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diff --git a/Firestore/core/src/firebase/firestore/model/maybe_document.h b/Firestore/core/src/firebase/firestore/model/maybe_document.h
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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.
+ */
+
+#ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_MODEL_MAYBE_DOCUMENT_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_MODEL_MAYBE_DOCUMENT_H_
+
+#include <functional>
+
+#include "Firestore/core/src/firebase/firestore/model/document_key.h"
+#include "Firestore/core/src/firebase/firestore/model/snapshot_version.h"
+
+namespace firebase {
+namespace firestore {
+namespace model {
+
+/**
+ * The result of a lookup for a given path may be an existing document or a
+ * tombstone that marks the path deleted.
+ */
+class MaybeDocument {
+ public:
+  /**
+   * All the different kinds of documents, including MaybeDocument and its
+   * subclasses. This is used to provide RTTI for documents.
+   */
+  enum class Type {
+    Unknown,
+    Document,
+    NoDocument,
+  };
+
+  MaybeDocument(DocumentKey key, SnapshotVersion version);
+
+  /** The runtime type of this document. */
+  Type type() const {
+    return type_;
+  }
+
+  /** The key for this document. */
+  const DocumentKey& key() const {
+    return key_;
+  }
+
+  /**
+   * Returns the version of this document if it exists or a version at which
+   * this document was guaranteed to not exist.
+   */
+  const SnapshotVersion& version() const {
+    return version_;
+  }
+
+ protected:
+  // Only allow subclass to set their types.
+  void set_type(Type type) {
+    type_ = type;
+  }
+
+  virtual bool Equals(const MaybeDocument& other) const;
+
+  friend bool operator==(const MaybeDocument& lhs, const MaybeDocument& rhs);
+
+ private:
+  Type type_ = Type::Unknown;
+  DocumentKey key_;
+  SnapshotVersion version_;
+};
+
+inline bool operator==(const MaybeDocument& lhs, const MaybeDocument& rhs) {
+  return lhs.Equals(rhs);
+}
+
+inline bool operator!=(const MaybeDocument& lhs, const MaybeDocument& rhs) {
+  return !(lhs == rhs);
+}
+
+/** Compares against another MaybeDocument by keys only. */
+struct DocumentKeyComparator : public std::less<MaybeDocument> {
+  bool operator()(const MaybeDocument& lhs, const MaybeDocument& rhs) const {
+    return lhs.key() < rhs.key();
+  }
+};
+
+}  // namespace model
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_MODEL_MAYBE_DOCUMENT_H_