Reimplemented `TakeWhile` and `DropWhile` in terms of `ListIndex`

* new higher order list operation `ListIndex` returns the index of the first list element satisfying a given predicate function
** this enables finding the split index required for both `TakeWhile` and `DropWhile`
** actual list manipulation is then performed using lower order `Take` or `Drop`
** analogously to `list-index` in SRFI-1
** added appropriate test case
* added higher order predicate utility function `utility::predicate_assurance`
** as its name implies this function enables assuring that a value satisfies a given predicate
*** if this is not the case a surrogate value is returned instead
** this is used to return a appropriate size value if the `ListIndex` call fails because no element satisfies its predicate
** `detail::predicate_negator` was moved from `Remove`'s implementation details into the newly introduced `utility` namespace
*** as it is required by both `TakeWhile` and `DropWhile` in addition to `Remove`
* continuation of 8e49cc6

1 files changed, 45 insertions, 0 deletions

diff --git a/src/list/operation/higher/list_index.h b/src/list/operation/higher/list_index.h
new file mode 100644
index 0000000..bbf43ee
--- /dev/null
+++ b/src/list/operation/higher/list_index.h
@@ -0,0 +1,45 @@
+#ifndef TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_LIST_INDEX_H_
+#define TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_LIST_INDEX_H_
+
+#include "operation/math.h"
+
+namespace tav {
+
+namespace detail {
+
+template <
+	template<typename> class Predicate,
+	typename                 List,
+	typename                 Index = Size<0>
+>
+struct index_of_first {
+	typedef If<
+		Eval<Predicate<Head<List>>>,
+		Index,
+		Eval<index_of_first<
+			Predicate,
+			Tail<List>,
+			Add<Index, Size<1>>
+		>>
+	> type;
+};
+
+template <
+	template<typename> class Predicate,
+	typename                 Index
+>
+struct index_of_first<Predicate, void, Index> {
+	typedef void type;
+};
+
+}
+
+template <
+	template<typename> class Predicate,
+	typename                 List
+>
+using ListIndex = Eval<detail::index_of_first<Predicate, List>>;
+
+}
+
+#endif  // TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_LIST_INDEX_H_