168 lines
6.1 KiB
C++
168 lines
6.1 KiB
C++
// Copyright 2014 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#include "base/hash/hash.h"
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#include "base/rand_util.h"
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#include "base/third_party/cityhash/city.h"
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#include "build/build_config.h"
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// Definition in base/third_party/superfasthash/superfasthash.c. (Third-party
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// code did not come with its own header file, so declaring the function here.)
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// Note: This algorithm is also in Blink under Source/wtf/StringHasher.h.
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extern "C" uint32_t SuperFastHash(const char* data, int len);
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namespace base {
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namespace {
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size_t FastHashImpl(base::span<const uint8_t> data) {
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// We use the updated CityHash within our namespace (not the deprecated
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// version from third_party/smhasher).
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#if defined(ARCH_CPU_64_BITS)
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return base::internal::cityhash_v111::CityHash64(
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reinterpret_cast<const char*>(data.data()), data.size());
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#else
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return base::internal::cityhash_v111::CityHash32(
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reinterpret_cast<const char*>(data.data()), data.size());
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#endif
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}
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// Implement hashing for pairs of at-most 32 bit integer values.
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// When size_t is 32 bits, we turn the 64-bit hash code into 32 bits by using
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// multiply-add hashing. This algorithm, as described in
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// Theorem 4.3.3 of the thesis "Über die Komplexität der Multiplikation in
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// eingeschränkten Branchingprogrammmodellen" by Woelfel, is:
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//
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// h32(x32, y32) = (h64(x32, y32) * rand_odd64 + rand16 * 2^16) % 2^64 / 2^32
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//
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// Contact danakj@chromium.org for any questions.
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size_t HashInts32Impl(uint32_t value1, uint32_t value2) {
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uint64_t value1_64 = value1;
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uint64_t hash64 = (value1_64 << 32) | value2;
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if (sizeof(size_t) >= sizeof(uint64_t))
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return static_cast<size_t>(hash64);
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uint64_t odd_random = 481046412LL << 32 | 1025306955LL;
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uint32_t shift_random = 10121U << 16;
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hash64 = hash64 * odd_random + shift_random;
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size_t high_bits =
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static_cast<size_t>(hash64 >> (8 * (sizeof(uint64_t) - sizeof(size_t))));
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return high_bits;
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}
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// Implement hashing for pairs of up-to 64-bit integer values.
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// We use the compound integer hash method to produce a 64-bit hash code, by
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// breaking the two 64-bit inputs into 4 32-bit values:
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// http://opendatastructures.org/versions/edition-0.1d/ods-java/node33.html#SECTION00832000000000000000
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// Then we reduce our result to 32 bits if required, similar to above.
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size_t HashInts64Impl(uint64_t value1, uint64_t value2) {
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uint32_t short_random1 = 842304669U;
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uint32_t short_random2 = 619063811U;
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uint32_t short_random3 = 937041849U;
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uint32_t short_random4 = 3309708029U;
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uint32_t value1a = static_cast<uint32_t>(value1 & 0xffffffff);
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uint32_t value1b = static_cast<uint32_t>((value1 >> 32) & 0xffffffff);
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uint32_t value2a = static_cast<uint32_t>(value2 & 0xffffffff);
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uint32_t value2b = static_cast<uint32_t>((value2 >> 32) & 0xffffffff);
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uint64_t product1 = static_cast<uint64_t>(value1a) * short_random1;
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uint64_t product2 = static_cast<uint64_t>(value1b) * short_random2;
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uint64_t product3 = static_cast<uint64_t>(value2a) * short_random3;
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uint64_t product4 = static_cast<uint64_t>(value2b) * short_random4;
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uint64_t hash64 = product1 + product2 + product3 + product4;
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if (sizeof(size_t) >= sizeof(uint64_t))
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return static_cast<size_t>(hash64);
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uint64_t odd_random = 1578233944LL << 32 | 194370989LL;
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uint32_t shift_random = 20591U << 16;
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hash64 = hash64 * odd_random + shift_random;
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size_t high_bits =
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static_cast<size_t>(hash64 >> (8 * (sizeof(uint64_t) - sizeof(size_t))));
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return high_bits;
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}
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// The random seed is used to perturb the output of base::FastHash() and
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// base::HashInts() so that it is only deterministic within the lifetime of a
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// process. This prevents inadvertent dependencies on the underlying
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// implementation, e.g. anything that persists the hash value and expects it to
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// be unchanging will break.
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//
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// Note: this is the same trick absl uses to generate a random seed. This is
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// more robust than using base::RandBytes(), which can fail inside a sandboxed
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// environment. Note that without ASLR, the seed won't be quite as random...
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#if DCHECK_IS_ON()
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constexpr const void* kSeed = &kSeed;
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#endif
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template <typename T>
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T Scramble(T input) {
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#if DCHECK_IS_ON()
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return HashInts64Impl(input, reinterpret_cast<uintptr_t>(kSeed));
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#else
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return input;
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#endif
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}
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} // namespace
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size_t FastHash(base::span<const uint8_t> data) {
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return Scramble(FastHashImpl(data));
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}
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uint32_t Hash(const void* data, size_t length) {
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// Currently our in-memory hash is the same as the persistent hash. The
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// split between in-memory and persistent hash functions is maintained to
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// allow the in-memory hash function to be updated in the future.
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return PersistentHash(data, length);
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}
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uint32_t Hash(const std::string& str) {
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return PersistentHash(as_bytes(make_span(str)));
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}
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uint32_t Hash(const string16& str) {
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return PersistentHash(as_bytes(make_span(str)));
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}
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uint32_t PersistentHash(span<const uint8_t> data) {
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// This hash function must not change, since it is designed to be persistable
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// to disk.
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if (data.size() > static_cast<size_t>(std::numeric_limits<int>::max())) {
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NOTREACHED();
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return 0;
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}
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return ::SuperFastHash(reinterpret_cast<const char*>(data.data()),
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static_cast<int>(data.size()));
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}
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uint32_t PersistentHash(const void* data, size_t length) {
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return PersistentHash(make_span(static_cast<const uint8_t*>(data), length));
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}
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uint32_t PersistentHash(const std::string& str) {
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return PersistentHash(str.data(), str.size());
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}
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size_t HashInts32(uint32_t value1, uint32_t value2) {
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return Scramble(HashInts32Impl(value1, value2));
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}
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// Implement hashing for pairs of up-to 64-bit integer values.
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// We use the compound integer hash method to produce a 64-bit hash code, by
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// breaking the two 64-bit inputs into 4 32-bit values:
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// http://opendatastructures.org/versions/edition-0.1d/ods-java/node33.html#SECTION00832000000000000000
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// Then we reduce our result to 32 bits if required, similar to above.
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size_t HashInts64(uint64_t value1, uint64_t value2) {
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return Scramble(HashInts64Impl(value1, value2));
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}
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} // namespace base
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