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diff --git a/sync/engine/entity_tracker.h b/sync/engine/entity_tracker.h
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+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef SYNC_ENGINE_ENTITY_TRACKER_H_
+#define SYNC_ENGINE_ENTITY_TRACKER_H_
+
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/time/time.h"
+#include "sync/base/sync_export.h"
+#include "sync/protocol/sync.pb.h"
+
+namespace syncer {
+
+// Manages the pending commit and update state for an entity on the sync
+// thread.
+//
+// It should be considered a helper class internal to the
+// ModelTypeSyncWorker.
+//
+// Maintains the state associated with a particular sync entity which is
+// necessary for decision-making on the sync thread.  It can track pending
+// commit state, received update state, and can detect conflicts.
+//
+// This object may or may not contain state associated with a pending commit.
+// If no commit is pending, the |is_commit_pending_| flag will be set to false
+// and many of this object's fields will be cleared.
+class SYNC_EXPORT EntityTracker {
+ public:
+  ~EntityTracker();
+
+  // Initialize a new entity based on an update response.
+  static EntityTracker* FromServerUpdate(const std::string& id_string,
+                                         const std::string& client_tag_hash,
+                                         int64 version);
+
+  // Initialize a new entity based on a commit request.
+  static EntityTracker* FromCommitRequest(
+      const std::string& id_string,
+      const std::string& client_tag_hash,
+      int64 sequence_number,
+      int64 base_version,
+      base::Time ctime,
+      base::Time mtime,
+      const std::string& non_unique_name,
+      bool deleted,
+      const sync_pb::EntitySpecifics& specifics);
+
+  // Returns true if this entity should be commited to the server.
+  bool IsCommitPending() const;
+
+  // Populates a sync_pb::SyncEntity for a commit.  Also sets the
+  // |sequence_number|, so we can track it throughout the commit process.
+  void PrepareCommitProto(sync_pb::SyncEntity* commit_entity,
+                          int64* sequence_number) const;
+
+  // Updates this entity with data from the latest version that the
+  // model asked us to commit.  May clobber state related to the
+  // model's previous commit attempt(s).
+  void RequestCommit(const std::string& id,
+                     const std::string& client_tag_hash,
+                     int64 sequence_number,
+                     int64 base_version,
+                     base::Time ctime,
+                     base::Time mtime,
+                     const std::string& non_unique_name,
+                     bool deleted,
+                     const sync_pb::EntitySpecifics& specifics);
+
+  // Handles the receipt of a commit response.
+  //
+  // Since commits happen entirely on the sync thread, we can safely assume
+  // that our item's state at the end of the commit is the same as it was at
+  // the start.
+  void ReceiveCommitResponse(const std::string& response_id,
+                             int64 response_version,
+                             int64 sequence_number);
+
+  // Handles receipt of an update from the server.
+  void ReceiveUpdate(int64 version);
+
+ private:
+  // Initializes received update state.  Does not initialize state related to
+  // pending commits and sets |is_commit_pending_| to false.
+  EntityTracker(const std::string& id,
+                const std::string& client_tag_hash,
+                int64 highest_commit_response_version,
+                int64 highest_gu_response_version);
+
+  // Initializes all fields.  Sets |is_commit_pending_| to true.
+  EntityTracker(const std::string& id,
+                const std::string& client_tag_hash,
+                int64 highest_commit_response_version,
+                int64 highest_gu_response_version,
+                bool is_commit_pending,
+                int64 sequence_number,
+                int64 base_version,
+                base::Time ctime,
+                base::Time mtime,
+                const std::string& non_unique_name,
+                bool deleted,
+                const sync_pb::EntitySpecifics& specifics);
+
+  // Checks if the current state indicates a conflict.
+  //
+  // This can be true only while a call to this object is in progress.
+  // Conflicts are always cleared before the method call ends.
+  bool IsInConflict() const;
+
+  // Checks if the server knows about this item.
+  bool IsServerKnown() const;
+
+  // Clears flag and optionally clears state associated with a pending commit.
+  void ClearPendingCommit();
+
+  // The ID for this entry.  May be empty if the entry has never been committed.
+  std::string id_;
+
+  // The hashed client tag for this entry.
+  std::string client_tag_hash_;
+
+  // The highest version seen in a commit response for this entry.
+  int64 highest_commit_response_version_;
+
+  // The highest version seen in a GU response for this entry.
+  int64 highest_gu_response_version_;
+
+  // Flag that indicates whether or not we're waiting for a chance to commit
+  // this item.
+  bool is_commit_pending_;
+
+  // Used to track in-flight commit requests on the model thread.  All we need
+  // to do here is return it back to the model thread when the pending commit
+  // is completed and confirmed.  Not valid if no commit is pending.
+  int64 sequence_number_;
+
+  // The following fields are valid only when a commit is pending.
+  // This is where we store the data that is to be sent up to the server
+  // at the next possible opportunity.
+  int64 base_version_;
+  base::Time ctime_;
+  base::Time mtime_;
+  std::string non_unique_name_;
+  bool deleted_;
+  sync_pb::EntitySpecifics specifics_;
+
+  DISALLOW_COPY_AND_ASSIGN(EntityTracker);
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_ENGINE_ENTITY_TRACKER_H_