hashing: New utils for hashing multiple values

src/v/hashing/utils.h

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2024 Redpanda Data, Inc.
+ *
+ * Use of this software is governed by the Business Source License
+ * included in the file licenses/BSL.md
+ *
+ * As of the Change Date specified in that file, in accordance with
+ * the Business Source License, use of this software will be governed
+ * by the Apache License, Version 2.0
+ */
+
+#pragma once
+
+#include <absl/hash/hash.h>
+#include <boost/functional/hash.hpp>
+
+#include <functional>
+
+namespace hash {
+namespace impl {
+/// Pulled from utils/functional.h to prevent making v::utils a dependency of
+/// v::hashing
+template<typename>
+inline constexpr bool always_false_v = false;
+
+template<typename T>
+concept is_absl_hashable = requires(T const& t) {
+    { absl::Hash<T>{}(t) } -> std::convertible_to<std::size_t>;
+};
+
+template<typename T>
+concept is_std_hashable = requires(T const& t) {
+    { std::hash<T>{}(t) } -> std::convertible_to<std::size_t>;
+};
+
+template<typename T>
+concept is_boost_hashable = requires(T const& t) {
+    { boost::hash<T>{}(t) } -> std::convertible_to<std::size_t>;
+};
+
+template<typename T>
+size_t combine(size_t& seed, T const& t) {
+    if constexpr (is_std_hashable<T>) {
+        boost::hash_combine(seed, std::hash<T>{}(t));
+    } else if constexpr (is_absl_hashable<T>) {
+        boost::hash_combine(seed, absl::Hash<T>{}(t));
+    } else if constexpr (is_boost_hashable<T>) {
+        boost::hash_combine(seed, boost::hash<T>{}(t));
+    } else {
+        static_assert(always_false_v<T>, "No hasher found for T");
+    }
+    return seed;
+}
+
+} // namespace impl
+
+template<typename... Args>
+size_t combine(size_t& seed, Args&&... args) {
+    return (impl::combine(seed, std::forward<Args>(args)), ...);
+}
+
+} // namespace hash