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, 16 insertions, 26 deletions

diff --git a/src/list/operation/higher/drop_while.h b/src/list/operation/higher/drop_while.h
index 021698f..1207020 100644
--- a/src/list/operation/higher/drop_while.h
+++ b/src/list/operation/higher/drop_while.h
@@ -1,38 +1,28 @@
 #ifndef TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_DROP_WHILE_H_
 #define TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_DROP_WHILE_H_
 
-#include "conditional/if.h"
+#include "list_index.h"
+#include "list/operation/drop.h"
+#include "utility/predicate.h"
 
 namespace tav {
 
-namespace detail {
-
-template <
-	template<typename> class Predicate,
-	typename                 Current
->
-struct DropWhile {
-	typedef If<
-		Eval<Predicate<Head<Current>>>,
-		Eval<DropWhile<Predicate, Tail<Current>>>,
-		Current
-	> type;
-};
-
-template <
-	template<typename> class Predicate
->
-struct DropWhile<Predicate, void> {
-	typedef void type;
-};
-
-}
-
 template <
 	template<typename> class Predicate,
-	typename                 Current
+	typename                 List
 >
-using DropWhile = Eval<detail::DropWhile<Predicate, Current>>;
+using DropWhile = Drop<
+	typename utility::predicate_assurance<
+		utility::predicate_negator<std::is_void>::template function,
+		Length<List>
+	>::template assure<
+		ListIndex<
+			utility::predicate_negator<Predicate>::template function,
+			List
+		>
+	>,
+	List
+>;
 
 }