From 899efcc2d690fc03cb0fa2c38b8eee9b8bcc3900 Mon Sep 17 00:00:00 2001
From: Christos Houtouridis <houtouridis.ch@gmail.com>
Date: Sun, 13 Oct 2019 19:54:08 +0300
Subject: [PATCH] meta: Change complexity of typelist<>::times<> from O(N) to
 O(logN)

---
 include/utl/meta/typelist.h | 42 ++++++++++++++++++++++++-------------
 1 file changed, 27 insertions(+), 15 deletions(-)

diff --git a/include/utl/meta/typelist.h b/include/utl/meta/typelist.h
index 23d39b4..713d32b 100644
--- a/include/utl/meta/typelist.h
+++ b/include/utl/meta/typelist.h
@@ -76,17 +76,29 @@ namespace meta {
       }
       // ======= times utility =======
    private:
-      template<size_t N, typename L, typename ...T>
-      struct times_ { };
-
-      template<size_t N, typename ...L>
-      struct times_<N, typelist<L...>, Ts...>  {
-         // append one and recurse
-         using type = type_<
-            if_c <N != 0,
-               times_<N-1, typelist<L..., Ts...>, Ts...>,
-               typelist<L...>
-            >>;
+      template <typename... > struct cat_ { };
+      template <typename... L1, typename... L2>
+      struct cat_<typelist<L1...>, typelist<L2...>> {
+         using type = typelist<L1..., L2...>;
+      };
+
+      template <size_t N, typename ...T>
+      struct times_ {
+         //static_assert( N >= 0, "Cannot make typelist of negative length" );
+         using type = eval<
+            cat_<
+               eval<times_<N/2, T...>>,
+               eval<times_<N - N/2, T...>>
+            >
+         >;
+      };
+      template <typename ...T>
+      struct times_<1, T...> {
+         using type = typelist<T...>;
+      };
+      template <typename ...T>
+      struct times_<0, T...> {
+         using type = typelist<>;
       };
    public:
       /*!
@@ -98,11 +110,11 @@ namespace meta {
        *                    typelist<int, char, int, char>
        *       >, "" );
        * \endcode
-       * complexity \f$ O(N) \f$
+       * complexity \f$ O(logN) \f$
        */
       template<size_t N>
-      using times = type_<
-            times_<N, typelist<>, Ts...>
+      using times = eval<
+         times_<N, Ts...>
       >;
    };
 
@@ -226,7 +238,7 @@ namespace meta {
    /*!
     * Return the \p N th element in the \c meta::typelist \p List.
     *
-    * Complexity \f$ O(N) \f$.
+    * Complexity \f$ O(logN) \f$.
     */
    template <typename List, index_t N>
    using at_c = eval<