From 78e954aa5864aed6d3c60d94813fec87f92aaa08 Mon Sep 17 00:00:00 2001
From: bolero-MURAKAMI <bolero.murakami@gmail.com>
Date: Sat, 11 Apr 2015 12:19:12 +0900
Subject: [PATCH] reimplementation types::fold

---
 sprout/type/algorithm/fold.hpp | 43 +++++++++++++++++++++++++---------
 1 file changed, 32 insertions(+), 11 deletions(-)

diff --git a/sprout/type/algorithm/fold.hpp b/sprout/type/algorithm/fold.hpp
index 273d37a7..419adc1a 100644
--- a/sprout/type/algorithm/fold.hpp
+++ b/sprout/type/algorithm/fold.hpp
@@ -8,7 +8,6 @@
 #ifndef SPROUT_TYPE_ALGORITHM_FOLD_HPP
 #define SPROUT_TYPE_ALGORITHM_FOLD_HPP
 
-#include <sprout/config.hpp>
 #include <sprout/workaround/std/cstddef.hpp>
 #include <sprout/type/apply.hpp>
 #include <sprout/type/tuple.hpp>
@@ -18,30 +17,52 @@ namespace sprout {
 	namespace types {
 		namespace detail {
 			template<
-				typename Tuple, typename T, typename BinaryOp, std::size_t I,
-				bool Valid = (I != sprout::types::tuple_size<Tuple>::value)
+				typename Tuple, typename T, typename BinaryOp, std::size_t First, std::size_t Last,
+				std::size_t Pivot,
+				bool C0 = (Pivot == 0)
 			>
 			struct fold_impl;
-			template<typename Tuple, typename T, typename BinaryOp, std::size_t I>
-			struct fold_impl<Tuple, T, BinaryOp, I, false>
-				: public sprout::identity<T>
+			template<
+				typename Tuple, typename T, typename BinaryOp, std::size_t First, std::size_t Last,
+				std::size_t Pivot
+			>
+			struct fold_impl<Tuple, T, BinaryOp, First, Last, Pivot, true>
+				: public sprout::types::apply<BinaryOp, T, typename sprout::types::tuple_element<First, Tuple>::type>
 			{};
-			template<typename Tuple, typename T, typename BinaryOp, std::size_t I>
-			struct fold_impl<Tuple, T, BinaryOp, I, true>
+			template<
+				typename Tuple, typename T, typename BinaryOp, std::size_t First, std::size_t Last,
+				std::size_t Pivot
+			>
+			struct fold_impl<Tuple, T, BinaryOp, First, Last, Pivot, false>
 				: public sprout::types::detail::fold_impl<
 					Tuple,
-					typename sprout::types::apply<BinaryOp, T, typename sprout::types::tuple_element<I, Tuple>::type>::type,
+					typename sprout::types::detail::fold_impl<
+						Tuple,
+						T,
+						BinaryOp,
+						First, First + Pivot,
+						Pivot / 2
+					>::type,
 					BinaryOp,
-					I + 1
+					First + Pivot, Last,
+					(Last - First - Pivot) / 2
 				>
 			{};
+			template<typename Tuple, typename T, typename BinaryOp, std::size_t Size = sprout::types::tuple_size<Tuple>::value>
+			struct fold
+				: public sprout::types::detail::fold_impl<Tuple, T, BinaryOp, 0, Size, Size / 2>
+			{};
+			template<typename Tuple, typename T, typename BinaryOp>
+			struct fold<Tuple, T, BinaryOp, 0>
+				: public sprout::identity<T>
+			{};
 		}	// namespace detail
 		//
 		// fold
 		//
 		template<typename Tuple, typename T, typename BinaryOp>
 		struct fold
-			: public sprout::types::detail::fold_impl<Tuple, T, BinaryOp, 0>
+			: public sprout::types::detail::fold<Tuple, T, BinaryOp>
 		{};
 
 #if SPROUT_USE_TEMPLATE_ALIASES