low_level_hash.cc

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// Copyright 2020 The Abseil Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
 
#include "absl/hash/internal/low_level_hash.h"
 
#include "absl/base/internal/unaligned_access.h"
#include "absl/numeric/bits.h"
#include "absl/numeric/int128.h"
 
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace hash_internal {
 
static uint64_t Mix(uint64_t v0, uint64_t v1) {
#if !defined(__aarch64__)
  // The default bit-mixer uses 64x64->128-bit multiplication.
  absl::uint128 p = v0;
  p *= v1;
  return absl::Uint128Low64(p) ^ absl::Uint128High64(p);
#else
  // The default bit-mixer above would perform poorly on some ARM microarchs,
  // where calculating a 128-bit product requires a sequence of two
  // instructions with a high combined latency and poor throughput.
  // Instead, we mix bits using only 64-bit arithmetic, which is faster.
  uint64_t p = v0 ^ absl::rotl(v1, 40);
  p *= v1 ^ absl::rotl(v0, 39);
  return p ^ (p >> 11);
#endif
}
 
uint64_t LowLevelHash(const void* data, size_t len, uint64_t seed,
                      const uint64_t salt[]) {
  const uint8_t* ptr = static_cast<const uint8_t*>(data);
  uint64_t starting_length = static_cast<uint64_t>(len);
  uint64_t current_state = seed ^ salt[0];
 
  if (len > 64) {
    // If we have more than 64 bytes, we're going to handle chunks of 64
    // bytes at a time. We're going to build up two separate hash states
    // which we will then hash together.
    uint64_t duplicated_state = current_state;
 
    do {
      uint64_t a = absl::base_internal::UnalignedLoad64(ptr);
      uint64_t b = absl::base_internal::UnalignedLoad64(ptr + 8);
      uint64_t c = absl::base_internal::UnalignedLoad64(ptr + 16);
      uint64_t d = absl::base_internal::UnalignedLoad64(ptr + 24);
      uint64_t e = absl::base_internal::UnalignedLoad64(ptr + 32);
      uint64_t f = absl::base_internal::UnalignedLoad64(ptr + 40);
      uint64_t g = absl::base_internal::UnalignedLoad64(ptr + 48);
      uint64_t h = absl::base_internal::UnalignedLoad64(ptr + 56);
 
      uint64_t cs0 = Mix(a ^ salt[1], b ^ current_state);
      uint64_t cs1 = Mix(c ^ salt[2], d ^ current_state);
      current_state = (cs0 ^ cs1);
 
      uint64_t ds0 = Mix(e ^ salt[3], f ^ duplicated_state);
      uint64_t ds1 = Mix(g ^ salt[4], h ^ duplicated_state);
      duplicated_state = (ds0 ^ ds1);
 
      ptr += 64;
      len -= 64;
    } while (len > 64);
 
    current_state = current_state ^ duplicated_state;
  }
 
  // We now have a data `ptr` with at most 64 bytes and the current state
  // of the hashing state machine stored in current_state.
  while (len > 16) {
    uint64_t a = absl::base_internal::UnalignedLoad64(ptr);
    uint64_t b = absl::base_internal::UnalignedLoad64(ptr + 8);
 
    current_state = Mix(a ^ salt[1], b ^ current_state);
 
    ptr += 16;
    len -= 16;
  }
 
  // We now have a data `ptr` with at most 16 bytes.
  uint64_t a = 0;
  uint64_t b = 0;
  if (len > 8) {
    // When we have at least 9 and at most 16 bytes, set A to the first 64
    // bits of the input and B to the last 64 bits of the input. Yes, they will
    // overlap in the middle if we are working with less than the full 16
    // bytes.
    a = absl::base_internal::UnalignedLoad64(ptr);
    b = absl::base_internal::UnalignedLoad64(ptr + len - 8);
  } else if (len > 3) {
    // If we have at least 4 and at most 8 bytes, set A to the first 32
    // bits and B to the last 32 bits.
    a = absl::base_internal::UnalignedLoad32(ptr);
    b = absl::base_internal::UnalignedLoad32(ptr + len - 4);
  } else if (len > 0) {
    // If we have at least 1 and at most 3 bytes, read all of the provided
    // bits into A, with some adjustments.
    a = ((ptr[0] << 16) | (ptr[len >> 1] << 8) | ptr[len - 1]);
    b = 0;
  } else {
    a = 0;
    b = 0;
  }
 
  uint64_t w = Mix(a ^ salt[1], b ^ current_state);
  uint64_t z = salt[1] ^ starting_length;
  return Mix(w, z);
}
 
}  // namespace hash_internal
ABSL_NAMESPACE_END
}  // namespace absl