commit 403cda2c32f6ea27e4ad075910eb347c4a4e4963
author William A. Kennington III <wak@google.com> Mon Dec 12 15:52:35 2022 -0800
committer William A. Kennington III <wak@google.com> Thu May 25 14:59:48 2023 -0700
tree 3763063d4f4740a10859ade26295d2ab5ce364c1
parent 48a470a410d51455de862e1ac23e283a49e3d358

hash: Add function for efficiently combining hashes

You can now call stdplus::hashMulti(...) with multiple arguments and get
a single resulting hash value.

Change-Id: I82d91f3372daeea941f6b9e8d57a224b5806527d
Signed-off-by: William A. Kennington III <wak@google.com>

include/meson.build

diff --git a/include/meson.build b/include/meson.build
index c3bfdfe..358bc32 100644
--- a/include/meson.build
+++ b/include/meson.build
@@ -6,6 +6,7 @@
   'stdplus/flags.hpp',
   'stdplus/handle/copyable.hpp',
   'stdplus/handle/managed.hpp',
+  'stdplus/hash.hpp',
   'stdplus/pinned.hpp',
   'stdplus/raw.hpp',
   'stdplus/signal.hpp',

include/stdplus/hash.hpp

diff --git a/include/stdplus/hash.hpp b/include/stdplus/hash.hpp
new file mode 100644
index 0000000..44cc6a6
--- /dev/null
+++ b/include/stdplus/hash.hpp
@@ -0,0 +1,68 @@
+#pragma once
+#include <cstddef>
+#include <utility>
+
+namespace std
+{
+template <class Key>
+struct hash;
+}
+
+namespace stdplus
+{
+
+template <class Key>
+struct hash;
+
+namespace detail
+{
+
+constexpr std::size_t updateSeed(std::size_t seed, std::size_t v) noexcept
+{
+    return seed ^ (v + 0x9e3779b9 + (seed << 6) + (seed >> 2));
+}
+
+template <typename T>
+constexpr std::size_t
+    hashMultiS(std::size_t seed,
+               const T& t) noexcept(noexcept(hash<T>{}(std::declval<T>())))
+{
+    return updateSeed(seed, hash<T>{}(t));
+}
+
+template <typename T, typename... Ts>
+constexpr std::size_t
+    hashMultiS(std::size_t seed, const T& t, const Ts&... ts) noexcept(
+        (noexcept(hashMultiS(0, std::declval<T>())) &&
+         ...&& noexcept(hashMultiS(0, std::declval<Ts>()))))
+{
+    return hashMultiS(hashMultiS(seed, t), ts...);
+}
+
+} // namespace detail
+
+constexpr std::size_t hashMulti() noexcept
+{
+    return 0;
+}
+
+template <typename T>
+constexpr std::size_t
+    hashMulti(const T& t) noexcept(noexcept(hash<T>{}(std::declval<T>())))
+{
+    return hash<T>{}(t);
+}
+
+template <typename T, typename... Ts>
+constexpr std::size_t hashMulti(const T& t, const Ts&... ts) noexcept(
+    noexcept(hashMulti(std::declval<T>())) && noexcept(
+        detail::hashMultiS(0, std::declval<Ts>()...)))
+{
+    return detail::hashMultiS(hashMulti(t), ts...);
+}
+
+template <class Key>
+struct hash : std::hash<Key>
+{};
+
+} // namespace stdplus