factor out and unit test parts of the DHT routing table logic that ha…

…ndles the affinity of nodes to insert. Specifically, make sure the even distribution within routing table buckets works correctly

include/libtorrent/kademlia/routing_table.hpp

@@ -80,7 +80,7 @@ struct ipv6_hash
 	}
 };
 
-struct ip_set
+struct TORRENT_EXTRA_EXPORT ip_set
 {
 	void insert(address const& addr);
 	bool exists(address const& addr) const;
@@ -97,12 +97,23 @@ struct ip_set
 		return m_ip4s == rh.m_ip4s && m_ip6s == rh.m_ip6s;
 	}
 
+	std::size_t size() const { return m_ip4s.size() + m_ip6s.size(); }
+
 	// these must be multisets because there can be multiple routing table
 	// entries for a single IP when restrict_routing_ips is set to false
 	std::unordered_multiset<address_v4::bytes_type, ipv4_hash> m_ip4s;
 	std::unordered_multiset<address_v6::bytes_type, ipv6_hash> m_ip6s;
 };
 
+// Each routing table bucket represents node IDs with a certain number of bits
+// of prefix in common with our own node ID. Each bucket fits 8 nodes (and
+// sometimes more, closer to the top). In order to minimize the number of hops
+// necessary to traverse the DHT, we want the nodes in our buckets to be spread
+// out across all possible "sub-branches". This is what the "classify" refers
+// to. The 3 (or more) bits following the shared bit prefix.
+TORRENT_EXTRA_EXPORT std::uint8_t  classify_prefix(int bucket_idx, bool last_bucket
+	, int bucket_size, node_id nid);
+
 // differences in the implementation from the description in
 // the paper:
 //
@@ -308,6 +319,14 @@ class TORRENT_EXTRA_EXPORT routing_table
 	int const m_bucket_size;
 };
 
+TORRENT_EXTRA_EXPORT routing_table::add_node_status_t
+replace_node_impl(node_entry const& e, bucket_t& b, ip_set& ips
+	, int bucket_index, int bucket_size_limit, bool last_bucket
+#ifndef TORRENT_DISABLE_LOGGING
+	, dht_logger* log
+#endif
+	);
+
 } } // namespace libtorrent::dht
 
 #endif // ROUTING_TABLE_HPP