gecko-dev/gfx/gl/AutoMappable.h

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef MOZILLA_AUTO_MAPPABLE_H
#define MOZILLA_AUTO_MAPPABLE_H

// Here be dragons.

#include <functional>

namespace mozilla::gfx {

template <class T>
size_t Hash(const T&);

template <class T>
struct StaticStdHasher {
  static auto HashImpl(const T& v) { return std::hash<T>()(v); }
};

template <class T>
struct StaticHasher {
  static auto HashImpl(const T& v) { return v.hash(); }
};
template <class T>
struct StaticHasher<std::optional<T>> {
  static size_t HashImpl(const std::optional<T>& v) {
    if (!v) return 0;
    return Hash(*v);
  }
};
template <>
struct StaticHasher<int> : public StaticStdHasher<int> {};
template <>
struct StaticHasher<bool> : public StaticStdHasher<bool> {};
template <>
struct StaticHasher<float> : public StaticStdHasher<float> {};

template <class T>
size_t Hash(const T& v) {
  return StaticHasher<T>::HashImpl(v);
}

//-
// From Boost:
// https://www.boost.org/doc/libs/1_37_0/doc/html/hash/reference.html#boost.hash_combine

inline size_t HashCombine(size_t seed, const size_t hash) {
  seed ^= hash + 0x9e3779b9 + (seed << 6) + (seed >> 2);
  return seed;
}

// -
// See
// https://codereview.stackexchange.com/questions/136770/hashing-a-tuple-in-c17

template <class... Args, size_t... Ids>
size_t HashTupleN(const std::tuple<Args...>& tup,
                  const std::index_sequence<Ids...>&) {
  size_t seed = 0;
  for (const auto& hash : {Hash(std::get<Ids>(tup))...}) {
    seed = HashCombine(seed, hash);
  }
  return seed;
}

template <class... Args>
size_t HashTuple(const std::tuple<Args...>& tup) {
  return HashTupleN(tup, std::make_index_sequence<sizeof...(Args)>());
}

// -

template <class T>
auto MembersEq(const T& a, const T& b) {
  const auto atup = a.Members();
  const auto btup = b.Members();
  return atup == btup;
}

template <class T>
auto MembersLt(const T& a, const T& b) {
  const auto atup = a.Members();
  const auto btup = b.Members();
  return atup == btup;
}

template <class T>
auto MembersHash(const T& a) {
  const auto atup = a.Members();
  return HashTuple(atup);
}

template <class T>
struct MembersHasher final {
  auto operator()(const T& v) const { return v.hash(); }
};

/** E.g.:
struct Foo {
    int i;
    bool b;

   auto Members() const { return std::tie(i, b); }
    INLINE_AUTO_MAPPABLE(Foo)
};
std::unordered_set<T, T::Hasher> easy;
**/
#define INLINE_DERIVE_MEMBERS_EQ(T)                \
  friend bool operator==(const T& a, const T& b) { \
    return mozilla::gfx::MembersEq(a, b);          \
  }                                                \
  friend bool operator!=(const T& a, const T& b) { return !operator==(a, b); }
#define INLINE_AUTO_MAPPABLE(T)                                          \
  friend bool operator<(const T& a, const T& b) {                        \
    return mozilla::gfx::MembersLt(a, b);                                \
  }                                                                      \
  INLINE_DERIVE_MEMBERS_EQ(T)                                            \
  size_t hash() const {                                                  \
    return mozilla::gfx::MembersHash(*reinterpret_cast<const T*>(this)); \
  }                                                                      \
  using Hasher = mozilla::gfx::MembersHasher<T>;

// -

/** E.g.:
```
struct Foo : public AutoMappable<Foo> {
    int i;
    bool b;

    auto Members() const { return std::tie(i, b); }
};
std::unordered_set<T, T::Hasher> easy;
```
`easy.insert({{}, 2, true});`
The initial {} is needed for aggregate initialization of AutoMappable<Foo>.
Use INLINE_AUTO_MAPPABLE if this is too annoying.
**/
template <class T>
struct AutoMappable {
  INLINE_AUTO_MAPPABLE(T)
};

}  // namespace mozilla::gfx

#endif  // MOZILLA_AUTO_MAPPABLE_H