Special case merging ArrayTagSets and TagLists (#986)

Special case merging an ArrayTagSet with another to avoid the
overhead of sorting, deduping, and copying. Since an ArrayTagSet
is immutable, already sorted, and deduped, we can skip directly
to merging..

Additionally, add handling for merging of an ArrayTagSet with a
TagList, where we can avoid some extra copying and allocating of
Tags.

spectator-api/src/main/java/com/netflix/spectator/api/ArrayTagSet.java

@@ -22,6 +22,7 @@
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.concurrent.ConcurrentMap;
 import java.util.function.BiPredicate;
 import java.util.function.Predicate;
@@ -150,6 +151,11 @@ ArrayTagSet addAll(Iterable<Tag> ts) {
         checkForNullValues(newTags);
         return addAll(newTags, newTags.length);
       }
+    } else if (ts instanceof TagList) {
+      if (ts instanceof ArrayTagSet) {
+        return addAll((ArrayTagSet) ts);
+      }
+     return addAll((TagList) ts);
     } else {
       List<Tag> data = new ArrayList<>();
       for (Tag t : ts) {
@@ -210,6 +216,33 @@ private ArrayTagSet addAll(String[] ts, int tsLength) {
     }
   }
 
+  private ArrayTagSet addAll(ArrayTagSet ts) {
+    if (ts == this || ts.isEmpty()) {
+      return this;
+    }
+    if (isEmpty()) {
+      return ts;
+    }
+    String[] newTags = new String[length + ts.length];
+    int newLength = merge(newTags, tags, length, ts.tags, ts.length);
+    return new ArrayTagSet(newTags, newLength);
+  }
+
+  private ArrayTagSet addAll(TagList ts) {
+    int size = ts.size();
+
+    if (size == 0) {
+      return this;
+    }
+    String[] newTags = new String[size * 2];
+    int j = 0;
+    for (int i = 0; i < size; i++) {
+      newTags[j++] = Objects.requireNonNull(ts.getKey(i), "tag keys cannot be null");
+      newTags[j++] = Objects.requireNonNull(ts.getValue(i), "tag values cannot be null");
+    }
+    return addAll(newTags, newTags.length);
+  }
+
   /** Add a collection of tags to the set. */
   ArrayTagSet addAll(Tag[] ts) {
     return addAll(ts, ts.length);

spectator-api/src/test/java/com/netflix/spectator/api/ArrayTagSetTest.java

@@ -226,6 +226,27 @@ public void testMergeTag() {
     Assertions.assertEquals(expected, initial.addAll(update));
   }
 
+  @Test
+  public void testMergeWithSelf() {
+    ArrayTagSet tags = ArrayTagSet.create("k1", "v1");
+    ArrayTagSet updated = tags.addAll(tags);
+    Assertions.assertSame(tags, updated);
+  }
+
+  @Test
+  public void testMergeWithEmpty() {
+    ArrayTagSet tags = ArrayTagSet.create("k1", "v1");
+    ArrayTagSet updated = tags.addAll(ArrayTagSet.EMPTY);
+    Assertions.assertSame(tags, updated);
+  }
+
+  @Test
+  public void testEmptyMergeWithNonEmpty() {
+    ArrayTagSet tags = ArrayTagSet.create("k1", "v1");
+    ArrayTagSet updated = ArrayTagSet.EMPTY.addAll(tags);
+    Assertions.assertSame(tags, updated);
+  }
+
   @Test
   public void testMergeTagWithSameKey() {
     ArrayTagSet initial = ArrayTagSet.create("k1", "v1");
@@ -503,4 +524,53 @@ public void compareToDifferentSizes() {
     Assertions.assertEquals(-1, a.compareTo(b));
     Assertions.assertEquals(1, b.compareTo(a));
   }
+
+  @Test
+  public void mergeTagList() {
+    TagList tagList = new TagList() {
+      @Override
+      public String getKey(int i) {
+        return "k" + ++i;
+      }
+
+      @Override
+      public String getValue(int i) {
+        return "v" + ++i;
+      }
+
+      @Override
+      public int size() {
+        return 3;
+      }
+    };
+    ArrayTagSet tags = ArrayTagSet.create("k1", "v1", "k2", "v2");
+    ArrayTagSet updated = tags.addAll(tagList);
+    ArrayTagSet expected = ArrayTagSet.create("k1", "v1", "k2", "v2", "k3", "v3");
+    Assertions.assertEquals(expected, updated);
+  }
+
+  @Test
+  public void mergeEmptyTagList() {
+    TagList empty = new TagList() {
+      @Override
+      public String getKey(int i) {
+        throw new IndexOutOfBoundsException();
+      }
+
+      @Override
+      public String getValue(int i) {
+        throw new IndexOutOfBoundsException();
+      }
+
+      @Override
+      public int size() {
+        return 0;
+      }
+    };
+
+    ArrayTagSet tags = ArrayTagSet.create("k1", "v1", "k2", "v2");
+    ArrayTagSet updated = tags.addAll(empty);
+
+    Assertions.assertSame(tags, updated);
+  }
 }