1 // Copyright 2020 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 #ifndef UTIL_HASHING_H_ 6 #define UTIL_HASHING_H_ 7 8 namespace openscreen { 9 10 // Computes the aggregate hash of the provided hashable objects. 11 // Seed must initially use a large prime between 2^63 and 2^64 as a starting 12 // value, or the result of a previous call to this function. 13 template <typename... T> ComputeAggregateHash(uint64_t seed,const T &...objs)14uint64_t ComputeAggregateHash(uint64_t seed, const T&... objs) { 15 auto hash_combiner = [](uint64_t seed, uint64_t hash_value) -> uint64_t { 16 static const uint64_t kMultiplier = UINT64_C(0x9ddfea08eb382d69); 17 uint64_t a = (hash_value ^ seed) * kMultiplier; 18 a ^= (a >> 47); 19 uint64_t b = (seed ^ a) * kMultiplier; 20 b ^= (b >> 47); 21 b *= kMultiplier; 22 return b; 23 }; 24 25 uint64_t result = seed; 26 std::vector<uint64_t> hashes{std::hash<T>()(objs)...}; 27 for (uint64_t hash : hashes) { 28 result = hash_combiner(result, hash); 29 } 30 return result; 31 } 32 33 template <typename... T> ComputeAggregateHash(const T &...objs)34uint64_t ComputeAggregateHash(const T&... objs) { 35 // This value is taken from absl::Hash implementation. 36 constexpr uint64_t default_seed = UINT64_C(0xc3a5c85c97cb3127); 37 return ComputeAggregateHash(default_seed, objs...); 38 } 39 40 struct PairHash { 41 template <typename TFirst, typename TSecond> operatorPairHash42 size_t operator()(const std::pair<TFirst, TSecond>& pair) const { 43 return ComputeAggregateHash(pair.first, pair.second); 44 } 45 }; 46 47 } // namespace openscreen 48 49 #endif // UTIL_HASHING_H_ 50