selecting peers use formula: max(MIN, min(MAX, sqrt(x))) (starcoinorg…

…#2309)

Co-authored-by: jolestar <jolestar@gmail.com>

network/src/service.rs

@@ -17,7 +17,7 @@ use network_api::messages::{
 use network_api::peer_score::{BlockBroadcastEntry, HandleState, LinearScore, Score};
 use network_api::{NetworkActor, PeerMessageHandler};
 use network_p2p::{Event, NetworkWorker};
-use rand::RngCore;
+use rand::prelude::SliceRandom;
 use starcoin_config::NodeConfig;
 use starcoin_crypto::HashValue;
 use starcoin_network_rpc::NetworkRpcService;
@@ -572,15 +572,42 @@ fn select_random_peers(
 ) -> Vec<PeerId> {
     let (min_peers, max_peers) = peer_num_range.into_inner();
     let peers_len = peers.len();
-    // sqrt(x)/x scaled to max u32
-    let fraction = ((peers_len as f64).powf(-0.5) * (u32::max_value() as f64).round()) as u32;
-    let small = peers_len < (min_peers as usize);
+    // take sqrt(x) peers
+    let mut count = (peers_len as f64).powf(0.5).round() as u32;
+    count = count.min(max_peers).max(min_peers);
 
     let mut random = rand::thread_rng();
-    peers
-        .keys()
-        .cloned()
-        .filter(|_| small || random.next_u32() < fraction)
-        .take(max_peers as usize)
-        .collect()
+    let mut peer_ids: Vec<_> = peers.keys().cloned().collect();
+    peer_ids.shuffle(&mut random);
+    peer_ids.truncate(count as usize);
+    peer_ids
+}
+
+#[cfg(test)]
+mod test {
+    use crate::service::{select_random_peers, Peer};
+    use network_api::{PeerId, PeerInfo};
+    use starcoin_types::startup_info::ChainInfo;
+    use std::collections::HashMap;
+
+    fn create_peers(n: u32) -> HashMap<PeerId, Peer> {
+        (0..n)
+            .map(|_| {
+                let peer_id = PeerId::random();
+                let peer = Peer::new(PeerInfo::new(peer_id.clone(), ChainInfo::random()));
+                (peer_id, peer)
+            })
+            .collect()
+    }
+
+    #[test]
+    fn test_select_peer() {
+        assert_eq!(select_random_peers(1..=3, &create_peers(2)).len(), 1);
+        assert_eq!(select_random_peers(2..=5, &create_peers(9)).len(), 3);
+        assert_eq!(select_random_peers(8..=128, &create_peers(3)).len(), 3);
+        assert_eq!(select_random_peers(8..=128, &create_peers(4)).len(), 4);
+        assert_eq!(select_random_peers(8..=128, &create_peers(10)).len(), 8);
+        assert_eq!(select_random_peers(8..=128, &create_peers(25)).len(), 8);
+        assert_eq!(select_random_peers(8..=128, &create_peers(64)).len(), 8);
+    }
 }