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, 34 insertions, 0 deletions

diff --git a/src/utility/predicate.h b/src/utility/predicate.h
new file mode 100644
index 0000000..f06fcee
--- /dev/null
+++ b/src/utility/predicate.h
@@ -0,0 +1,34 @@
+#ifndef TYPEASVALUE_SRC_UTILITY_PREDICATE_H_
+#define TYPEASVALUE_SRC_UTILITY_PREDICATE_H_
+
+#include "conditional/if.h"
+#include "operation/logic.h"
+
+namespace tav {
+
+namespace utility {
+
+template <
+	template<typename> class Predicate,
+	typename                 Surrogate
+>
+struct predicate_assurance {
+	template <typename Value>
+	using assure = If<
+		Eval<Predicate<Value>>,
+		Value,
+		Surrogate
+	>;
+};
+
+template <template<typename> class Predicate>
+struct predicate_negator {
+	template <typename Element>
+	using function = Not<Predicate<Element>>;
+};
+
+}
+
+}
+
+#endif  // TYPEASVALUE_SRC_UTILITY_PREDICATE_H_