Implements hello generation in sync module

beacon_node/network/src/sync/simple_sync.rs

@@ -1,4 +1,5 @@
 use crate::beacon_chain::BeaconChain;
+use libp2p::rpc::HelloMessage;
 use libp2p::PeerId;
 use std::collections::HashMap;
 use std::sync::Arc;
@@ -23,8 +24,13 @@ pub enum SyncState {
 //TODO: Decide for HELLO messages whether its better to keep current in RAM or build on the fly
 //when asked.
 pub struct SimpleSync {
+    /// A reference to the underlying beacon chain.
+    chain: Arc<BeaconChain>,
+    /// A mapping of Peers to their respective PeerSyncInfo.
     known_peers: HashMap<PeerId, PeerSyncInfo>,
+    /// The current state of the syncing protocol.
     state: SyncState,
+    /// The network id, for quick HELLO RPC message lookup.
     network_id: u8,
 }
 
@@ -34,6 +40,20 @@ impl SimpleSync {
             known_peers: HashMap::new(),
             state: SyncState::Idle,
             network_id: beacon_chain.get_spec().network_id,
+            chain: beacon_chain,
+        }
+    }
+
+    /// Generates our current state in the form of a HELLO RPC message.
+    pub fn generate_hello(&self) -> HelloMessage {
+        let state = &self.chain.get_state();
+        //TODO: Paul to verify the logic of these fields.
+        HelloMessage {
+            network_id: self.network_id,
+            latest_finalized_root: state.finalized_root.clone(),
+            latest_finalized_epoch: state.finalized_epoch,
+            best_root: state.latest_block_roots[0], // 0 or len of vec?
+            best_slot: state.slot,
         }
     }
 }