From 8f2f623520b6684a0144fcdf5d465849c2c127cc Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Thu, 5 Apr 2018 12:23:12 +1000
Subject: [PATCH] tuple/value: add zip implementation

---
 test/tuple.cpp  | 13 ++++++++++++
 tuple/value.hpp | 53 ++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 65 insertions(+), 1 deletion(-)

diff --git a/test/tuple.cpp b/test/tuple.cpp
index 37cb0134..a2de49c0 100644
--- a/test/tuple.cpp
+++ b/test/tuple.cpp
@@ -3,6 +3,7 @@
 
 #include "tap.hpp"
 
+#include <array>
 #include <typeindex>
 #include <vector>
 
@@ -64,5 +65,17 @@ main ()
         tap.expect (std::is_same<dst_t, std::tuple<int>>::value, "tuple type mapping");
     }
 
+
+    {
+        std::tuple a (1, 2);
+        std::array<char,2> b { 'a', 'b' };
+        std::tuple c (
+            std::tuple(1, 'a'),
+            std::tuple(2, 'b')
+        );
+
+        tap.expect_eq (c, util::tuple::value::zip (a, b), "tuple zipping");
+    }
+
     return tap.status ();
 }
diff --git a/tuple/value.hpp b/tuple/value.hpp
index 95454ce6..0d66fbbc 100644
--- a/tuple/value.hpp
+++ b/tuple/value.hpp
@@ -72,7 +72,58 @@ namespace util::tuple::value {
     {
         return detail::map (IndicesV{}, func, arg);
     }
-};
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    // return a tuple that contains the 'nth' element from each of the supplied
+    // tuple-like arguments
+    template <size_t N, typename ...TupleT>
+    auto
+    nth (TupleT&&...args)
+    {
+        return std::make_tuple (
+            std::get<N> (args)...
+        );
+    }
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    namespace detail {
+        template <typename ...TupleT, size_t ...Elements>
+        auto
+        zip (std::index_sequence<Elements...>, TupleT &&...args)
+        {
+            return std::make_tuple (
+                nth<Elements> (args...)...
+            );
+        }
+    };
+
+
+    /// converts m arguments of n-tuples into one m-tuple of n-tuples by taking
+    /// successive elements from each argument tuple.
+    ///
+    /// eg, zip (('a', 1.f), (0, nullptr)) becomes (('a', 0), (1.f, nullptr))
+    template <typename ...Args >
+    auto
+    zip (Args &&...args)
+    {
+        static_assert (sizeof...(args) > 0);
+        // ensure all the types have the same static size
+        using all_t = std::tuple<std::decay_t<Args>...>;
+        using first_t = std::tuple_element_t<0,all_t>;
+
+        static constexpr auto arity = std::tuple_size_v<first_t>;
+        static_assert ((
+            (arity == std::tuple_size_v<std::decay_t<Args>>) && ...
+        ));
+
+        return detail::zip (
+            std::make_index_sequence<arity> {},
+            std::forward<Args> (args)...
+        );
+    }
+}
 
 
 #endif