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diff --git a/chromium/components/federated_learning/sim_hash.h b/chromium/components/federated_learning/sim_hash.h
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+++ b/chromium/components/federated_learning/sim_hash.h
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+// Copyright 2020 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 COMPONENTS_FEDERATED_LEARNING_SIM_HASH_H_
+#define COMPONENTS_FEDERATED_LEARNING_SIM_HASH_H_
+
+#include <set>
+#include <unordered_set>
+
+namespace federated_learning {
+
+// A 2^64 bit vector
+class LargeBitVector {
+ public:
+  LargeBitVector();
+  LargeBitVector(const LargeBitVector&);
+  ~LargeBitVector();
+
+  void SetBit(uint64_t pos);
+  const std::set<uint64_t>& PositionsOfSetBits() const;
+
+ private:
+  // Sparse representation of a 2^64 bit vector. Each number in
+  // |positions_of_set_bits_| represents the position of a bit that is being
+  // set.
+  std::set<uint64_t> positions_of_set_bits_;
+};
+
+// Set the two seeds used for generating the random gaussian.
+void SetSeedsForTesting(uint64_t seed1, uint64_t seed2);
+
+// SimHash a 2^64 bit vector to an |output_dimensions| bit number.
+// |output_dimensions| must be greater than 0 and no greater than 64.
+uint64_t SimHashBits(const LargeBitVector& input, size_t output_dimensions);
+
+// SimHash a set of strings to an |output_dimensions| bit number.
+// |output_dimensions| must be greater than 0 and no greater than 64.
+uint64_t SimHashStrings(const std::unordered_set<std::string>& input,
+                        size_t output_dimensions);
+
+}  // namespace federated_learning
+
+#endif  // COMPONENTS_FEDERATED_LEARNING_SIM_HASH_H_