--- nono/lib/test_mystring.cpp	2026/04/29 17:04:57	1.1.1.2
+++ nono/lib/test_mystring.cpp	2026/04/29 17:05:08	1.1.1.3
@@ -201,6 +201,7 @@ class Test_MyString : public TestClass
 		TESTDEF(startwith_ignorecase_1);
 		TESTDEF(string_split_1);
 		TESTDEF(string_split_2);
+		TESTDEF(format_number_1);
 	}
 
 	void string_ltrim_1() {
@@ -379,6 +380,27 @@ class Test_MyString : public TestClass
 			xp_eq(exp, string_split(src, ',', num), t);
 		}
 	}
+
+	void format_number_1() {
+		TestTable<uint64, std::string> table = {
+			{ 0,			"0" },
+			{ 1,			"1" },
+			{ 999,			"999" },
+			{ 1000,			"1,000" },
+			{ 12345,		"12,345" },
+			{ 123456,		"123,456" },
+			{ 9876543,		"9,876,543" },
+			{ 987654321,	"987,654,321" },
+			{ 1000000000,	"1,000,000,000" },
+		};
+
+		for (const auto& t : table) {
+			auto        val = t.get<0>();
+			const auto& exp = t.get<1>();
+
+			xp_eq(exp, format_number(val), t);
+		}
+	}
 };
 
 int