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diff --git a/Firestore/core/src/firebase/firestore/immutable/map_entry.h b/Firestore/core/src/firebase/firestore/immutable/map_entry.h
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+++ b/Firestore/core/src/firebase/firestore/immutable/map_entry.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_IMMUTABLE_MAP_ENTRY_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_IMMUTABLE_MAP_ENTRY_H_
+
+#include <functional>
+#include <utility>
+
+namespace firebase {
+namespace firestore {
+namespace immutable {
+
+/**
+ * Compares two keys out of a map entry.
+ *
+ * @tparam K The type of the first value in the pair.
+ * @tparam V The type of the second value in the pair.
+ * @tparam C The comparator for use for values of type K
+ */
+template <typename K, typename V, typename C = std::less<K>>
+struct KeyComparator {
+  typedef std::pair<K, V> pair_type;
+
+  explicit KeyComparator(const C& comparator = C())
+      : key_comparator_(comparator) {
+  }
+
+  bool operator()(const K& lhs, const pair_type& rhs) const noexcept {
+    return key_comparator_(lhs, rhs.first);
+  }
+
+  bool operator()(const pair_type& lhs, const K& rhs) const noexcept {
+    return key_comparator_(lhs.first, rhs);
+  }
+
+  bool operator()(const pair_type& lhs, const pair_type& rhs) const noexcept {
+    return key_comparator_(lhs.first, rhs.first);
+  }
+
+ private:
+  C key_comparator_;
+};
+
+}  // namespace immutable
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_IMMUTABLE_MAP_ENTRY_H_