chromium/base/hash/hash.cc

// Copyright 2014 The Chromium Authors
// 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 <cstddef>
#include <cstdint>
#include <limits>
#include <string>
#include <string_view>

#include "base/containers/span.h"
#include "base/dcheck_is_on.h"
#include "base/notreached.h"
#include "base/numerics/safe_conversions.h"
#include "base/third_party/cityhash/city.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<const uint8_t> data) { … }

// 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 [email protected] for any questions.
size_t HashInts32Impl(uint32_t value1, uint32_t 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 HashInts64Impl(uint64_t value1, uint64_t value2) { … }

// 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 = …;
#endif

template <typename T>
T Scramble(T input) { … }

}  // namespace

size_t FastHash(base::span<const uint8_t> data) { … }

uint32_t Hash(base::span<const uint8_t> data) { … }

uint32_t Hash(const std::string& str) { … }

uint32_t PersistentHash(span<const uint8_t> data) { … }

uint32_t PersistentHash(std::string_view str) { … }

size_t HashInts32(uint32_t value1, uint32_t value2) { … }

size_t HashInts64(uint64_t value1, uint64_t value2) { … }

}  // namespace base