tuple/type: add prefix query

test/tuple/type.cpp

@@ -99,5 +99,17 @@ main (void)
         tap.expect (std::is_same<dst_t, std::tuple<int>>::value, "tuple type mapping");
     }
 
+    {
+        // Test that prefix extraction of a tuple works as expected.
+        using full_t = std::tuple<i08, i16, i32, i64>;
+
+        static constexpr std::size_t LENGTH = 2;
+        using prefix_t = cruft::tuple::type::prefix_t<LENGTH, full_t>;
+
+        using expected_t = std::tuple<i08, i16>;
+
+        tap.expect (std::is_same_v<prefix_t, expected_t>, "tuple prefix");
+    }
+
     return tap.status ();
 }

tuple/type.hpp

@@ -335,4 +335,37 @@ namespace cruft::tuple::type {
     //-------------------------------------------------------------------------
     template <typename TupleT, std::size_t IndexV>
     using nth_t = typename nth<TupleT, IndexV>::type;
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// Extract the first CountV types of the tuple-like-object and store them
+    /// in a tuple definition.
+    template <
+        std::size_t CountV,
+        typename TupleT,
+        typename Indices = std::make_index_sequence<CountV>
+    >
+    struct prefix;
+
+
+    //-------------------------------------------------------------------------
+    template <
+        std::size_t CountV,
+        typename TupleT,
+        std::size_t ...IndexV
+    >
+    struct prefix<
+        CountV,
+        TupleT,
+        std::index_sequence<IndexV...>
+    > {
+        using type = std::tuple<
+            std::tuple_element_t<IndexV, TupleT>...
+        >;
+    };
+
+
+    //-------------------------------------------------------------------------
+    template <std::size_t CountV, typename TupleT>
+    using prefix_t = typename prefix<CountV, TupleT>::type;
 };