Implemented `Nth` function

* as its name implies this function returns the _nth_ value of a given _Cons_ structure
* added appropriate test case

2 files changed, 28 insertions, 0 deletions

diff --git a/src/list/operation/basic.h b/src/list/operation/basic.h
index 1e05a50..cee4cbc 100644
--- a/src/list/operation/basic.h
+++ b/src/list/operation/basic.h
@@ -1,6 +1,8 @@
 #ifndef TYPEASVALUE_SRC_LIST_OPERATION_BASIC_H_
 #define TYPEASVALUE_SRC_LIST_OPERATION_BASIC_H_
 
+#include "operation/math.h"
+
 namespace tav {
 
 template <typename Cons>
@@ -16,6 +18,23 @@ struct Length<void> {
 	typedef Size<0> type;
 };
 
+template <
+	typename Index,
+	typename Cons
+>
+struct Nth {
+	typedef If<
+		equal_value<Index, Size<0>>::value,
+		Head<Cons>,
+		typename Nth<Substract<Index, Size<1>>, Tail<Cons>>::type
+	> type;
+};
+
+template <typename Index>
+struct Nth<Index, void> {
+	typedef void type;
+};
+
 }
 
 #endif  // TYPEASVALUE_SRC_LIST_OPERATION_BASIC_H_
diff --git a/test.cc b/test.cc
index 6ce3691..e8f7c47 100644
--- a/test.cc
+++ b/test.cc
@@ -53,6 +53,15 @@ TEST_F(TypeAsValueTest, List) {
 	EXPECT_EQ(2, ( tav::Head<tav::Tail<tav::List<tav::Int<1>, tav::Int<2>, tav::Int<3>>::type>>::value ));
 }
 
+TEST_F(TypeAsValueTest, ListNth) {
+	// (nth 0 (list 1))
+	EXPECT_EQ(1, ( tav::Nth<tav::Size<0>, tav::List<tav::Int<1>>::type>::type::value ));
+	// (nth 0 (list 1 2))
+	EXPECT_EQ(1, ( tav::Nth<tav::Size<0>, tav::List<tav::Int<1>, tav::Int<2>>::type>::type::value ));
+	// (nth 1 (list 1 2))
+	EXPECT_EQ(2, ( tav::Nth<tav::Size<1>, tav::List<tav::Int<1>, tav::Int<2>>::type>::type::value ));
+}
+
 TEST_F(TypeAsValueTest, ListConcatenate) {
 	// (length (concatenate (list 1) (list 2)))
 	EXPECT_EQ(2, ( tav::Length<tav::Concatenate<tav::List<tav::Int<1>>::type, tav::List<tav::Int<2>>::type>::type>::type::value ));