Make RNG seedable

Cargo.toml

@@ -5,4 +5,5 @@ resolver = "2"
 [workspace.dependencies]
 tokio = { version = "1.38.0", features = ["full"] }
 axum = "0.7.5"
-serde = { version = "1.0.204", features = ["derive"] }
+serde = { version = "1.0.204", features = ["derive"] }
+rand = { version = "0.8.5", features = ["small_rng"] }

crates/chatbot/Cargo.toml

@@ -4,4 +4,5 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
-tokio = { workspace = true }
+tokio = { workspace = true }
+rand = { workspace = true }

crates/chatbot/src/lib.rs

@@ -1,12 +1,22 @@
-use std::time::Duration;
+use rand::{rngs::SmallRng, Rng, SeedableRng};
+use std::{cell::RefCell, time::Duration};
 use tokio::time::sleep;
 
+thread_local! {
+    static RNG: RefCell<SmallRng> = RefCell::new(SmallRng::from_entropy());
+}
+
+/// Seeds the thread-local RNG used by [`gen_random_number`].
+pub fn seed_rng(seed: u64) {
+    RNG.with(|rng| *rng.borrow_mut() = SmallRng::seed_from_u64(seed));
+}
+
 /// Generates a random `usize`.
 ///
 /// Warning: may take a few seconds!
 pub async fn gen_random_number() -> usize {
     sleep(Duration::from_secs(2)).await;
-    4
+    RNG.with(|rng| rng.borrow_mut().gen())
 }
 
 /// Generates a list of possible responses given the current chat.

crates/server/Cargo.toml

@@ -8,3 +8,7 @@ anyhow = "1.0.86"
 axum = { workspace = true }
 serde = { workspace = true }
 tokio = { workspace = true }
+chatbot = { path = "../chatbot" }
+
+[dev-dependencies]
+rand = { workspace = true }