1 files changed, 148 insertions, 0 deletions
diff --git a/gfx/gl/AutoMappable.h b/gfx/gl/AutoMappable.h
new file mode 100644
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+++ b/gfx/gl/AutoMappable.h
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+/* 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