Change Builder to accept an rng seed

src/builder.rs

@@ -1,6 +1,6 @@
 use std::{ops::RangeInclusive, time::SystemTime};
 
-use rand::{RngCore, SeedableRng};
+use rand::{rngs::SmallRng, SeedableRng};
 
 use crate::*;
 
@@ -50,27 +50,15 @@ use crate::*;
 ///
 /// ## Entropy and randomness
 ///
-/// By default, the builder will use its own `rng` to determine the variation
-/// in random factors that affect the simulation, like message latency, and
-/// failure distributions. To make your tests deterministic, you can use your
-/// own seeded `rng` provider when building the simulation through
-/// `build_with_rng`.
-///
-/// For example:
-///
-/// ```
-/// use rand::rngs::SmallRng;
-/// use rand::SeedableRng;
-///
-/// let rng = SmallRng::seed_from_u64(0);
-/// let sim = turmoil::Builder::new().build_with_rng(Box::new(rng));
-/// ```
+/// Turmoil uses a random number generator to determine the variation in random
+/// factors that affect the simulation, like message latency, and failure
+/// distributions. By default, this rng is randomly seeded. To make your tests
+/// deterministic, you can provide your own seed through [`Builder::rng_seed`].
 pub struct Builder {
     config: Config,
-
     ip_version: IpVersion,
-
     link: config::Link,
+    rng_seed: Option<u64>,
 }
 
 impl Default for Builder {
@@ -88,6 +76,7 @@ impl Builder {
                 latency: Some(config::Latency::default()),
                 message_loss: Some(config::MessageLoss::default()),
             },
+            rng_seed: None,
         }
     }
 
@@ -178,24 +167,26 @@ impl Builder {
         self
     }
 
-    /// Build a simulation with the settings from the builder.
-    ///
-    /// This will use default rng with entropy from the device running.
-    pub fn build<'a>(&self) -> Sim<'a> {
-        self.build_with_rng(Box::new(rand::rngs::SmallRng::from_entropy()))
+    /// Seed for random numbers generated by turmoil.
+    pub fn rng_seed(&mut self, value: u64) -> &mut Self {
+        self.rng_seed = Some(value);
+        self
     }
 
-    /// Build a sim with a provided `rng`.
-    ///
-    /// This allows setting the random number generator used to fuzz
-    pub fn build_with_rng<'a>(&self, rng: Box<dyn RngCore>) -> Sim<'a> {
+    /// Build a simulation with the settings from the builder.
+    pub fn build<'a>(&self) -> 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 rng = match self.rng_seed {
+            Some(seed) => SmallRng::seed_from_u64(seed),
+            None => SmallRng::from_entropy(),
+        };
+
         let world = World::new(
             self.link.clone(),
-            rng,
+            Box::new(rng),
             self.ip_version.iter(),
             self.config.tick,
         );

src/world.rs

@@ -34,7 +34,7 @@ pub(crate) struct World {
     pub(crate) current: Option<IpAddr>,
 
     /// Random number generator used for all decisions. To make execution
-    /// determinstic, reuse the same seed.
+    /// deterministic, reuse the same seed.
     pub(crate) rng: Box<dyn RngCore>,
 
     /// Run duration for each host on every step.