sets.difference(values(), other.values()));
+        if (other instanceof EncodableDiscreteResources) {
+            return of(parent, Sets.difference(this.values(), other.values()));
+        } else if (other instanceof EmptyDiscreteResources) {
+            return this;
+        }
+
+        return DiscreteResources.of(Sets.difference(values(), other.values()));
     }
 
     @Override

@@ -59,7 +59,13 @@ final class GenericDiscreteResources implements DiscreteResources {
 
     @Override
     public DiscreteResources difference(DiscreteResources other) {
+        if (other instanceof GenericDiscreteResources) {
             return of(Sets.difference(this.values(), other.values()));
+        } else if (other instanceof EmptyDiscreteResources) {
+            return this;
+        }
+
+        return DiscreteResources.of(Sets.difference(this.values(), other.values()));
     }
 
     @Override

@@ -67,6 +67,15 @@ final class UnifiedDiscreteResources implements DiscreteResources {
 
     @Override
     public DiscreteResources difference(DiscreteResources other) {
+        if (other instanceof UnifiedDiscreteResources) {
+            UnifiedDiscreteResources cast = (UnifiedDiscreteResources) other;
+            return new UnifiedDiscreteResources(
+                    this.generics.difference(cast.generics),
+                    this.encodables.difference(cast.encodables));
+        } else if (other instanceof EmptyDiscreteResources) {
+            return this;
+        }
+
         return of(Sets.difference(this.values(), other.values()));
     }