add `ttl` to TOTP

examples/ttl.rs

@@ -0,0 +1,23 @@
+use totp_rs::{Algorithm, TOTP};
+
+fn main() {
+    let totp = TOTP::new(
+        Algorithm::SHA1,
+        6,
+        1,
+        30,
+        "my-secret".to_string(),
+        None,
+        "account".to_string(),
+    ).unwrap();
+
+    loop {
+        println!(
+            "code {}\t ttl {}\t valid until: {}",
+            totp.generate_current().unwrap(),
+            totp.ttl().unwrap(),
+            totp.next_step_current().unwrap()
+        );
+        std::thread::sleep(std::time::Duration::from_secs(1));
+    }
+}

src/lib.rs

@@ -233,6 +233,12 @@ impl<T: AsRef<[u8]>> TOTP<T> {
         Ok(self.next_step(t))
     }
 
+    /// Give the ttl (in seconds) of the current token
+    pub fn ttl(&self) -> Result<u64, SystemTimeError> {
+        let t = system_time()?;
+        Ok(self.step - (t % self.step))
+    }
+
     /// Generate a token from the current system time
     pub fn generate_current(&self) -> Result<String, SystemTimeError> {
         let t = system_time()?;