utils: add helpers for assoc-pairs

Add some unit-test, too.

lib/utils/mu-utils.hh

@@ -326,7 +326,7 @@ typename Sequence::const_iterator seq_find_if(const Sequence& seq, UnaryPredicat
 }
 
 /**
- * Is at least pred(element) true for at least one element of sequence
+ * Is pred(element) true for at least one element of sequence?
  *
  * @param seq sequence
  * @param pred a predicate
@@ -391,6 +391,49 @@ void seq_for_each(const Sequence& seq, UnaryOp op) {
 	std::for_each(seq.cbegin(), seq.cend(), op);
 }
 
+/**
+ * array of associated pair elements -- like an alist
+ * but based on std::array and thus can be constexpr
+ */
+template<typename T1, typename T2, std::size_t N>
+      using AssocPairs = std::array<std::pair<T1, T2>, N>;
+
+/**
+  * Get the first value of the pair where the second element is @param s.
+  *
+  * @param p AssocPairs
+  * @param s some second pair value
+  *
+  * @return the matching first pair value, or Nothing if not found.
+  */
+template<typename P>
+constexpr Option<typename P::value_type::first_type>
+to_first(const P& p, typename P::value_type::second_type s)
+{
+	for (const auto& item: p)
+		if (item.second == s)
+			return item.first;
+	return Nothing;
+}
+
+/**
+  * Get the second value of the pair where the first element is @param f.
+  *
+  * @param p AssocPairs
+  * @param f some first pair value
+  *
+  * @return the matching second pair value, or Nothing if not found.
+  */
+template<typename P>
+constexpr Option<typename P::value_type::second_type>
+to_second(const P& p, typename P::value_type::first_type f)
+{
+	for (const auto& item: p)
+		if (item.first == f)
+			return item.second;
+	return Nothing;
+}
+
 
 /**
  * Convert string view in something printable with %*s

lib/utils/tests/test-utils.cc

@@ -298,6 +298,33 @@ test_error()
 	g_clear_error(&err);
 }
 
+enum struct TestEnum { A, B, C };
+constexpr AssocPairs<TestEnum, std::string_view, 3>
+test_epairs = {{
+	{TestEnum::A, "a"},
+	{TestEnum::B, "b"},
+	{TestEnum::C, "c"},
+}};
+
+static constexpr Option<std::string_view>
+to_name(TestEnum te)
+{
+	return to_second(test_epairs, te);
+}
+
+static constexpr Option<TestEnum>
+to_type(std::string_view name)
+{
+	return to_first(test_epairs, name);
+
+}
+
+static void
+test_enum_pairs(void)
+{
+	assert_equal(to_name(TestEnum::A).value(), "a");
+	g_assert_true(to_type("c").value() ==  TestEnum::C);
+}
 
 int
 main(int argc, char* argv[])
@@ -317,6 +344,7 @@ main(int argc, char* argv[])
 	g_test_add_func("/utils/to-from-lexnum", test_to_from_lexnum);
 	g_test_add_func("/utils/locale-workaround", test_locale_workaround);
 	g_test_add_func("/utils/error", test_error);
+	g_test_add_func("/utils/enum-pairs", test_enum_pairs);
 
 	return g_test_run();
 }