Fix subtraction overflow bug with latency (#147)

Previously if you specify a `min_message_latency` greater than the default
`max_message_latency`, the test would fail with a tokio error: `overflow
when subtracting durations`. This change updates the builder to assert
that the min/max latencies are valid on build.

src/builder.rs

@@ -64,20 +64,12 @@ impl Builder {
     }
 
     pub fn min_message_latency(&mut self, value: Duration) -> &mut Self {
-        self.link
-            .latency
-            .as_mut()
-            .expect("`Latency` missing")
-            .min_message_latency = value;
+        self.link.latency_mut().min_message_latency = value;
         self
     }
 
     pub fn max_message_latency(&mut self, value: Duration) -> &mut Self {
-        self.link
-            .latency
-            .as_mut()
-            .expect("`MessageLoss` missing")
-            .max_message_latency = value;
+        self.link.latency_mut().max_message_latency = value;
         self
     }
 
@@ -106,7 +98,27 @@ impl Builder {
     }
 
     pub fn build_with_rng<'a>(&self, rng: Box<dyn RngCore>) -> Sim<'a> {
+        if self.link.latency().max_message_latency < self.link.latency().min_message_latency {
+            panic!("Maximum message latency must be greater than minimum.");
+        }
+
         let world = World::new(self.link.clone(), rng, self.ip_version.iter());
         Sim::new(self.config.clone(), world)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use std::time::Duration;
+
+    use crate::Builder;
+
+    #[test]
+    #[should_panic]
+    fn invalid_latency() {
+        let _sim = Builder::new()
+            .min_message_latency(Duration::from_millis(100))
+            .max_message_latency(Duration::from_millis(50))
+            .build();
+    }
+}

src/top.rs

@@ -149,6 +149,7 @@ struct Link {
     now: Instant,
 }
 
+/// States that a link between two nodes can be in.
 enum State {
     /// The link is healthy.
     Healthy,
@@ -268,6 +269,7 @@ impl Topology {
     }
 }
 
+/// Represents a message sent between two hosts on the network.
 struct Sent {
     src: SocketAddr,
     dst: SocketAddr,