Add helper functions for invoice expiry

Author: benthecarman

lightning-invoice/src/lib.rs

@@ -1213,6 +1213,11 @@ impl Invoice {
 		self.signed_invoice.recover_payee_pub_key().expect("was checked by constructor").0
 	}
 
+	/// Returns the Duration since the Unix epoch at which the invoice expires.
+	pub fn expires_at(&self) -> Option<Duration> {
+		self.duration_since_epoch().checked_add(self.expiry_time())
+	}
+
 	/// Returns the invoice's expiry time, if present, otherwise [`DEFAULT_EXPIRY_TIME`].
 	pub fn expiry_time(&self) -> Duration {
 		self.signed_invoice.expiry_time()
@@ -1235,6 +1240,21 @@ impl Invoice {
 		}
 	}
 
+	/// Returns the Duration remaining until the invoice expires.
+	#[cfg(feature = "std")]
+	pub fn expiration_remaining(&self) -> Option<Duration> {
+		 match self.timestamp().elapsed() {
+			 Ok(elapsed) => self.expiry_time().checked_sub(elapsed),
+			 Err(_) => None,
+		 }
+	}
+
+	/// Returns the Duration remaining until the invoice expires given the current time.
+	/// `time` is the timestamp as a duration since the Unix epoch.
+	pub fn expiration_remaining_from_epoch(&self, time: Duration) -> Option<Duration> {
+		self.expires_at().map(|x| x.checked_sub(time)).flatten()
+	}
+
 	/// Returns whether the expiry time would pass at the given point in time.
 	/// `at_time` is the timestamp as a duration since the Unix epoch.
 	pub fn would_expire(&self, at_time: Duration) -> bool {