feat(crypto_core): add function for unbiased random numbers

Author: kurnevsky

src/toxcore/crypto_core.rs

@@ -62,6 +62,19 @@ pub fn random_usize() -> usize {
     random_u64() as usize
 }
 
+/// Return unbiased random number from `[0, limit)` interval.
+pub fn random_limit_usize(limit: usize) -> usize {
+    // TODO: possibly migrate to rand crate, it has more performant version
+    // of this algorithm implemented with UniformSampler trait
+    let cap = usize::max_value() - usize::max_value() % limit;
+    loop {
+        let n = random_usize();
+        if n < cap {
+            return n % limit;
+        }
+    }
+}
+
 /** Check if Tox public key `PUBLICKEYBYTES` is valid. Should be used only for
     input validation.
 
@@ -310,6 +323,13 @@ pub mod tests {
         assert_ne!(a, b);
     }
 
+    #[test]
+    fn random_limit_usize_test() {
+        crypto_init().unwrap();
+        let n = random_limit_usize(7);
+        assert!(n < 7);
+    }
+
     #[test]
     fn public_key_valid_test() {
         crypto_init().unwrap();

src/toxcore/dht/server/mod.rs

@@ -508,10 +508,10 @@ impl Server {
             return Box::new(future::ok(()))
         }
 
-        let mut random_node_idx = random_usize() % good_nodes.len();
+        let mut random_node_idx = random_limit_usize(good_nodes.len());
         // Increase probability of sending packet to a close node (has lower index)
         if random_node_idx != 0 {
-            random_node_idx -= random_usize() % (random_node_idx + 1);
+            random_node_idx -= random_limit_usize(random_node_idx + 1);
         }
 
         let random_node = &good_nodes[random_node_idx];