components/url_pattern_index/uint64_hasher.h

// Copyright 2016 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.

// Based on "v8/src/base/functional.cc".
// See: Thomas Wang, Integer Hash Functions.
// https://gist.github.com/badboy/6267743
// TODO(pkalinnikov): Consider moving the implementation into base/.

#ifndef COMPONENTS_URL_PATTERN_INDEX_UINT64_HASHER_H_
#define COMPONENTS_URL_PATTERN_INDEX_UINT64_HASHER_H_

#include <stddef.h>
#include <stdint.h>

#include <type_traits>

namespace url_pattern_index {

template <typename T>
typename std::enable_if<sizeof(T) == 4, T>::type Uint64Hash(uint64_t v) {
  // "64 bit to 32 bit Hash Functions"
  v = ~v + (v << 18);  // v = (v << 18) - v - 1;
  v = v ^ (v >> 31);
  v = v * 21;  // v = (v + (v << 2)) + (v << 4);
  v = v ^ (v >> 11);
  v = v + (v << 6);
  v = v ^ (v >> 22);
  return static_cast<T>(v);
}

template <typename T>
typename std::enable_if<sizeof(T) == 8, T>::type Uint64Hash(uint64_t v) {
  // "64 bit Mix Functions"
  v = ~v + (v << 21);  // v = (v << 21) - v - 1;
  v = v ^ (v >> 24);
  v = (v + (v << 3)) + (v << 8);  // v * 265
  v = v ^ (v >> 14);
  v = (v + (v << 2)) + (v << 4);  // v * 21
  v = v ^ (v >> 28);
  v = v + (v << 31);
  return static_cast<T>(v);
}

// Note: A Uint64ToUint64Hasher variant is currently not needed.
// Note: Be careful about a variant that hashes differently in 32-bit vs. 64-bit
// processes (i.e., using |size_t| where the below uses |uint32_t|). Such a
// variant will break compatibility for tables that are built and accessed in
// processes of differing bitness, which is a real-world concern for users of
// this component (see crbug.com/1174797 for details).
class Uint64ToUint32Hasher {
 public:
  uint32_t operator()(uint64_t v) const { return Uint64Hash<uint32_t>(v); }
};

}  // namespace url_pattern_index

#endif  // COMPONENTS_URL_PATTERN_INDEX_UINT64_HASHER_H_