distribution: handle sqrt peer view updates (#2871)

* distribution: handle sqrt peer view updates

* someone please put rustc into my brain

* guide updates

polkadot/node/subsystem-util/src/lib.rs

@@ -216,12 +216,26 @@ pub fn choose_random_sqrt_subset<T>(mut v: Vec<T>, min: usize) -> Vec<T> {
 	let mut rng = rand::thread_rng();
 	v.shuffle(&mut rng);
 
-	let len_sqrt = (v.len() as f64).sqrt() as usize;
-	let len = std::cmp::max(min, len_sqrt);
+	let len = max_of_min_and_sqrt_len(v.len(), min);
 	v.truncate(len);
 	v
 }
 
+/// Returns bool with a probability of `max(len.sqrt(), min) / len`
+/// being true.
+pub fn gen_ratio_sqrt_subset(len: usize, min: usize) -> bool {
+	use rand::Rng as _;
+	let mut rng = rand::thread_rng();
+	let threshold = max_of_min_and_sqrt_len(len, min);
+	let n = rng.gen_range(0..len);
+	n < threshold
+}
+
+fn max_of_min_and_sqrt_len(len: usize, min: usize) -> usize {
+	let len_sqrt = (len as f64).sqrt() as usize;
+	std::cmp::max(min, len_sqrt)
+}
+
 /// Local validator information
 ///
 /// It can be created if the local node is a validator in the context of a particular