Avoid map resizing in PrivateViewConfig

Properly size the LinkedHashMaps in the createState method to avoid rehashing / resizing.
Additionally, if instrumentation is not enabled, just use the singleton empty map.

To facilitate this change, add an Iterables utility class with a "size" helper to determine
the size of the passed in Iterable; this is necessary because the Config::keys method returns
Iterable, but in practice is typically a Collection.

Finally, add a marker annotation, Internal, to indicate classes such as Maps and Iterables that
are not meant to be used by users of Archaius and may not have stable APIs.

archaius2-core/src/main/java/com/netflix/archaius/Internal.java

@@ -0,0 +1,20 @@
+package com.netflix.archaius;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import static java.lang.annotation.ElementType.CONSTRUCTOR;
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.ElementType.PACKAGE;
+import static java.lang.annotation.ElementType.TYPE;
+
+/**
+ * Annotation to convey classes and other elements not meant to be part of the public API, but are public by necessity;
+ * such elements should not be depended on by consumers as there is no guarantee of API stability.
+ */
+@Retention(RetentionPolicy.RUNTIME)
+@Target(value = {CONSTRUCTOR, METHOD, TYPE, FIELD, PACKAGE})
+public @interface Internal {
+}

archaius2-core/src/main/java/com/netflix/archaius/config/PrivateViewConfig.java

@@ -15,14 +15,16 @@
  */
 package com.netflix.archaius.config;
 
-import java.util.LinkedHashMap;
+import java.util.Collections;
 import java.util.Map;
 import java.util.Map.Entry;
 
 import com.netflix.archaius.api.Config;
 import com.netflix.archaius.api.ConfigListener;
 import com.netflix.archaius.api.Decoder;
 import com.netflix.archaius.api.StrInterpolator;
+import com.netflix.archaius.util.Iterables;
+import com.netflix.archaius.util.Maps;
 
 /**
  * View into another Config that allows usage of a private {@link Decoder}, {@link StrInterpolator}, and
@@ -68,9 +70,11 @@ private void updateState(Config config) {
     }
 
     private CachedState createState(Config config) {
-        Map<String, Object> data = new LinkedHashMap<>();
-        Map<String, Config> instrumentedKeys = new LinkedHashMap<>();
         boolean instrumented = config.instrumentationEnabled();
+        int size = Iterables.size(config.keys());
+        final Map<String, Object> data = Maps.newLinkedHashMap(size);
+        final Map<String, Config> instrumentedKeys = instrumented ? Maps.newHashMap(size) : Collections.emptyMap();
+
         config.forEachPropertyUninstrumented((k, v) -> {
             data.put(k, v);
             if (instrumented) {

archaius2-core/src/main/java/com/netflix/archaius/util/Iterables.java

@@ -0,0 +1,24 @@
+package com.netflix.archaius.util;
+
+import com.netflix.archaius.Internal;
+
+import java.util.Collection;
+
+@Internal
+public final class Iterables {
+    private Iterables() {}
+
+    /**
+     * Returns the number of elements in {@code iterable}.
+     */
+    public static int size(Iterable<?> iterable) {
+        if (iterable instanceof Collection<?>) {
+            return ((Collection<?>) iterable).size();
+        }
+        int size = 0;
+        for (Object ignored : iterable) {
+            size++;
+        }
+        return size;
+    }
+}

archaius2-core/src/main/java/com/netflix/archaius/util/Maps.java

@@ -1,8 +1,11 @@
 package com.netflix.archaius.util;
 
+import com.netflix.archaius.Internal;
+
 import java.util.HashMap;
 import java.util.LinkedHashMap;
 
+@Internal
 public final class Maps {
     private Maps() {}
 

archaius2-core/src/test/java/com/netflix/archaius/util/IterablesTest.java

@@ -0,0 +1,22 @@
+package com.netflix.archaius.util;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
+import org.junit.jupiter.api.Test;
+
+import java.util.Collections;
+import java.util.stream.Stream;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class IterablesTest {
+    @Test
+    public void testSize() {
+        assertEquals(0, Iterables.size(ImmutableList.of()));
+        assertEquals(1, Iterables.size(ImmutableList.of(1)));
+        assertEquals(2, Iterables.size(ImmutableSet.of(1, 2)));
+        assertEquals(0, Iterables.size(Collections::emptyIterator));
+        assertEquals(1, Iterables.size(() -> Stream.<Object>of("foo").iterator()));
+        assertEquals(3, Iterables.size(() -> ImmutableList.<Object>of(1, 2, 3).iterator()));
+    }
+}