1. Add code samples to comments

2. Updated Null/Nan semantics for online execution
3. Validate for duplicated aliases
4. Add new functions

Author: wu-hui

firebase-firestore/api.txt

@@ -400,12 +400,8 @@ public void testCannotPerformMoreThanMaxAggregations() {
 
     assertThat(e, instanceOf(FirebaseFirestoreException.class));
     FirebaseFirestoreException firestoreException = (FirebaseFirestoreException) e;
-    if (isRunningAgainstEmulator()) {
-      assertEquals(FirebaseFirestoreException.Code.UNKNOWN, firestoreException.getCode());
-    } else {
-      assertEquals(FirebaseFirestoreException.Code.INVALID_ARGUMENT, firestoreException.getCode());
-      assertTrue(firestoreException.getMessage().contains("maximum number of aggregations"));
-    }
+    assertEquals(FirebaseFirestoreException.Code.INVALID_ARGUMENT, firestoreException.getCode());
+    assertTrue(firestoreException.getMessage().contains("maximum number of aggregations"));
   }
 
   @Test

firebase-firestore/src/androidTest/java/com/google/firebase/firestore/AggregationTest.java

@@ -38,8 +38,8 @@
 import com.google.firebase.firestore.FirebaseFirestoreSettings;
 import com.google.firebase.firestore.ListenerRegistration;
 import com.google.firebase.firestore.MetadataChanges;
+import com.google.firebase.firestore.Pipeline;
 import com.google.firebase.firestore.PipelineResult;
-import com.google.firebase.firestore.PipelineSnapshot;
 import com.google.firebase.firestore.Query;
 import com.google.firebase.firestore.QuerySnapshot;
 import com.google.firebase.firestore.Source;
@@ -468,7 +468,7 @@ public static List<Map<String, Object>> querySnapshotToValues(QuerySnapshot quer
   }
 
   public static List<Map<String, Object>> pipelineSnapshotToValues(
-      PipelineSnapshot pipelineSnapshot) {
+      Pipeline.Snapshot pipelineSnapshot) {
     List<Map<String, Object>> res = new ArrayList<>();
     for (PipelineResult result : pipelineSnapshot) {
       res.add(result.getData());
@@ -484,7 +484,7 @@ public static List<String> querySnapshotToIds(QuerySnapshot querySnapshot) {
     return res;
   }
 
-  public static List<String> pipelineSnapshotToIds(PipelineSnapshot pipelineResults) {
+  public static List<String> pipelineSnapshotToIds(Pipeline.Snapshot pipelineResults) {
     List<String> res = new ArrayList<>();
     for (PipelineResult result : pipelineResults) {
       DocumentReference ref = result.getRef();
@@ -597,7 +597,7 @@ public static void checkQueryAndPipelineResultsMatch(Query query, String... expe
     } catch (Exception e) {
       throw new RuntimeException("Classic Query FAILED", e);
     }
-    PipelineSnapshot docsFromPipeline;
+    Pipeline.Snapshot docsFromPipeline;
     try {
       docsFromPipeline = waitFor(query.getFirestore().pipeline().createFrom(query).execute());
     } catch (Exception e) {

firebase-firestore/src/androidTest/java/com/google/firebase/firestore/PipelineTest.java

@@ -14,7 +14,7 @@
 
 package com.google.firebase.firestore;
 
-import static com.google.firebase.firestore.pipeline.Expr.field;
+import static com.google.firebase.firestore.pipeline.Expression.field;
 
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
@@ -232,7 +232,7 @@ private AverageAggregateField(@NonNull FieldPath fieldPath) {
     @NonNull
     @Override
     AliasedAggregate toPipeline() {
-      return field(getFieldPath()).avg().alias(getAlias());
+      return field(getFieldPath()).average().alias(getAlias());
     }
   }
 }