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diff --git a/Firestore/core/src/firebase/firestore/immutable/keys_view.h b/Firestore/core/src/firebase/firestore/immutable/keys_view.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_KEYS_VIEW_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_IMMUTABLE_KEYS_VIEW_H_
+
+#include <utility>
+
+#include "Firestore/core/src/firebase/firestore/util/iterator_adaptors.h"
+#include "Firestore/core/src/firebase/firestore/util/range.h"
+
+namespace firebase {
+namespace firestore {
+namespace immutable {
+namespace impl {
+
+template <typename Iterator>
+using KeysRange = util::range<util::iterator_first<Iterator>>;
+
+template <typename Iterator>
+KeysRange<Iterator> MakeKeysRange(Iterator begin, Iterator end) {
+  auto keys_begin = util::make_iterator_first(begin);
+  auto keys_end = util::make_iterator_first(end);
+  return util::make_range(keys_begin, keys_end);
+}
+
+/**
+ * Returns a view of the keys of the given key-value range.
+ */
+template <typename Range>
+auto KeysView(const Range& range) -> KeysRange<decltype(std::begin(range))> {
+  return MakeKeysRange(std::begin(range), std::end(range));
+}
+
+template <typename Range, typename K>
+auto KeysViewFrom(const Range& range, const K& key)
+    -> KeysRange<decltype(range.lower_bound(key))> {
+  auto keys_begin = util::make_iterator_first(range.lower_bound(key));
+  auto keys_end = util::make_iterator_first(std::end(range));
+  return util::make_range(keys_begin, keys_end);
+}
+
+/**
+ * Returns a view of keys of the given key-value range that are greater than or
+ * equal to the given start_key and less than the given end_key.
+ *
+ * If `end_key` is less than or equal to `start_key`, creates empty range.
+ */
+template <typename Range, typename K, typename C>
+auto KeysViewIn(const Range& range,
+                const K& start_key,
+                const K& end_key,
+                const C& comparator)
+    -> KeysRange<decltype(range.lower_bound(start_key))> {
+  // Forward iterators can't ever reach the end if the end is behind the start:
+  // they just keep incrementing until address space runs out. Adjust the range
+  // accordingly.
+  bool empty_range = !comparator(start_key, end_key);
+  if (empty_range) {
+    return MakeKeysRange(std::end(range), std::end(range));
+  }
+
+  auto range_begin = range.lower_bound(start_key);
+  auto range_end = range.lower_bound(end_key);
+  return MakeKeysRange(std::move(range_begin), std::move(range_end));
+}
+
+}  // namespace impl
+}  // namespace immutable
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_IMMUTABLE_KEYS_VIEW_H_