diff --git a/src/main.zig b/src/main.zig
index 9a3b622..e1b5cdf 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -62,6 +62,7 @@ const OtpAuthUrl = struct { name: []const u8, secretEncoded: []const u8, url: []
 const Authenticator = struct {
     url: OtpAuthUrl,
 
+    /// algorithm based on https://www.rfc-editor.org/rfc/rfc6238
     pub fn code(self: Authenticator, allocator: Allocator, timeFunc: *const fn () i64) ![]const u8 {
         if (self.url.name.len > 0) {}
 
@@ -331,6 +332,54 @@ test "authenticator generate code with 10 digits and zero padding" {
     try std.testing.expectEqualStrings("0844221464", code);
 }
 
+test "testcases from https://www.rfc-editor.org/rfc/rfc6238#appendix-A" {
+    const secretForSha1 = "GEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQ"; // plain: "12345678901234567890"  hex: "3132333435363738393031323334353637383930"
+
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8 }, _closure_return_59, "94287082");
+
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8 }, _closure_return_1111111109, "07081804");
+
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8 }, _closure_return_1111111111, "14050471");
+
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8 }, _closure_return_1234567890, "89005924");
+
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8 }, _closure_return_2000000000, "69279037");
+
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8 }, _closure_return_20000000000, "65353130");
+}
+
+fn testTOTP(otpAuthUrl: OtpAuthUrl, timeFunc: *const fn () i64, expected: []const u8) !void {
+    const authenticator = Authenticator{ .url = otpAuthUrl };
+
+    const code = try authenticator.code(std.testing.allocator, timeFunc);
+    defer std.testing.allocator.free(code);
+    try std.testing.expectEqualStrings(expected, code);
+}
+
 fn _closure_return_1725695340() i64 {
     return 1725695340;
 }
+
+fn _closure_return_59() i64 {
+    return 59;
+}
+
+fn _closure_return_1111111109() i64 {
+    return 1111111109;
+}
+
+fn _closure_return_1111111111() i64 {
+    return 1111111111;
+}
+
+fn _closure_return_1234567890() i64 {
+    return 1234567890;
+}
+
+fn _closure_return_2000000000() i64 {
+    return 2000000000;
+}
+
+fn _closure_return_20000000000() i64 {
+    return 20000000000;
+}