string: add less comparator with transform

string.hpp

@@ -302,6 +302,55 @@ namespace cruft {
 }
 
 
+namespace cruft::string {
+    template <char (*TransformV)(char) noexcept>
+    struct less {
+        bool operator() (char const *a, char const *b) const noexcept
+        {
+            for (; a && b; ++a, ++b) {
+                auto const &a_val = TransformV (*a);
+                auto const &b_val = TransformV (*b);
+
+                if (a_val < b_val)
+                    return true;
+                if (a_val > b_val)
+                    return false;
+            }
+
+            return a == nullptr && b != nullptr;
+        }
+
+        bool operator() (std::string const &a, std::string const & b) const noexcept
+        {
+            return (*this) (std::string_view (a), std::string_view (b));
+        }
+
+        bool operator() (std::string_view a, std::string_view b) const noexcept
+        {
+            auto a_cursor = a.begin ();
+            auto b_cursor = b.begin ();
+
+            for ( ; a_cursor != a.end () && b_cursor != b.end (); ++a_cursor, ++b_cursor) {
+                auto const &a_val = TransformV (*a_cursor);
+                auto const &b_val = TransformV (*b_cursor);
+                if (a_val < b_val)
+                    return true;
+                if (a_val > b_val)
+                    return false;
+            }
+
+
+            if (a_cursor == a.end () && b_cursor != b.end ())
+                return true;
+            return false;
+        }
+    };
+
+
+    using less_lower = less<ascii::try_lower>;
+}
+
+
 namespace cruft::string::equality {
     ///////////////////////////////////////////////////////////////////////////
     /// A case comparator that tests equality on a string after a