string: add string_less comparator

string.hpp

@@ -11,6 +11,10 @@
 #include "debug.hpp"
 #include "view.hpp"
 
+#include <string>
+#include <type_traits>
+
+
 namespace cruft {
     std::string to_utf8 (const wchar_t*);
     std::string to_utf8 (const std::wstring&);
@@ -194,4 +198,21 @@ namespace cruft {
 
         return val;
     }
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// A comparator for string-like objects that uses strcmp rather than
+    /// pointer comparison.
+    ///
+    /// TODO: handle string and string_view objects
+    struct string_less {
+        template <
+            typename CharT,
+            typename = std::void_t<typename std::char_traits<CharT>::char_type>
+        >
+        bool operator() (CharT const *a, CharT const *b) const noexcept
+        {
+            return strcmp (a, b) < 0;
+        }
+    };
 }

test/string.cpp

@@ -143,6 +143,27 @@ test_contains (cruft::TAP::logger &tap)
 }
 
 
+///////////////////////////////////////////////////////////////////////////////
+void test_comparator (cruft::TAP::logger &tap)
+{
+    cruft::string_less cmp;
+
+    struct {
+        char const *a;
+        char const *b;
+        bool less;
+        char const *message;
+    } const TESTS[] = {
+        { "a", "b",  true,  "single char less-than" },
+        { "b", "a",  false, "single char not-less-than" },
+        { "a", "aa", true,  "single/double char less-than" },
+    };
+
+    for (auto const &t: TESTS)
+        tap.expect_eq (cmp (t.a, t.b), t.less, "string_less, %!", t.message);
+}
+
+
 ///////////////////////////////////////////////////////////////////////////////
 int
 main (int, char**)
@@ -152,6 +173,7 @@ main (int, char**)
     test_tokeniser (tap);
     test_position (tap);
     test_contains (tap);
+    test_comparator (tap);
 
     return tap.status ();
 }