env/ceres-solver/internal/ceres/pair_hash.h

// Ceres Solver - A fast non-linear least squares minimizer
// Copyright 2022 Google Inc. All rights reserved.
// http://ceres-solver.org/
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// Author: keir@google.com (Keir Mierle)
//
// A hasher for std::pair<T, T>.

#ifndef CERES_INTERNAL_PAIR_HASH_H_
#define CERES_INTERNAL_PAIR_HASH_H_

#include <cstddef>
#include <cstdint>
#include <functional>
#include <utility>

#include "ceres/internal/export.h"

namespace ceres::internal {

#if defined(_WIN32) && !defined(__MINGW64__) && !defined(__MINGW32__)
#define GG_LONGLONG(x) x##I64
#define GG_ULONGLONG(x) x##UI64
#else
#define GG_LONGLONG(x) x##LL
#define GG_ULONGLONG(x) x##ULL
#endif

// The hash function is due to Bob Jenkins (see
// http://burtleburtle.net/bob/hash/index.html). Each mix takes 36 instructions,
// in 18 cycles if you're lucky. On x86 architectures, this requires 45
// instructions in 27 cycles, if you're lucky.
//
// clang-format off
//
// 32bit version
inline void hash_mix(uint32_t& a, uint32_t& b, uint32_t& c) {
  a -= b; a -= c; a ^= (c>>13);
  b -= c; b -= a; b ^= (a<<8);
  c -= a; c -= b; c ^= (b>>13);
  a -= b; a -= c; a ^= (c>>12);
  b -= c; b -= a; b ^= (a<<16);
  c -= a; c -= b; c ^= (b>>5);
  a -= b; a -= c; a ^= (c>>3);
  b -= c; b -= a; b ^= (a<<10);
  c -= a; c -= b; c ^= (b>>15);
}

// 64bit version
inline void hash_mix(uint64_t& a, uint64_t& b, uint64_t& c) {
  a -= b; a -= c; a ^= (c>>43);
  b -= c; b -= a; b ^= (a<<9);
  c -= a; c -= b; c ^= (b>>8);
  a -= b; a -= c; a ^= (c>>38);
  b -= c; b -= a; b ^= (a<<23);
  c -= a; c -= b; c ^= (b>>5);
  a -= b; a -= c; a ^= (c>>35);
  b -= c; b -= a; b ^= (a<<49);
  c -= a; c -= b; c ^= (b>>11);
}
// clang-format on

inline uint32_t Hash32NumWithSeed(uint32_t num, uint32_t c) {
  // The golden ratio; an arbitrary value.
  uint32_t b = 0x9e3779b9UL;
  hash_mix(num, b, c);
  return c;
}

inline uint64_t Hash64NumWithSeed(uint64_t num, uint64_t c) {
  // More of the golden ratio.
  uint64_t b = GG_ULONGLONG(0xe08c1d668b756f82);
  hash_mix(num, b, c);
  return c;
}

// Hasher for STL pairs. Requires hashers for both members to be defined.
struct pair_hash {
 public:
  template <typename T>
  std::size_t operator()(const std::pair<T, T>& p) const {
    const std::size_t h1 = std::hash<T>()(p.first);
    const std::size_t h2 = std::hash<T>()(p.second);
    // The decision below is at compile time
    return (sizeof(h1) <= sizeof(uint32_t)) ? Hash32NumWithSeed(h1, h2)
                                            : Hash64NumWithSeed(h1, h2);
  }
};

}  // namespace ceres::internal

#endif  // CERES_INTERNAL_PAIR_HASH_H_