// Copyright 2014 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. #include "base/hash/hash.h" #include "base/check_op.h" #include "base/notreached.h" #include "base/rand_util.h" #include "base/third_party/cityhash/city.h" #include "build/build_config.h" // Definition in base/third_party/superfasthash/superfasthash.c. (Third-party // code did not come with its own header file, so declaring the function here.) // Note: This algorithm is also in Blink under Source/wtf/StringHasher.h. extern "C" uint32_t SuperFastHash(const char* data, int len); namespace base { namespace { size_t FastHashImpl(base::span data) { // We use the updated CityHash within our namespace (not the deprecated // version from third_party/smhasher). if constexpr (sizeof(size_t) > 4) { return base::internal::cityhash_v111::CityHash64( reinterpret_cast(data.data()), data.size()); } else { return base::internal::cityhash_v111::CityHash32( reinterpret_cast(data.data()), data.size()); } } // Implement hashing for pairs of at-most 32 bit integer values. // When size_t is 32 bits, we turn the 64-bit hash code into 32 bits by using // multiply-add hashing. This algorithm, as described in // Theorem 4.3.3 of the thesis "Über die Komplexität der Multiplikation in // eingeschränkten Branchingprogrammmodellen" by Woelfel, is: // // h32(x32, y32) = (h64(x32, y32) * rand_odd64 + rand16 * 2^16) % 2^64 / 2^32 // // Contact danakj@chromium.org for any questions. size_t HashInts32Impl(uint32_t value1, uint32_t value2) { uint64_t value1_64 = value1; uint64_t hash64 = (value1_64 << 32) | value2; if (sizeof(size_t) >= sizeof(uint64_t)) return static_cast(hash64); uint64_t odd_random = 481046412LL << 32 | 1025306955LL; uint32_t shift_random = 10121U << 16; hash64 = hash64 * odd_random + shift_random; size_t high_bits = static_cast(hash64 >> (8 * (sizeof(uint64_t) - sizeof(size_t)))); return high_bits; } // Implement hashing for pairs of up-to 64-bit integer values. // We use the compound integer hash method to produce a 64-bit hash code, by // breaking the two 64-bit inputs into 4 32-bit values: // http://opendatastructures.org/versions/edition-0.1d/ods-java/node33.html#SECTION00832000000000000000 // Then we reduce our result to 32 bits if required, similar to above. size_t HashInts64Impl(uint64_t value1, uint64_t value2) { uint32_t short_random1 = 842304669U; uint32_t short_random2 = 619063811U; uint32_t short_random3 = 937041849U; uint32_t short_random4 = 3309708029U; uint32_t value1a = static_cast(value1 & 0xffffffff); uint32_t value1b = static_cast((value1 >> 32) & 0xffffffff); uint32_t value2a = static_cast(value2 & 0xffffffff); uint32_t value2b = static_cast((value2 >> 32) & 0xffffffff); uint64_t product1 = static_cast(value1a) * short_random1; uint64_t product2 = static_cast(value1b) * short_random2; uint64_t product3 = static_cast(value2a) * short_random3; uint64_t product4 = static_cast(value2b) * short_random4; uint64_t hash64 = product1 + product2 + product3 + product4; if (sizeof(size_t) >= sizeof(uint64_t)) return static_cast(hash64); uint64_t odd_random = 1578233944LL << 32 | 194370989LL; uint32_t shift_random = 20591U << 16; hash64 = hash64 * odd_random + shift_random; size_t high_bits = static_cast(hash64 >> (8 * (sizeof(uint64_t) - sizeof(size_t)))); return high_bits; } // The random seed is used to perturb the output of base::FastHash() and // base::HashInts() so that it is only deterministic within the lifetime of a // process. This prevents inadvertent dependencies on the underlying // implementation, e.g. anything that persists the hash value and expects it to // be unchanging will break. // // Note: this is the same trick absl uses to generate a random seed. This is // more robust than using base::RandBytes(), which can fail inside a sandboxed // environment. Note that without ASLR, the seed won't be quite as random... #if DCHECK_IS_ON() constexpr const void* kSeed = &kSeed; #endif template T Scramble(T input) { #if DCHECK_IS_ON() return HashInts64Impl(input, reinterpret_cast(kSeed)); #else return input; #endif } } // namespace size_t FastHash(base::span data) { return Scramble(FastHashImpl(data)); } uint32_t Hash(const void* data, size_t length) { // Currently our in-memory hash is the same as the persistent hash. The // split between in-memory and persistent hash functions is maintained to // allow the in-memory hash function to be updated in the future. return PersistentHash(data, length); } uint32_t Hash(const std::string& str) { return PersistentHash(as_bytes(make_span(str))); } uint32_t Hash(const std::u16string& str) { return PersistentHash(as_bytes(make_span(str))); } uint32_t PersistentHash(span data) { // This hash function must not change, since it is designed to be persistable // to disk. if (data.size() > static_cast(std::numeric_limits::max())) { NOTREACHED(); return 0; } return ::SuperFastHash(reinterpret_cast(data.data()), static_cast(data.size())); } uint32_t PersistentHash(const void* data, size_t length) { return PersistentHash(make_span(static_cast(data), length)); } uint32_t PersistentHash(const std::string& str) { return PersistentHash(str.data(), str.size()); } size_t HashInts32(uint32_t value1, uint32_t value2) { return Scramble(HashInts32Impl(value1, value2)); } // Implement hashing for pairs of up-to 64-bit integer values. // We use the compound integer hash method to produce a 64-bit hash code, by // breaking the two 64-bit inputs into 4 32-bit values: // http://opendatastructures.org/versions/edition-0.1d/ods-java/node33.html#SECTION00832000000000000000 // Then we reduce our result to 32 bits if required, similar to above. size_t HashInts64(uint64_t value1, uint64_t value2) { return Scramble(HashInts64Impl(value1, value2)); } } // namespace base