hash/tuple: add a std compatible tuple hashing object

CMakeLists.txt

@@ -427,6 +427,8 @@ list (
     hash/siphash.hpp
     hash/table.cpp
     hash/table.hpp
+    hash/tuple.cpp
+    hash/tuple.hpp
     hash/wang.hpp
     hash/xxhash.cpp
     hash/xxhash.hpp
@@ -755,6 +757,7 @@ if (TESTS)
         hash/murmur
         hash/siphash
         hash/table
+        hash/tuple
         hash/xxhash
         hton
         io

hash/tuple.cpp

@@ -0,0 +1 @@
+#include "./tuple.hpp"

hash/tuple.hpp

@@ -0,0 +1,37 @@
+#pragma once
+
+#include "../hash.hpp"
+#include "./mix.hpp"
+
+#include <tuple>
+
+
+namespace cruft::hash {
+    /// Computes a hash for a tuple type by elementwise hashing, then variadic mixing.
+    ///
+    /// \tparam ElementT  The per-element hash type
+    /// \tparam MixT      The mixing hash type. Must support variadic operator()
+    template <
+        typename ElementT,
+        typename MixT = cruft::hash::mixer
+    >
+    class tuple {
+    public:
+        template <typename ...ValueT>
+        auto
+        operator() (std::tuple<ValueT...> const &val)
+        {
+            return compute (val, std::make_index_sequence<sizeof... (ValueT)> {});
+        }
+
+    private:
+        template <typename ...ValueT, std::size_t ...IndexV>
+        auto
+        compute (std::tuple<ValueT...> const &val, std::index_sequence<IndexV...>)
+        {
+            return MixT {} (
+                ElementT {} (std::get<IndexV> (val))...
+            );
+        }
+    };
+}

test/hash/tuple.cpp

@@ -0,0 +1,47 @@
+#include "hash/tuple.hpp"
+
+#include "tap.hpp"
+
+#include <string_view>
+
+
+// A "hash" that returns the argument's size.
+struct size_hash {
+    template <typename ValueT>
+    std::size_t
+    operator () (ValueT const &val) const
+    { return std::size (val); }
+};
+
+
+// A 'mix' function that just sums the arguments.
+struct sum_mix {
+    template <typename ...ValueT>
+    std::size_t
+    operator () (ValueT ...val) const
+    {
+        return (val + ...);
+    }
+};
+
+
+int
+main ()
+{
+    using namespace std::string_view_literals;
+
+    cruft::hash::tuple<size_hash, sum_mix> h;
+
+    cruft::TAP::logger tap;
+    tap.expect_eq (
+        h (
+            std::make_tuple (
+                "foo"sv, std::vector<int> (5)
+            )
+        ),
+        8u,
+        "size/sum tuple hash"
+    );
+
+    return tap.status ();
+}