Query performance: use a custom Iterator impl

For queries that primarily list the targets in a package or set of
packages, this can be on the critical path. I've seen significant
performance improvements with this change (after a number of other
fixes), and the implementation is not significantly more complex than
before, so I'm calling it a net win.

Also add some basic tests for the values() collection.

PiperOrigin-RevId: 249656957

src/main/java/com/google/devtools/build/lib/collect/ImmutableSortedKeyMap.java

@@ -17,7 +17,6 @@
 import static java.util.stream.Collectors.joining;
 
 import com.google.common.base.Preconditions;
-import com.google.common.collect.AbstractIterator;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Streams;
 import java.util.AbstractCollection;
@@ -28,6 +27,7 @@
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.Map;
+import java.util.NoSuchElementException;
 import java.util.Set;
 import java.util.stream.Stream;
 import javax.annotation.Nullable;
@@ -153,13 +153,18 @@ public Iterator<V> iterator() {
       if (isEmpty()) {
         return Collections.emptyIterator();
       }
-      return new AbstractIterator<V>() {
+      return new Iterator<V>() {
         private int currentIndex = 0;
 
         @Override
-        protected V computeNext() {
+        public boolean hasNext() {
+          return currentIndex < values.length;
+        }
+
+        @Override
+        public V next() {
           if (currentIndex >= values.length) {
-            return endOfData();
+            throw new NoSuchElementException();
           }
           return values[currentIndex++];
         }

src/test/java/com/google/devtools/build/lib/collect/ImmutableSortedKeyMapTest.java

@@ -20,6 +20,7 @@
 import com.google.common.testing.NullPointerTester;
 import java.io.Serializable;
 import java.util.Collections;
+import java.util.Iterator;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import org.junit.Test;
@@ -249,4 +250,26 @@ public void toStringTest() {
     map = ImmutableSortedKeyMap.of();
     assertThat(map.toString()).isEqualTo("{}");
   }
+
+  @Test
+  public void emptyValuesCollectionTest() {
+    Map<String, Integer> map = ImmutableSortedKeyMap.of();
+    assertThat(map.values().size()).isEqualTo(0);
+    assertThat(map.values()).containsExactly();
+    Iterator<Integer> it = map.values().iterator();
+    assertThat(it.hasNext()).isFalse();
+  }
+
+  @Test
+  public void valuesCollectionTest() {
+    Map<String, Integer> map = ImmutableSortedKeyMap.of("a", 1, "b", 2);
+    assertThat(map.values().size()).isEqualTo(2);
+    assertThat(map.values()).containsExactly(1, 2);
+    Iterator<Integer> it = map.values().iterator();
+    assertThat(it.hasNext()).isTrue();
+    assertThat(it.next()).isEqualTo(1);
+    assertThat(it.hasNext()).isTrue();
+    assertThat(it.next()).isEqualTo(2);
+    assertThat(it.hasNext()).isFalse();
+  }
 }