diff --git a/sprout/complex.hpp b/sprout/complex.hpp
index 26df2016..9b8bd196 100644
--- a/sprout/complex.hpp
+++ b/sprout/complex.hpp
@@ -3,14 +3,9 @@
 
 #include <cmath>
 #include <sprout/config.hpp>
+#include <sprout/math/constants.hpp>
 
 namespace sprout {
-	namespace detail {
-		template<typename T>
-		SPROUT_CONSTEXPR T pi_div_two() {
-			return 1.5707963267948966192313216916397514L;
-		}
-	}	// namespace detail
 	template<typename T>
 	class complex;
 	template<typename T>
@@ -94,7 +89,7 @@ namespace sprout {
 		complex<T>& operator*=(complex<X> const& rhs) {
 			return *this = complex<T>(
 				re_ * rhs.real() - im_ * rhs.imag(),
-				re_ * rhs.imag() + im_ * rhs.imag()
+				re_ * rhs.imag() + im_ * rhs.real()
 				);
 		}
 		template<typename X>
@@ -153,7 +148,7 @@ namespace sprout {
 	SPROUT_CONSTEXPR sprout::complex<T> operator*(sprout::complex<T> const& lhs, sprout::complex<T> const& rhs) {
 		return sprout::complex<T>(
 			lhs.real() * rhs.real() - lhs.imag() * rhs.imag(),
-			lhs.real() * rhs.imag() + lhs.imag() * rhs.imag()
+			lhs.real() * rhs.imag() + lhs.imag() * rhs.real()
 			);
 	}
 	template<typename T>
@@ -364,7 +359,7 @@ namespace sprout {
 	namespace detail {
 		template<typename T>
 		SPROUT_CONSTEXPR sprout::complex<T> acos_impl(sprout::complex<T> const& t) {
-			return sprout::complex<T>(sprout::detail::pi_div_two<T>() - t.real(), -t.imag());
+			return sprout::complex<T>(sprout::math::pi_div_two<T>() - t.real(), -t.imag());
 		}
 	}	// namespace detail
 	template<typename T>
diff --git a/sprout/fft/bitrev_table.hpp b/sprout/fft/bitrev_table.hpp
deleted file mode 100644
index 1929212b..00000000
--- a/sprout/fft/bitrev_table.hpp
+++ /dev/null
@@ -1,8 +0,0 @@
-#ifndef SPROUT_FFT_BITREV_TABLE_HPP
-#define SPROUT_FFT_BITREV_TABLE_HPP
-
-#include <sprout/config.hpp>
-#include <sprout/fft/fixed/bitrev_table.hpp>
-#include <sprout/fft/fit/bitrev_table.hpp>
-
-#endif	// #ifndef SPROUT_FFT_BITREV_TABLE_HPP
diff --git a/sprout/functional/dft.hpp b/sprout/functional/dft.hpp
new file mode 100644
index 00000000..0e7ba62f
--- /dev/null
+++ b/sprout/functional/dft.hpp
@@ -0,0 +1,11 @@
+#ifndef SPROUT_FUNCTIONAL_DFT_HPP
+#define SPROUT_FUNCTIONAL_DFT_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/functional/dft/dft.hpp>
+#include <sprout/functional/dft/idft.hpp>
+#include <sprout/functional/dft/dft_element.hpp>
+#include <sprout/functional/dft/idft_element.hpp>
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_DFT_HPP
+
diff --git a/sprout/functional/dft/detail/dft_element_gen.hpp b/sprout/functional/dft/detail/dft_element_gen.hpp
new file mode 100644
index 00000000..498ad5a3
--- /dev/null
+++ b/sprout/functional/dft/detail/dft_element_gen.hpp
@@ -0,0 +1,40 @@
+#ifndef SPROUT_FUNCTIONAL_DFT_DETAIL_DFT_ELEMENT_GEN_HPP
+#define SPROUT_FUNCTIONAL_DFT_DETAIL_DFT_ELEMENT_GEN_HPP
+
+#include <cmath>
+#include <iterator>
+#include <type_traits>
+#include <sprout/config.hpp>
+#include <sprout/iterator/operation.hpp>
+
+namespace sprout {
+	namespace detail {
+		template<typename InputIterator>
+		SPROUT_CONSTEXPR inline typename std::iterator_traits<InputIterator>::value_type dft_element_gen(
+			InputIterator first,
+			InputIterator last,
+			typename std::iterator_traits<InputIterator>::value_type::value_type arg,
+			typename std::iterator_traits<InputIterator>::difference_type k = 0,
+			typename std::iterator_traits<InputIterator>::value_type value = typename std::iterator_traits<InputIterator>::value_type(),
+			typename std::iterator_traits<InputIterator>::value_type::value_type theta = typename std::iterator_traits<InputIterator>::value_type::value_type()
+			)
+		{
+			typedef typename std::iterator_traits<InputIterator>::value_type value_type;
+			using std::cos;
+			using std::sin;
+			return first != last
+				? value + sprout::detail::dft_element_gen(
+					sprout::next(first),
+					last,
+					arg,
+					k + 1,
+					*first * value_type(cos(theta), sin(theta)),
+					arg * (k + 1)
+					)
+				: value
+				;
+		}
+	}	// namespace detail
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_DFT_DETAIL_DFT_ELEMENT_GEN_HPP
diff --git a/sprout/functional/dft/dft.hpp b/sprout/functional/dft/dft.hpp
new file mode 100644
index 00000000..e567013e
--- /dev/null
+++ b/sprout/functional/dft/dft.hpp
@@ -0,0 +1,8 @@
+#ifndef SPROUT_FUNCTIONAL_DFT_DFT_HPP
+#define SPROUT_FUNCTIONAL_DFT_DFT_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/functional/dft/fixed/dft.hpp>
+#include <sprout/functional/dft/fit/dft.hpp>
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_DFT_DFT_HPP
diff --git a/sprout/functional/dft/dft_element.hpp b/sprout/functional/dft/dft_element.hpp
new file mode 100644
index 00000000..890f08f9
--- /dev/null
+++ b/sprout/functional/dft/dft_element.hpp
@@ -0,0 +1,48 @@
+#ifndef SPROUT_FUNCTIONAL_DFT_FIXED_DFT_ELEMENT_HPP
+#define SPROUT_FUNCTIONAL_DFT_FIXED_DFT_ELEMENT_HPP
+
+#include <iterator>
+#include <sprout/config.hpp>
+#include <sprout/functional/dft/detail/dft_element_gen.hpp>
+#include <sprout/math/constants.hpp>
+#include HDR_ITERATOR_SSCRISK_CEL_OR_SPROUT_DETAIL
+
+namespace sprout {
+	namespace detail {
+		template<typename InputIterator, typename Size>
+		SPROUT_CONSTEXPR inline typename std::iterator_traits<InputIterator>::value_type dft_element_impl(
+			InputIterator first,
+			InputIterator last,
+			typename std::iterator_traits<InputIterator>::difference_type i,
+			Size size
+			)
+		{
+			typedef typename std::iterator_traits<InputIterator>::value_type value_type;
+			typedef typename value_type::value_type elem_type;
+			return sprout::detail::dft_element_gen(
+				first,
+				last,
+				-(2 * sprout::math::pi<elem_type>() * i / size)
+				);
+		}
+	}	// namespace detail
+	//
+	// dft_element
+	//
+	template<typename InputIterator>
+	SPROUT_CONSTEXPR inline typename std::iterator_traits<InputIterator>::value_type dft_element(
+		InputIterator first,
+		InputIterator last,
+		typename std::iterator_traits<InputIterator>::difference_type i
+		)
+	{
+		return sprout::detail::dft_element_impl(
+			first,
+			last,
+			i,
+			NS_SSCRISK_CEL_OR_SPROUT_DETAIL::distance(first, last)
+			);
+	}
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_DFT_FIXED_DFT_ELEMENT_HPP
diff --git a/sprout/functional/dft/fit/dft.hpp b/sprout/functional/dft/fit/dft.hpp
new file mode 100644
index 00000000..1955e803
--- /dev/null
+++ b/sprout/functional/dft/fit/dft.hpp
@@ -0,0 +1,46 @@
+#ifndef SPROUT_FUNCTIONAL_DFT_FIT_DFT_HPP
+#define SPROUT_FUNCTIONAL_DFT_FIT_DFT_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/fixed_container/traits.hpp>
+#include <sprout/fixed_container/functions.hpp>
+#include <sprout/functional/dft/fixed/dft.hpp>
+#include <sprout/algorithm/fit/result_of.hpp>
+#include <sprout/sub_array.hpp>
+#include HDR_ALGORITHM_SSCRISK_CEL_OR_SPROUT_DETAIL
+#include HDR_ITERATOR_SSCRISK_CEL_OR_SPROUT_DETAIL
+
+namespace sprout {
+	namespace fit {
+		namespace detail {
+			template<typename InputIterator, typename Result>
+			SPROUT_CONSTEXPR inline typename sprout::fit::result_of::algorithm<Result>::type dft_impl(
+				InputIterator first,
+				InputIterator last,
+				Result const& result,
+				typename sprout::fixed_container_traits<Result>::difference_type offset
+				)
+			{
+				return sprout::sub_copy(
+					sprout::get_fixed(sprout::fixed::dft(first, last, result)),
+					offset,
+					offset + NS_SSCRISK_CEL_OR_SPROUT_DETAIL::min(NS_SSCRISK_CEL_OR_SPROUT_DETAIL::distance(first, last), sprout::size(result))
+					);
+			}
+		}	// namespace detail
+		//
+		// dft
+		//
+		template<typename InputIterator, typename Result>
+		SPROUT_CONSTEXPR inline typename sprout::fit::result_of::algorithm<Result>::type dft(
+			InputIterator first,
+			InputIterator last,
+			Result const& result
+			)
+		{
+			return sprout::fit::detail::dft_impl(first, last, result, sprout::fixed_begin_offset(result));
+		}
+	}	// namespace fit
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_DFT_FIT_DFT_HPP
diff --git a/sprout/functional/dft/fit/idft.hpp b/sprout/functional/dft/fit/idft.hpp
new file mode 100644
index 00000000..3cfbe81d
--- /dev/null
+++ b/sprout/functional/dft/fit/idft.hpp
@@ -0,0 +1,46 @@
+#ifndef SPROUT_FUNCTIONAL_DFT_FIT_IDFT_HPP
+#define SPROUT_FUNCTIONAL_DFT_FIT_IDFT_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/fixed_container/traits.hpp>
+#include <sprout/fixed_container/functions.hpp>
+#include <sprout/functional/dft/fixed/idft.hpp>
+#include <sprout/algorithm/fit/result_of.hpp>
+#include <sprout/sub_array.hpp>
+#include HDR_ALGORITHM_SSCRISK_CEL_OR_SPROUT_DETAIL
+#include HDR_ITERATOR_SSCRISK_CEL_OR_SPROUT_DETAIL
+
+namespace sprout {
+	namespace fit {
+		namespace detail {
+			template<typename InputIterator, typename Result>
+			SPROUT_CONSTEXPR inline typename sprout::fit::result_of::algorithm<Result>::type idft_impl(
+				InputIterator first,
+				InputIterator last,
+				Result const& result,
+				typename sprout::fixed_container_traits<Result>::difference_type offset
+				)
+			{
+				return sprout::sub_copy(
+					sprout::get_fixed(sprout::fixed::idft(first, last, result)),
+					offset,
+					offset + NS_SSCRISK_CEL_OR_SPROUT_DETAIL::min(NS_SSCRISK_CEL_OR_SPROUT_DETAIL::distance(first, last), sprout::size(result))
+					);
+			}
+		}	// namespace detail
+		//
+		// idft
+		//
+		template<typename InputIterator, typename Result>
+		SPROUT_CONSTEXPR inline typename sprout::fit::result_of::algorithm<Result>::type idft(
+			InputIterator first,
+			InputIterator last,
+			Result const& result
+			)
+		{
+			return sprout::fit::detail::idft_impl(first, last, result, sprout::fixed_begin_offset(result));
+		}
+	}	// namespace fit
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_DFT_FIT_IDFT_HPP
diff --git a/sprout/functional/dft/fixed/dft.hpp b/sprout/functional/dft/fixed/dft.hpp
new file mode 100644
index 00000000..0d1f4525
--- /dev/null
+++ b/sprout/functional/dft/fixed/dft.hpp
@@ -0,0 +1,72 @@
+#ifndef SPROUT_FUNCTIONAL_DFT_FIXED_DFT_HPP
+#define SPROUT_FUNCTIONAL_DFT_FIXED_DFT_HPP
+
+#include <cstddef>
+#include <sprout/config.hpp>
+#include <sprout/index_tuple.hpp>
+#include <sprout/fixed_container/traits.hpp>
+#include <sprout/fixed_container/functions.hpp>
+#include <sprout/iterator/operation.hpp>
+#include <sprout/algorithm/fixed/result_of.hpp>
+#include <sprout/functional/dft/dft_element.hpp>
+#include HDR_ITERATOR_SSCRISK_CEL_OR_SPROUT_DETAIL
+
+namespace sprout {
+	namespace fixed {
+		namespace detail {
+			template<typename InputIterator, typename Result, std::ptrdiff_t... Indexes>
+			SPROUT_CONSTEXPR inline typename sprout::fixed::result_of::algorithm<Result>::type dft_impl(
+				InputIterator first,
+				InputIterator last,
+				Result const& result,
+				sprout::index_tuple<Indexes...>,
+				typename sprout::fixed_container_traits<Result>::difference_type offset,
+				typename sprout::fixed_container_traits<Result>::size_type size,
+				typename sprout::fixed_container_traits<Result>::size_type input_size
+				)
+			{
+				return sprout::remake_clone<Result>(
+					result,
+					sprout::size(result),
+					(Indexes >= offset && Indexes < offset + size && Indexes < offset + input_size
+						? sprout::detail::dft_element_impl(first, last, Indexes - offset, input_size)
+						: *sprout::next(sprout::fixed_begin(result), Indexes)
+						)...
+					);
+			}
+			template<typename InputIterator, typename Result>
+			SPROUT_CONSTEXPR inline typename sprout::fixed::result_of::algorithm<Result>::type dft(
+				InputIterator first,
+				InputIterator last,
+				Result const& result
+				)
+			{
+				return sprout::fixed::detail::dft_impl(
+					first,
+					last,
+					result,
+					typename sprout::index_range<0, sprout::fixed_container_traits<Result>::fixed_size>::type(),
+					sprout::fixed_begin_offset(result),
+					sprout::size(result),
+					NS_SSCRISK_CEL_OR_SPROUT_DETAIL::distance(first, last)
+					);
+			}
+		}	// namespace detail
+		//
+		// dft
+		//
+		template<typename InputIterator, typename Result>
+		SPROUT_CONSTEXPR inline typename sprout::fixed::result_of::algorithm<Result>::type dft(
+			InputIterator first,
+			InputIterator last,
+			Result const& result
+			)
+		{
+			return sprout::fixed::detail::dft(first, last, result);
+		}
+	}	// namespace fixed
+
+	using sprout::fixed::dft;
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_DFT_FIXED_DFT_HPP
diff --git a/sprout/functional/dft/fixed/idft.hpp b/sprout/functional/dft/fixed/idft.hpp
new file mode 100644
index 00000000..db90b584
--- /dev/null
+++ b/sprout/functional/dft/fixed/idft.hpp
@@ -0,0 +1,72 @@
+#ifndef SPROUT_FUNCTIONAL_DFT_FIXED_IDFT_HPP
+#define SPROUT_FUNCTIONAL_DFT_FIXED_IDFT_HPP
+
+#include <cstddef>
+#include <sprout/config.hpp>
+#include <sprout/index_tuple.hpp>
+#include <sprout/fixed_container/traits.hpp>
+#include <sprout/fixed_container/functions.hpp>
+#include <sprout/iterator/operation.hpp>
+#include <sprout/algorithm/fixed/result_of.hpp>
+#include <sprout/functional/dft/idft_element.hpp>
+#include HDR_ITERATOR_SSCRISK_CEL_OR_SPROUT_DETAIL
+
+namespace sprout {
+	namespace fixed {
+		namespace detail {
+			template<typename InputIterator, typename Result, std::ptrdiff_t... Indexes>
+			SPROUT_CONSTEXPR inline typename sprout::fixed::result_of::algorithm<Result>::type idft_impl(
+				InputIterator first,
+				InputIterator last,
+				Result const& result,
+				sprout::index_tuple<Indexes...>,
+				typename sprout::fixed_container_traits<Result>::difference_type offset,
+				typename sprout::fixed_container_traits<Result>::size_type size,
+				typename sprout::fixed_container_traits<Result>::size_type input_size
+				)
+			{
+				return sprout::remake_clone<Result>(
+					result,
+					sprout::size(result),
+					(Indexes >= offset && Indexes < offset + size && Indexes < offset + input_size
+						? sprout::detail::idft_element_impl(first, last, Indexes - offset, input_size)
+						: *sprout::next(sprout::fixed_begin(result), Indexes)
+						)...
+					);
+			}
+			template<typename InputIterator, typename Result>
+			SPROUT_CONSTEXPR inline typename sprout::fixed::result_of::algorithm<Result>::type idft(
+				InputIterator first,
+				InputIterator last,
+				Result const& result
+				)
+			{
+				return sprout::fixed::detail::idft_impl(
+					first,
+					last,
+					result,
+					typename sprout::index_range<0, sprout::fixed_container_traits<Result>::fixed_size>::type(),
+					sprout::fixed_begin_offset(result),
+					sprout::size(result),
+					NS_SSCRISK_CEL_OR_SPROUT_DETAIL::distance(first, last)
+					);
+			}
+		}	// namespace detail
+		//
+		// idft
+		//
+		template<typename InputIterator, typename Result>
+		SPROUT_CONSTEXPR inline typename sprout::fixed::result_of::algorithm<Result>::type idft(
+			InputIterator first,
+			InputIterator last,
+			Result const& result
+			)
+		{
+			return sprout::fixed::detail::idft(first, last, result);
+		}
+	}	// namespace fixed
+
+	using sprout::fixed::idft;
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_DFT_FIXED_IDFT_HPP
diff --git a/sprout/functional/dft/idft.hpp b/sprout/functional/dft/idft.hpp
new file mode 100644
index 00000000..6fb05b7a
--- /dev/null
+++ b/sprout/functional/dft/idft.hpp
@@ -0,0 +1,8 @@
+#ifndef SPROUT_FUNCTIONAL_DFT_IDFT_HPP
+#define SPROUT_FUNCTIONAL_DFT_IDFT_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/functional/dft/fixed/idft.hpp>
+#include <sprout/functional/dft/fit/idft.hpp>
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_DFT_IDFT_HPP
diff --git a/sprout/functional/dft/idft_element.hpp b/sprout/functional/dft/idft_element.hpp
new file mode 100644
index 00000000..b4be4e01
--- /dev/null
+++ b/sprout/functional/dft/idft_element.hpp
@@ -0,0 +1,50 @@
+#ifndef SPROUT_FUNCTIONAL_DFT_FIXED_IDFT_ELEMENT_HPP
+#define SPROUT_FUNCTIONAL_DFT_FIXED_IDFT_ELEMENT_HPP
+
+#include <iterator>
+#include <sprout/config.hpp>
+#include <sprout/functional/dft/detail/dft_element_gen.hpp>
+#include <sprout/math/constants.hpp>
+#include HDR_ITERATOR_SSCRISK_CEL_OR_SPROUT_DETAIL
+
+namespace sprout {
+	namespace detail {
+		template<typename InputIterator, typename Size>
+		SPROUT_CONSTEXPR inline typename std::iterator_traits<InputIterator>::value_type idft_element_impl(
+			InputIterator first,
+			InputIterator last,
+			typename std::iterator_traits<InputIterator>::difference_type i,
+			Size size
+			)
+		{
+			typedef typename std::iterator_traits<InputIterator>::value_type value_type;
+			typedef typename value_type::value_type elem_type;
+			return sprout::detail::dft_element_gen(
+				first,
+				last,
+				2 * sprout::math::pi<elem_type>() * i / size
+				)
+				/ static_cast<elem_type>(size)
+				;
+		}
+	}	// namespace detail
+	//
+	// idft_element
+	//
+	template<typename InputIterator>
+	SPROUT_CONSTEXPR inline typename std::iterator_traits<InputIterator>::value_type idft_element(
+		InputIterator first,
+		InputIterator last,
+		typename std::iterator_traits<InputIterator>::difference_type i
+		)
+	{
+		return sprout::detail::idft_element_impl(
+			first,
+			last,
+			i,
+			NS_SSCRISK_CEL_OR_SPROUT_DETAIL::distance(first, last)
+			);
+	}
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_DFT_FIXED_IDFT_ELEMENT_HPP
diff --git a/sprout/functional/fft.hpp b/sprout/functional/fft.hpp
new file mode 100644
index 00000000..a6bd8c96
--- /dev/null
+++ b/sprout/functional/fft.hpp
@@ -0,0 +1,8 @@
+#ifndef SPROUT_FUNCTIONAL_FFT_HPP
+#define SPROUT_FUNCTIONAL_FFT_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/functional/fft/bitrev_table.hpp>
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_FFT_HPP
+
diff --git a/sprout/functional/fft/bitrev_table.hpp b/sprout/functional/fft/bitrev_table.hpp
new file mode 100644
index 00000000..75a30652
--- /dev/null
+++ b/sprout/functional/fft/bitrev_table.hpp
@@ -0,0 +1,8 @@
+#ifndef SPROUT_FUNCTIONAL_FFT_BITREV_TABLE_HPP
+#define SPROUT_FUNCTIONAL_FFT_BITREV_TABLE_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/functional/fft/fixed/bitrev_table.hpp>
+#include <sprout/functional/fft/fit/bitrev_table.hpp>
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_FFT_BITREV_TABLE_HPP
diff --git a/sprout/fft/fit/bitrev_table.hpp b/sprout/functional/fft/fit/bitrev_table.hpp
similarity index 79%
rename from sprout/fft/fit/bitrev_table.hpp
rename to sprout/functional/fft/fit/bitrev_table.hpp
index b35ccd24..30f6eeaf 100644
--- a/sprout/fft/fit/bitrev_table.hpp
+++ b/sprout/functional/fft/fit/bitrev_table.hpp
@@ -1,11 +1,11 @@
-#ifndef SPROUT_FFT_FIT_BITREV_TABLE_HPP
-#define SPROUT_FFT_FIT_BITREV_TABLE_HPP
+#ifndef SPROUT_FUNCTIONAL_FFT_FIT_BITREV_TABLE_HPP
+#define SPROUT_FUNCTIONAL_FFT_FIT_BITREV_TABLE_HPP
 
 #include <type_traits>
 #include <sprout/config.hpp>
 #include <sprout/fixed_container/traits.hpp>
 #include <sprout/fixed_container/functions.hpp>
-#include <sprout/fft/fixed/bitrev_table.hpp>
+#include <sprout/functional/fft/fixed/bitrev_table.hpp>
 #include <sprout/algorithm/fit/result_of.hpp>
 #include <sprout/sub_array.hpp>
 
@@ -38,4 +38,4 @@ namespace sprout {
 	}	// namespace fit
 }	// namespace sprout
 
-#endif	// #ifndef SPROUT_FFT_FIT_BITREV_TABLE_HPP
+#endif	// #ifndef SPROUT_FUNCTIONAL_FFT_FIT_BITREV_TABLE_HPP
diff --git a/sprout/fft/fixed/bitrev_table.hpp b/sprout/functional/fft/fixed/bitrev_table.hpp
similarity index 88%
rename from sprout/fft/fixed/bitrev_table.hpp
rename to sprout/functional/fft/fixed/bitrev_table.hpp
index 311e48eb..80e6ae21 100644
--- a/sprout/fft/fixed/bitrev_table.hpp
+++ b/sprout/functional/fft/fixed/bitrev_table.hpp
@@ -1,5 +1,5 @@
-#ifndef SPROUT_FFT_FIXED_BITREV_TABLE_HPP
-#define SPROUT_FFT_FIXED_BITREV_TABLE_HPP
+#ifndef SPROUT_FUNCTIONAL_FFT_FIXED_BITREV_TABLE_HPP
+#define SPROUT_FUNCTIONAL_FFT_FIXED_BITREV_TABLE_HPP
 
 #include <cstddef>
 #include <type_traits>
@@ -61,4 +61,4 @@ namespace sprout {
 	using sprout::fixed::bitrev_table;
 }	// namespace sprout
 
-#endif	// #ifndef SPROUT_FFT_FIXED_BITREV_TABLE_HPP
+#endif	// #ifndef SPROUT_FUNCTIONAL_FFT_FIXED_BITREV_TABLE_HPP
diff --git a/sprout/math/constants.hpp b/sprout/math/constants.hpp
new file mode 100644
index 00000000..87f2a385
--- /dev/null
+++ b/sprout/math/constants.hpp
@@ -0,0 +1,25 @@
+#ifndef SPROUT_MATH_CONSTANTS_HPP
+#define SPROUT_MATH_CONSTANTS_HPP
+
+#include <sprout/config.hpp>
+
+namespace sprout {
+	namespace math {
+		//
+		// pi
+		//
+		template<typename T>
+		SPROUT_CONSTEXPR inline T pi() {
+			return 3.141592653589793238462643383279502884197169399375105820974944L;
+		}
+		//
+		// pi_div_two
+		//
+		template<typename T>
+		SPROUT_CONSTEXPR inline T pi_div_two() {
+			return 1.570796326794896619231321691639751442098584699687552910487472L;
+		}
+	}	// namespace math
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_MATH_CONSTANTS_HPP