Port `SnapshotVersion` for the rest Firestore modulo (#1185)

* remove conversions in `Local` and `Remote`

* replace `FSTSnapshotVersion` in `Model`

* replace `FSTSnapshotVersion` in the rest

* let test check equal with C++ equal

* fix newly added code

* address changes

13 files changed, 84 insertions, 61 deletions

diff --git a/Firestore/Source/API/FSTUserDataConverter.h b/Firestore/Source/API/FSTUserDataConverter.h
index 98a65ae..a3d8a2d 100644
--- a/Firestore/Source/API/FSTUserDataConverter.h
+++ b/Firestore/Source/API/FSTUserDataConverter.h
@@ -27,7 +27,6 @@
 @class FSTObjectValue;
 @class FSTFieldValue;
 @class FSTMutation;
-@class FSTSnapshotVersion;
 
 NS_ASSUME_NONNULL_BEGIN
 
diff --git a/Firestore/Source/Core/FSTSyncEngine.mm b/Firestore/Source/Core/FSTSyncEngine.mm
index 775f865..8ec98e8 100644
--- a/Firestore/Source/Core/FSTSyncEngine.mm
+++ b/Firestore/Source/Core/FSTSyncEngine.mm
@@ -24,7 +24,6 @@
 
 #import "FIRFirestoreErrors.h"
 #import "Firestore/Source/Core/FSTQuery.h"
-#import "Firestore/Source/Core/FSTSnapshotVersion.h"
 #import "Firestore/Source/Core/FSTTransaction.h"
 #import "Firestore/Source/Core/FSTView.h"
 #import "Firestore/Source/Core/FSTViewSnapshot.h"
@@ -347,7 +346,7 @@ static const FSTListenSequenceNumber kIrrelevantSequenceNumber = -1;
     NSMutableDictionary<NSNumber *, FSTTargetChange *> *targetChanges =
         [NSMutableDictionary dictionary];
     FSTDeletedDocument *doc =
-        [FSTDeletedDocument documentWithKey:limboKey version:[FSTSnapshotVersion noVersion]];
+        [FSTDeletedDocument documentWithKey:limboKey version:SnapshotVersion::None()];
     FSTDocumentKeySet *limboDocuments = [[FSTDocumentKeySet keySet] setByAddingObject:doc.key];
     FSTRemoteEvent *event = [[FSTRemoteEvent alloc] initWithSnapshotVersion:SnapshotVersion::None()
                                                               targetChanges:targetChanges
diff --git a/Firestore/Source/Core/FSTTransaction.mm b/Firestore/Source/Core/FSTTransaction.mm
index 4aabd5a..57dcc48 100644
--- a/Firestore/Source/Core/FSTTransaction.mm
+++ b/Firestore/Source/Core/FSTTransaction.mm
@@ -23,7 +23,6 @@
 
 #import "FIRFirestoreErrors.h"
 #import "Firestore/Source/API/FSTUserDataConverter.h"
-#import "Firestore/Source/Core/FSTSnapshotVersion.h"
 #import "Firestore/Source/Model/FSTDocument.h"
 #import "Firestore/Source/Model/FSTDocumentKeySet.h"
 #import "Firestore/Source/Model/FSTMutation.h"
@@ -33,9 +32,11 @@
 
 #include "Firestore/core/src/firebase/firestore/model/document_key.h"
 #include "Firestore/core/src/firebase/firestore/model/precondition.h"
+#include "Firestore/core/src/firebase/firestore/model/snapshot_version.h"
 
 using firebase::firestore::model::DocumentKey;
 using firebase::firestore::model::Precondition;
+using firebase::firestore::model::SnapshotVersion;
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -53,7 +54,7 @@ NS_ASSUME_NONNULL_BEGIN
 @end
 
 @implementation FSTTransaction {
-  std::map<DocumentKey, FSTSnapshotVersion *> _readVersions;
+  std::map<DocumentKey, SnapshotVersion> _readVersions;
 }
 
 + (instancetype)transactionWithDatastore:(FSTDatastore *)datastore {
@@ -79,11 +80,11 @@ NS_ASSUME_NONNULL_BEGIN
 - (BOOL)recordVersionForDocument:(FSTMaybeDocument *)doc error:(NSError **)error {
   FSTAssert(error != nil, @"nil error parameter");
   *error = nil;
-  FSTSnapshotVersion *docVersion = doc.version;
+  SnapshotVersion docVersion = doc.version;
   if ([doc isKindOfClass:[FSTDeletedDocument class]]) {
     // For deleted docs, we must record an explicit no version to build the right precondition
     // when writing.
-    docVersion = [FSTSnapshotVersion noVersion];
+    docVersion = SnapshotVersion::None();
   }
   if (_readVersions.find(doc.key) == _readVersions.end()) {
     _readVersions[doc.key] = docVersion;
@@ -159,8 +160,8 @@ NS_ASSUME_NONNULL_BEGIN
     return Precondition::Exists(true);
   }
 
-  FSTSnapshotVersion *version = iter->second;
-  if ([version isEqual:[FSTSnapshotVersion noVersion]]) {
+  const SnapshotVersion &version = iter->second;
+  if (version == SnapshotVersion::None()) {
     // The document was read, but doesn't exist.
     // Return an error because the precondition is impossible
     if (error) {
@@ -200,7 +201,7 @@ NS_ASSUME_NONNULL_BEGIN
                            precondition:[self preconditionForDocumentKey:key]] ]];
   // Since the delete will be applied before all following writes, we need to ensure that the
   // precondition for the next write will be exists without timestamp.
-  _readVersions[key] = [FSTSnapshotVersion noVersion];
+  _readVersions[key] = SnapshotVersion::None();
 }
 
 - (void)commitWithCompletion:(FSTVoidErrorBlock)completion {
diff --git a/Firestore/Source/Local/FSTLocalSerializer.mm b/Firestore/Source/Local/FSTLocalSerializer.mm
index 652fa5f..8fa1278 100644
--- a/Firestore/Source/Local/FSTLocalSerializer.mm
+++ b/Firestore/Source/Local/FSTLocalSerializer.mm
@@ -103,8 +103,8 @@ using firebase::firestore::model::SnapshotVersion;
   FSTSerializerBeta *remoteSerializer = self.remoteSerializer;
 
   FSTObjectValue *data = [remoteSerializer decodedFields:document.fields];
-  const DocumentKey key = [remoteSerializer decodedDocumentKey:document.name];
-  const SnapshotVersion version = [remoteSerializer decodedVersion:document.updateTime];
+  DocumentKey key = [remoteSerializer decodedDocumentKey:document.name];
+  SnapshotVersion version = [remoteSerializer decodedVersion:document.updateTime];
   return [FSTDocument documentWithData:data key:key version:version hasLocalMutations:NO];
 }
 
@@ -122,8 +122,8 @@ using firebase::firestore::model::SnapshotVersion;
 - (FSTDeletedDocument *)decodedDeletedDocument:(FSTPBNoDocument *)proto {
   FSTSerializerBeta *remoteSerializer = self.remoteSerializer;
 
-  const DocumentKey key = [remoteSerializer decodedDocumentKey:proto.name];
-  const SnapshotVersion version = [remoteSerializer decodedVersion:proto.readTime];
+  DocumentKey key = [remoteSerializer decodedDocumentKey:proto.name];
+  SnapshotVersion version = [remoteSerializer decodedVersion:proto.readTime];
   return [FSTDeletedDocument documentWithKey:key version:version];
 }
 
@@ -188,7 +188,7 @@ using firebase::firestore::model::SnapshotVersion;
 
   FSTTargetID targetID = target.targetId;
   FSTListenSequenceNumber sequenceNumber = target.lastListenSequenceNumber;
-  const SnapshotVersion version = [remoteSerializer decodedVersion:target.snapshotVersion];
+  SnapshotVersion version = [remoteSerializer decodedVersion:target.snapshotVersion];
   NSData *resumeToken = target.resumeToken;
 
   FSTQuery *query;
diff --git a/Firestore/Source/Local/FSTLocalStore.mm b/Firestore/Source/Local/FSTLocalStore.mm
index a3c447f..3a324cd 100644
--- a/Firestore/Source/Local/FSTLocalStore.mm
+++ b/Firestore/Source/Local/FSTLocalStore.mm
@@ -318,8 +318,9 @@ NS_ASSUME_NONNULL_BEGIN
       } else {
         FSTLog(
             @"FSTLocalStore Ignoring outdated watch update for %s. "
-             "Current version: %@  Watch version: %@",
-            key.ToString().c_str(), existingDoc.version, doc.version);
+             "Current version: %s  Watch version: %s",
+            key.ToString().c_str(), existingDoc.version.timestamp().ToString().c_str(),
+            doc.version.timestamp().ToString().c_str());
       }
 
       // The document might be garbage because it was unreferenced by everything.
@@ -331,8 +332,7 @@ NS_ASSUME_NONNULL_BEGIN
     // events when we get permission denied errors while trying to resolve the state of a locally
     // cached document that is in limbo.
     const SnapshotVersion &lastRemoteVersion = [self.queryCache lastRemoteSnapshotVersion];
-    // TODO(zxu): convert to reference once SnapshotVersion is used in RemoteEvent.
-    const SnapshotVersion remoteVersion = remoteEvent.snapshotVersion;
+    const SnapshotVersion &remoteVersion = remoteEvent.snapshotVersion;
     if (remoteVersion != SnapshotVersion::None()) {
       FSTAssert(remoteVersion >= lastRemoteVersion,
                 @"Watch stream reverted to previous snapshot?? (%s < %s)",
diff --git a/Firestore/Source/Local/FSTQueryData.mm b/Firestore/Source/Local/FSTQueryData.mm
index 456d42a..e352101 100644
--- a/Firestore/Source/Local/FSTQueryData.mm
+++ b/Firestore/Source/Local/FSTQueryData.mm
@@ -74,7 +74,7 @@ NS_ASSUME_NONNULL_BEGIN
 
   FSTQueryData *other = (FSTQueryData *)object;
   return [self.query isEqual:other.query] && self.targetID == other.targetID &&
-         self.purpose == other.purpose && [self.snapshotVersion isEqual:other.snapshotVersion] &&
+         self.purpose == other.purpose && self.snapshotVersion == other.snapshotVersion &&
          [self.resumeToken isEqual:other.resumeToken];
 }
 
diff --git a/Firestore/Source/Model/FSTDocument.h b/Firestore/Source/Model/FSTDocument.h
index 47e4d28..0f8d4b3 100644
--- a/Firestore/Source/Model/FSTDocument.h
+++ b/Firestore/Source/Model/FSTDocument.h
@@ -18,10 +18,10 @@
 
 #include "Firestore/core/src/firebase/firestore/model/document_key.h"
 #include "Firestore/core/src/firebase/firestore/model/field_path.h"
+#include "Firestore/core/src/firebase/firestore/model/snapshot_version.h"
 
 @class FSTFieldValue;
 @class FSTObjectValue;
-@class FSTSnapshotVersion;
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -32,14 +32,13 @@ NS_ASSUME_NONNULL_BEGIN
 @interface FSTMaybeDocument : NSObject <NSCopying>
 - (id)init __attribute__((unavailable("Abstract base class")));
 - (const firebase::firestore::model::DocumentKey &)key;
-
-@property(nonatomic, readonly) FSTSnapshotVersion *version;
+- (const firebase::firestore::model::SnapshotVersion &)version;
 @end
 
 @interface FSTDocument : FSTMaybeDocument
 + (instancetype)documentWithData:(FSTObjectValue *)data
                              key:(firebase::firestore::model::DocumentKey)key
-                         version:(FSTSnapshotVersion *)version
+                         version:(firebase::firestore::model::SnapshotVersion)version
                hasLocalMutations:(BOOL)mutations;
 
 - (nullable FSTFieldValue *)fieldForPath:(const firebase::firestore::model::FieldPath &)path;
@@ -51,7 +50,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 @interface FSTDeletedDocument : FSTMaybeDocument
 + (instancetype)documentWithKey:(firebase::firestore::model::DocumentKey)key
-                        version:(FSTSnapshotVersion *)version;
+                        version:(firebase::firestore::model::SnapshotVersion)version;
 @end
 
 /** An NSComparator suitable for comparing docs using only their keys. */
diff --git a/Firestore/Source/Model/FSTDocument.mm b/Firestore/Source/Model/FSTDocument.mm
index 9898c2a..8ebc9ab 100644
--- a/Firestore/Source/Model/FSTDocument.mm
+++ b/Firestore/Source/Model/FSTDocument.mm
@@ -18,7 +18,6 @@
 
 #include <utility>
 
-#import "Firestore/Source/Core/FSTSnapshotVersion.h"
 #import "Firestore/Source/Model/FSTFieldValue.h"
 #import "Firestore/Source/Util/FSTAssert.h"
 
@@ -29,26 +28,28 @@
 namespace util = firebase::firestore::util;
 using firebase::firestore::model::DocumentKey;
 using firebase::firestore::model::FieldPath;
+using firebase::firestore::model::SnapshotVersion;
 
 NS_ASSUME_NONNULL_BEGIN
 
 @interface FSTMaybeDocument ()
 
 - (instancetype)initWithKey:(DocumentKey)key
-                    version:(FSTSnapshotVersion *)version NS_DESIGNATED_INITIALIZER;
+                    version:(SnapshotVersion)version NS_DESIGNATED_INITIALIZER;
 
 @end
 
 @implementation FSTMaybeDocument {
   DocumentKey _key;
+  SnapshotVersion _version;
 }
 
-- (instancetype)initWithKey:(DocumentKey)key version:(FSTSnapshotVersion *)version {
+- (instancetype)initWithKey:(DocumentKey)key version:(SnapshotVersion)version {
   FSTAssert(!!version, @"Version must not be nil.");
   self = [super init];
   if (self) {
     _key = std::move(key);
-    _version = version;
+    _version = std::move(version);
   }
   return self;
 }
@@ -62,25 +63,29 @@ NS_ASSUME_NONNULL_BEGIN
   return _key;
 }
 
+- (const SnapshotVersion &)version {
+  return _version;
+}
+
 @end
 
 @implementation FSTDocument
 
 + (instancetype)documentWithData:(FSTObjectValue *)data
                              key:(DocumentKey)key
-                         version:(FSTSnapshotVersion *)version
+                         version:(SnapshotVersion)version
                hasLocalMutations:(BOOL)mutations {
   return [[FSTDocument alloc] initWithData:data
                                        key:std::move(key)
-                                   version:version
+                                   version:std::move(version)
                          hasLocalMutations:mutations];
 }
 
 - (instancetype)initWithData:(FSTObjectValue *)data
                          key:(DocumentKey)key
-                     version:(FSTSnapshotVersion *)version
+                     version:(SnapshotVersion)version
            hasLocalMutations:(BOOL)mutations {
-  self = [super initWithKey:std::move(key) version:version];
+  self = [super initWithKey:std::move(key) version:std::move(version)];
   if (self) {
     _data = data;
     _localMutations = mutations;
@@ -110,8 +115,9 @@ NS_ASSUME_NONNULL_BEGIN
 }
 
 - (NSString *)description {
-  return [NSString stringWithFormat:@"<FSTDocument: key:%s version:%@ localMutations:%@ data:%@>",
-                                    self.key.ToString().c_str(), self.version,
+  return [NSString stringWithFormat:@"<FSTDocument: key:%s version:%s localMutations:%@ data:%@>",
+                                    self.key.ToString().c_str(),
+                                    self.version.timestamp().ToString().c_str(),
                                     self.localMutations ? @"YES" : @"NO", self.data];
 }
 
@@ -123,8 +129,8 @@ NS_ASSUME_NONNULL_BEGIN
 
 @implementation FSTDeletedDocument
 
-+ (instancetype)documentWithKey:(DocumentKey)key version:(FSTSnapshotVersion *)version {
-  return [[FSTDeletedDocument alloc] initWithKey:std::move(key) version:version];
++ (instancetype)documentWithKey:(DocumentKey)key version:(SnapshotVersion)version {
+  return [[FSTDeletedDocument alloc] initWithKey:std::move(key) version:std::move(version)];
 }
 
 - (BOOL)isEqual:(id)other {
diff --git a/Firestore/Source/Model/FSTMutation.h b/Firestore/Source/Model/FSTMutation.h
index 7261f30..0acec15 100644
--- a/Firestore/Source/Model/FSTMutation.h
+++ b/Firestore/Source/Model/FSTMutation.h
@@ -24,13 +24,15 @@
 #include "Firestore/core/src/firebase/firestore/model/field_path.h"
 #include "Firestore/core/src/firebase/firestore/model/field_transform.h"
 #include "Firestore/core/src/firebase/firestore/model/precondition.h"
+#include "Firestore/core/src/firebase/firestore/model/snapshot_version.h"
 #include "Firestore/core/src/firebase/firestore/model/transform_operations.h"
 
+#include "absl/types/optional.h"
+
 @class FSTDocument;
 @class FSTFieldValue;
 @class FSTMaybeDocument;
 @class FSTObjectValue;
-@class FSTSnapshotVersion;
 @class FIRTimestamp;
 
 NS_ASSUME_NONNULL_BEGIN
@@ -40,12 +42,12 @@ NS_ASSUME_NONNULL_BEGIN
 @interface FSTMutationResult : NSObject
 
 - (instancetype)init NS_UNAVAILABLE;
-- (instancetype)initWithVersion:(FSTSnapshotVersion *_Nullable)version
+- (instancetype)initWithVersion:(absl::optional<firebase::firestore::model::SnapshotVersion>)version
                transformResults:(NSArray<FSTFieldValue *> *_Nullable)transformResults
     NS_DESIGNATED_INITIALIZER;
 
 /** The version at which the mutation was committed or null for a delete. */
-@property(nonatomic, strong, readonly, nullable) FSTSnapshotVersion *version;
+- (const absl::optional<firebase::firestore::model::SnapshotVersion> &)version;
 
 /**
  * The resulting fields returned from the backend after a FSTTransformMutation has been committed.
diff --git a/Firestore/Source/Model/FSTMutation.mm b/Firestore/Source/Model/FSTMutation.mm
index 3432a7c..fdf6014 100644
--- a/Firestore/Source/Model/FSTMutation.mm
+++ b/Firestore/Source/Model/FSTMutation.mm
@@ -23,7 +23,6 @@
 
 #import "FIRTimestamp.h"
 
-#import "Firestore/Source/Core/FSTSnapshotVersion.h"
 #import "Firestore/Source/Model/FSTDocument.h"
 #import "Firestore/Source/Model/FSTFieldValue.h"
 #import "Firestore/Source/Util/FSTAssert.h"
@@ -36,6 +35,8 @@
 #include "Firestore/core/src/firebase/firestore/model/precondition.h"
 #include "Firestore/core/src/firebase/firestore/model/transform_operations.h"
 
+#include "absl/types/optional.h"
+
 using firebase::firestore::model::ArrayTransform;
 using firebase::firestore::model::DocumentKey;
 using firebase::firestore::model::FieldMask;
@@ -43,23 +44,30 @@ using firebase::firestore::model::FieldPath;
 using firebase::firestore::model::FieldTransform;
 using firebase::firestore::model::Precondition;
 using firebase::firestore::model::ServerTimestampTransform;
+using firebase::firestore::model::SnapshotVersion;
 using firebase::firestore::model::TransformOperation;
 
 NS_ASSUME_NONNULL_BEGIN
 
 #pragma mark - FSTMutationResult
 
-@implementation FSTMutationResult
+@implementation FSTMutationResult {
+  absl::optional<SnapshotVersion> _version;
+}
 
-- (instancetype)initWithVersion:(nullable FSTSnapshotVersion *)version
+- (instancetype)initWithVersion:(absl::optional<SnapshotVersion>)version
                transformResults:(nullable NSArray<FSTFieldValue *> *)transformResults {
   if (self = [super init]) {
-    _version = version;
+    _version = std::move(version);
     _transformResults = transformResults;
   }
   return self;
 }
 
+- (const absl::optional<SnapshotVersion> &)version {
+  return _version;
+}
+
 @end
 
 #pragma mark - FSTMutation
@@ -157,7 +165,7 @@ NS_ASSUME_NONNULL_BEGIN
     // If the document didn't exist before, create it.
     return [FSTDocument documentWithData:self.value
                                      key:self.key
-                                 version:[FSTSnapshotVersion noVersion]
+                                 version:SnapshotVersion::None()
                        hasLocalMutations:hasLocalMutations];
   }
 
@@ -239,10 +247,10 @@ NS_ASSUME_NONNULL_BEGIN
   if (!maybeDoc || [maybeDoc isMemberOfClass:[FSTDeletedDocument class]]) {
     // Precondition applied, so create the document if necessary
     const DocumentKey &key = maybeDoc ? maybeDoc.key : self.key;
-    FSTSnapshotVersion *version = maybeDoc ? maybeDoc.version : [FSTSnapshotVersion noVersion];
+    SnapshotVersion version = maybeDoc ? maybeDoc.version : SnapshotVersion::None();
     maybeDoc = [FSTDocument documentWithData:[FSTObjectValue objectValue]
                                          key:key
-                                     version:version
+                                     version:std::move(version)
                            hasLocalMutations:hasLocalMutations];
   }
 
@@ -556,7 +564,7 @@ serverTransformResultsWithBaseDocument:(nullable FSTMaybeDocument *)baseDocument
     FSTAssert([maybeDoc.key isEqual:self.key], @"Can only delete a document with the same key");
   }
 
-  return [FSTDeletedDocument documentWithKey:self.key version:[FSTSnapshotVersion noVersion]];
+  return [FSTDeletedDocument documentWithKey:self.key version:SnapshotVersion::None()];
 }
 
 @end
diff --git a/Firestore/Source/Model/FSTMutationBatch.h b/Firestore/Source/Model/FSTMutationBatch.h
index 3c82338..d3d68a9 100644
--- a/Firestore/Source/Model/FSTMutationBatch.h
+++ b/Firestore/Source/Model/FSTMutationBatch.h
@@ -21,12 +21,12 @@
 #import "Firestore/Source/Model/FSTDocumentVersionDictionary.h"
 
 #include "Firestore/core/src/firebase/firestore/model/document_key.h"
+#include "Firestore/core/src/firebase/firestore/model/snapshot_version.h"
 
 @class FSTMutation;
 @class FIRTimestamp;
 @class FSTMutationResult;
 @class FSTMutationBatchResult;
-@class FSTSnapshotVersion;
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -106,12 +106,13 @@ extern const FSTBatchID kFSTBatchIDUnknown;
  * (as docVersions).
  */
 + (instancetype)resultWithBatch:(FSTMutationBatch *)batch
-                  commitVersion:(FSTSnapshotVersion *)commitVersion
+                  commitVersion:(firebase::firestore::model::SnapshotVersion)commitVersion
                 mutationResults:(NSArray<FSTMutationResult *> *)mutationResults
                     streamToken:(nullable NSData *)streamToken;
 
+- (const firebase::firestore::model::SnapshotVersion &)commitVersion;
+
 @property(nonatomic, strong, readonly) FSTMutationBatch *batch;
-@property(nonatomic, strong, readonly) FSTSnapshotVersion *commitVersion;
 @property(nonatomic, strong, readonly) NSArray<FSTMutationResult *> *mutationResults;
 @property(nonatomic, strong, readonly, nullable) NSData *streamToken;
 @property(nonatomic, strong, readonly) FSTDocumentVersionDictionary *docVersions;
diff --git a/Firestore/Source/Model/FSTMutationBatch.mm b/Firestore/Source/Model/FSTMutationBatch.mm
index e62a72c..3c03bdc 100644
--- a/Firestore/Source/Model/FSTMutationBatch.mm
+++ b/Firestore/Source/Model/FSTMutationBatch.mm
@@ -16,9 +16,10 @@
 
 #import "Firestore/Source/Model/FSTMutationBatch.h"
 
+#include <utility>
+
 #import "FIRTimestamp.h"
 
-#import "Firestore/Source/Core/FSTSnapshotVersion.h"
 #import "Firestore/Source/Model/FSTDocument.h"
 #import "Firestore/Source/Model/FSTMutation.h"
 #import "Firestore/Source/Util/FSTAssert.h"
@@ -26,6 +27,7 @@
 #include "Firestore/core/src/firebase/firestore/model/document_key.h"
 
 using firebase::firestore::model::DocumentKey;
+using firebase::firestore::model::SnapshotVersion;
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -127,22 +129,24 @@ const FSTBatchID kFSTBatchIDUnknown = -1;
 
 @interface FSTMutationBatchResult ()
 - (instancetype)initWithBatch:(FSTMutationBatch *)batch
-                commitVersion:(FSTSnapshotVersion *)commitVersion
+                commitVersion:(SnapshotVersion)commitVersion
               mutationResults:(NSArray<FSTMutationResult *> *)mutationResults
                   streamToken:(nullable NSData *)streamToken
                   docVersions:(FSTDocumentVersionDictionary *)docVersions NS_DESIGNATED_INITIALIZER;
 @end
 
-@implementation FSTMutationBatchResult
+@implementation FSTMutationBatchResult {
+  SnapshotVersion _commitVersion;
+}
 
 - (instancetype)initWithBatch:(FSTMutationBatch *)batch
-                commitVersion:(FSTSnapshotVersion *)commitVersion
+                commitVersion:(SnapshotVersion)commitVersion
               mutationResults:(NSArray<FSTMutationResult *> *)mutationResults
                   streamToken:(nullable NSData *)streamToken
                   docVersions:(FSTDocumentVersionDictionary *)docVersions {
   if (self = [super init]) {
     _batch = batch;
-    _commitVersion = commitVersion;
+    _commitVersion = std::move(commitVersion);
     _mutationResults = mutationResults;
     _streamToken = streamToken;
     _docVersions = docVersions;
@@ -150,8 +154,12 @@ const FSTBatchID kFSTBatchIDUnknown = -1;
   return self;
 }
 
+- (const SnapshotVersion &)commitVersion {
+  return _commitVersion;
+}
+
 + (instancetype)resultWithBatch:(FSTMutationBatch *)batch
-                  commitVersion:(FSTSnapshotVersion *)commitVersion
+                  commitVersion:(SnapshotVersion)commitVersion
                 mutationResults:(NSArray<FSTMutationResult *> *)mutationResults
                     streamToken:(nullable NSData *)streamToken {
   FSTAssert(batch.mutations.count == mutationResults.count,
@@ -162,18 +170,18 @@ const FSTBatchID kFSTBatchIDUnknown = -1;
       [FSTDocumentVersionDictionary documentVersionDictionary];
   NSArray<FSTMutation *> *mutations = batch.mutations;
   for (NSUInteger i = 0; i < mutations.count; i++) {
-    FSTSnapshotVersion *_Nullable version = mutationResults[i].version;
+    absl::optional<SnapshotVersion> version = mutationResults[i].version;
     if (!version) {
       // deletes don't have a version, so we substitute the commitVersion
       // of the entire batch.
       version = commitVersion;
     }
 
-    docVersions = [docVersions dictionaryBySettingObject:version forKey:mutations[i].key];
+    docVersions = [docVersions dictionaryBySettingObject:version.value() forKey:mutations[i].key];
   }
 
   return [[FSTMutationBatchResult alloc] initWithBatch:batch
-                                         commitVersion:commitVersion
+                                         commitVersion:std::move(commitVersion)
                                        mutationResults:mutationResults
                                            streamToken:streamToken
                                            docVersions:docVersions];
diff --git a/Firestore/Source/Remote/FSTRemoteStore.mm b/Firestore/Source/Remote/FSTRemoteStore.mm
index 4dacc8e..c1ec9ff 100644
--- a/Firestore/Source/Remote/FSTRemoteStore.mm
+++ b/Firestore/Source/Remote/FSTRemoteStore.mm
@@ -317,7 +317,7 @@ static const int kMaxPendingWrites = 10;
     // using a resume token).
     [self.accumulatedChanges addObject:change];
     if (snapshotVersion == SnapshotVersion::None() ||
-        snapshotVersion < SnapshotVersion{[self.localStore lastRemoteSnapshotVersion]}) {
+        snapshotVersion < [self.localStore lastRemoteSnapshotVersion]) {
       return;
     }