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+/*
+ * Copyright 2018 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// TODO(rsgowman): This file isn't intended to be used just yet. It's just an
+// outline of what the API might eventually look like. Most of this was
+// shamelessly stolen and modified from rtdb's header file, melded with the
+// (java) firestore api.
+
+#ifndef FIRESTORE_CORE_INCLUDE_FIREBASE_FIRESTORE_DOCUMENT_REFERENCE_H_
+#define FIRESTORE_CORE_INCLUDE_FIREBASE_FIRESTORE_DOCUMENT_REFERENCE_H_
+
+#include <string>
+#include <unordered_map>
+
+#if defined(FIREBASE_USE_STD_FUNCTION)
+#include <functional>
+#endif
+
+// TODO(rsgowman): Note that RTDB uses:
+//   #if defined(FIREBASE_USE_MOVE_OPERATORS) || defined(DOXYGEN
+// to protect move operators from older compilers. But all our supported
+// compilers support this, so we've skipped the #if guard. This TODO comment is
+// here so we don't forget to mention this during the API review, and should be
+// removed once this note has migrated to the API review doc.
+
+// TODO(rsgowman): replace these forward decl's with appropriate includes (once
+// they exist)
+namespace firebase {
+class App;
+template <typename T>
+class Future;
+}  // namespace firebase
+
+namespace firebase {
+namespace firestore {
+
+// TODO(rsgowman): replace these forward decl's with appropriate includes (once
+// they exist)
+class FieldValue;
+class DocumentSnapshot;
+class Firestore;
+class Error;
+template <typename T>
+class EventListener;
+class ListenerRegistration;
+class CollectionReference;
+class DocumentListenOptions;
+// TODO(rsgowman): not quite a forward decl, but required to make the default
+// parameter to Set() "compile".
+class SetOptions {
+ public:
+  SetOptions();
+};
+
+// TODO(rsgowman): move this into the FieldValue header
+#ifdef STLPORT
+using MapFieldValue = std::tr1::unordered_map<std::string, FieldValue>;
+#else
+using MapFieldValue = std::unordered_map<std::string, FieldValue>;
+#endif
+
+/**
+ * A DocumentReference refers to a document location in a Firestore database and
+ * can be used to write, read, or listen to the location. There may or may not
+ * exist a document at the referenced location. A DocumentReference can also be
+ * used to create a CollectionReference to a subcollection.
+ *
+ * Create a DocumentReference via Firebase::Document(const string& path).
+ *
+ * Subclassing Note: Firestore classes are not meant to be subclassed except for
+ * use in test mocks. Subclassing is not supported in production code and new
+ * SDK releases may break code that does so.
+ */
+class DocumentReference {
+ public:
+  /**
+   * @brief Default constructor. This creates an invalid DocumentReference.
+   * Attempting to perform any operations on this reference will fail (and cause
+   * a crash) unless a valid DocumentReference has been assigned to it.
+   */
+  DocumentReference();
+
+  /**
+   * @brief Copy constructor. It's totally okay (and efficient) to copy
+   * DocumentReference instances, as they simply point to the same location in
+   * the database.
+   *
+   * @param[in] reference DocumentReference to copy from.
+   */
+  DocumentReference(const DocumentReference& reference);
+
+  /**
+   * @brief Move constructor. Moving is an efficient operation for
+   * DocumentReference instances.
+   *
+   * @param[in] reference DocumentReference to move data from.
+   */
+  DocumentReference(DocumentReference&& reference);
+
+  virtual ~DocumentReference();
+
+  /**
+   * @brief Copy assignment operator. It's totally okay (and efficient) to copy
+   * DocumentReference instances, as they simply point to the same location in
+   * the database.
+   *
+   * @param[in] reference DocumentReference to copy from.
+   *
+   * @returns Reference to the destination DocumentReference.
+   */
+  DocumentReference& operator=(const DocumentReference& reference);
+
+  /**
+   * @brief Move assignment operator. Moving is an efficient operation for
+   * DocumentReference instances.
+   *
+   * @param[in] reference DocumentReference to move data from.
+   *
+   * @returns Reference to the destination DocumentReference.
+   */
+  DocumentReference& operator=(DocumentReference&& reference);
+
+  /**
+   * @brief Returns the Firestore instance associated with this document
+   * reference.
+   *
+   * The pointer will remain valid indefinitely.
+   *
+   * @returns Firebase Firestore instance that this DocumentReference refers to.
+   */
+  virtual const Firestore* firestore() const;
+
+  /**
+   * @brief Returns the Firestore instance associated with this document
+   * reference.
+   *
+   * The pointer will remain valid indefinitely.
+   *
+   * @returns Firebase Firestore instance that this DocumentReference refers to.
+   */
+  virtual Firestore* firestore();
+
+  /**
+   * @brief Returns the string id of this document location.
+   *
+   * The pointer is only valid while the DocumentReference remains in memory.
+   *
+   * @returns String id of this document location, which will remain valid in
+   * memory until the DocumentReference itself goes away.
+   */
+  virtual const char* id() const;
+
+  /**
+   * @brief Returns the string id of this document location.
+   *
+   * @returns String id of this document location.
+   */
+  virtual std::string id_string() const;
+
+  /**
+   * @brief Returns the path of this document (relative to the root of the
+   * database) as a slash-separated string.
+   *
+   * The pointer is only valid while the DocumentReference remains in memory.
+   *
+   * @returns String path of this document location, which will remain valid in
+   * memory until the DocumentReference itself goes away.
+   */
+  virtual const char* path() const;
+
+  /**
+   * @brief Returns the path of this document (relative to the root of the
+   * database) as a slash-separated string.
+   *
+   * @returns String path of this document location.
+   */
+  virtual std::string path_string() const;
+
+  /**
+   * @brief Returns a CollectionReference to the collection that contains this
+   * document.
+   */
+  virtual CollectionReference get_parent() const;
+
+  /**
+   * @brief Returns a CollectionReference instance that refers to the
+   * subcollection at the specified path relative to this document.
+   *
+   * @param[in] collection_path A slash-separated relative path to a
+   * subcollection. The pointer only needs to be valid during this call.
+   *
+   * @return The CollectionReference instance.
+   */
+  virtual CollectionReference Collection(const char* collection_path) const;
+
+  /**
+   * @brief Returns a CollectionReference instance that refers to the
+   * subcollection at the specified path relative to this document.
+   *
+   * @param[in] collection_path A slash-separated relative path to a
+   * subcollection.
+   *
+   * @return The CollectionReference instance.
+   */
+  virtual CollectionReference Collection(
+      const std::string& collection_path) const;
+
+  /**
+   * @brief Reads the document referenced by this DocumentReference.
+   *
+   * @return A Future that will be resolved with the contents of the Document at
+   * this DocumentReference.
+   */
+  virtual Future<DocumentSnapshot> Get() const;
+
+  /**
+   * @brief Writes to the document referred to by this DocumentReference.
+   *
+   * If the document does not yet exist, it will be created. If you pass
+   * SetOptions, the provided data can be merged into an existing document.
+   *
+   * @param[in] data A map of the fields and values for the document.
+   * @param[in] options An object to configure the set behavior.
+   *
+   * @return A Future that will be resolved when the write finishes.
+   */
+  virtual Future<void> Set(const MapFieldValue& data,
+                           const SetOptions& options = SetOptions());
+
+  /**
+   * @brief Updates fields in the document referred to by this
+   * DocumentReference.
+   *
+   * If no document exists yet, the update will fail.
+   *
+   * @param[in] data A map of field / value pairs to update. Fields can contain
+   * dots to reference nested fields within the document.
+   *
+   * @return A Future that will be resolved when the write finishes.
+   */
+  virtual Future<void> Update(const MapFieldValue& data);
+
+  /**
+   * @brief Removes the document referred to by this DocumentReference.
+   *
+   * @return A Task that will be resolved when the delete completes.
+   */
+  virtual Future<void> Delete();
+
+  /**
+   * @brief Starts listening to the document referenced by this
+   * DocumentReference.
+   *
+   * @param[in] listener The event listener that will be called with the
+   * snapshots, which must remain in memory until you remove the listener from
+   * this DocumentReference. (Ownership is not transferred; you are responsible
+   * for making sure that listener is valid as long as this DocumentReference is
+   * valid and the listener is registered.)
+   *
+   * @return A registration object that can be used to remove the listener.
+   */
+  virtual ListenerRegistration AddSnapshotListener(
+      EventListener<DocumentSnapshot>* listener);
+
+  /**
+   * @brief Starts listening to the document referenced by this
+   * DocumentReference.
+   *
+   * @param[in] options The options to use for this listen.
+   * @param[in] listener The event listener that will be called with the
+   * snapshots, which must remain in memory until you remove the listener from
+   * this DocumentReference. (Ownership is not transferred; you are responsible
+   * for making sure that listener is valid as long as this DocumentReference is
+   * valid and the listener is registered.)
+   *
+   * @return A registration object that can be used to remove the listener.
+   */
+  virtual ListenerRegistration AddSnapshotListener(
+      const DocumentListenOptions& options,
+      EventListener<DocumentSnapshot>* listener);
+
+#if defined(FIREBASE_USE_STD_FUNCTION) || defined(DOXYGEN)
+  /**
+   * @brief Starts listening to the document referenced by this
+   * DocumentReference.
+   *
+   * @param[in] callback function or lambda to call. When this function is
+   * called, exactly one of the parameters will be non-null.
+   *
+   * @return A registration object that can be used to remove the listener.
+   *
+   * @note This method is not available when using STLPort on Android, as
+   * std::function is not supported on STLPort.
+   */
+  virtual ListenerRegistration AddSnapshotListener(
+      std::function<void(const DocumentSnapshot*, const Error*)> callback);
+
+  /**
+   * @brief Starts listening to the document referenced by this
+   * DocumentReference.
+   *
+   * @param[in] options The options to use for this listen.
+   * @param[in] callback function or lambda to call. When this function is
+   * called, exactly one of the parameters will be non-null.
+   *
+   * @return A registration object that can be used to remove the listener.
+   *
+   * @note This method is not available when using STLPort on Android, as
+   * std::function is not supported on STLPort.
+   */
+  virtual ListenerRegistration AddSnapshotListener(
+      const DocumentListenOptions& options,
+      std::function<void(const DocumentSnapshot*, const Error*)> callback);
+#endif  // defined(FIREBASE_USE_STD_FUNCTION) || defined(DOXYGEN)
+};
+
+// TODO(rsgowman): probably define and inline here.
+bool operator==(const DocumentReference& lhs, const DocumentReference& rhs);
+
+inline bool operator!=(const DocumentReference& lhs,
+                       const DocumentReference& rhs) {
+  return !(lhs == rhs);
+}
+
+// TODO(rsgowman): probably define and inline here.
+bool operator<(const DocumentReference& lhs, const DocumentReference& rhs);
+
+inline bool operator>(const DocumentReference& lhs,
+                      const DocumentReference& rhs) {
+  return rhs < lhs;
+}
+
+inline bool operator<=(const DocumentReference& lhs,
+                       const DocumentReference& rhs) {
+  return !(lhs > rhs);
+}
+
+inline bool operator>=(const DocumentReference& lhs,
+                       const DocumentReference& rhs) {
+  return !(lhs < rhs);
+}
+
+}  // namespace firestore
+}  // namespace firebase
+
+namespace std {
+// TODO(rsgowman): NB that specialization of std::hash deviates from the Google
+// C++ style guide. But we think this is probably ok in this case since:
+// a) It's the standard way of doing this outside of Google (as the style guide
+// itself points out), and
+// b) This has a straightfoward hash function anyway (just hash the path) so I
+// don't think the concerns in the style guide are going to bite us.
+//
+// Raise this concern during the API review.
+template <>
+struct hash<firebase::firestore::DocumentReference> {
+  std::size_t operator()(
+      const firebase::firestore::DocumentReference& doc_ref) const;
+};
+}  // namespace std
+
+#endif  // FIRESTORE_CORE_INCLUDE_FIREBASE_FIRESTORE_DOCUMENT_REFERENCE_H_