Implemented `Count` in terms of `Fold` and `Map`

* as its name implies this function counts the amount of elements satisfying a given _function_
* added appropriate test case

2 files changed, 35 insertions, 0 deletions

diff --git a/src/list/operation/higher/query.h b/src/list/operation/higher/query.h
index 29f65dc..c6eabc7 100644
--- a/src/list/operation/higher/query.h
+++ b/src/list/operation/higher/query.h
@@ -3,6 +3,7 @@
 
 #include "fold.h"
 #include "misc.h"
+#include "operation/math.h"
 #include "operation/logic.h"
 
 namespace tav {
@@ -35,6 +36,16 @@ using None = Not<
 	typename Any<Function, List>::type
 >;
 
+template <
+	template<typename> class Function,
+	typename                 List
+>
+using Count = Fold<
+	Add,
+	tav::Size<0>,
+	typename Map<Function, List>::type
+>;
+
 }
 
 #endif  // TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_QUERY_H_
diff --git a/test.cc b/test.cc
index 6476bb8..62c6f85 100644
--- a/test.cc
+++ b/test.cc
@@ -607,3 +607,27 @@ static_assert(
 	>::value,
 	"(list-tabulate 4 square) != (list 0 1 4 9)"
 );
+
+// list count
+
+static_assert(
+	std::is_same<
+		tav::Size<2>,
+		tav::Count<
+			tav::Odd,
+			tav::List<tav::Int<1>, tav::Int<2>, tav::Int<3>>::type
+		>::type
+	>::value,
+	"(count odd? (list 1 2 3)) != 2"
+);
+
+static_assert(
+	std::is_same<
+		tav::Size<0>,
+		tav::Count<
+			tav::Even,
+			tav::List<tav::Int<1>, tav::Int<3>, tav::Int<5>>::type
+		>::type
+	>::value,
+	"(count even? (list 1 3 5)) != 0"
+);