Merge bitcoin/bitcoin#22896: refactor: net: avoid duplicate map lookups to `mapLocalHost`

330d3aa1a2 refactor: net: avoid duplicate map lookups to `mapLocalHost` (Sebastian Falbesoner)

Pull request description:

  This simple refactoring PR aims to avoid duplicate lookups to `mapLocalHost`: instead of calling `count()` (to first find out whether a key is in the map) and then `operator[]` (to get the value to the passed key, or default-construct one if not found), use either
  * `find()` and dereference the returned iterator (for simple lookups), see https://www.cplusplus.com/reference/map/map/find/
  * `emplace()` and use the returned <iterator, inserted> pair (for lookups where a new element should be inserted if the key isn't found), see https://www.cplusplus.com/reference/map/map/emplace/

ACKs for top commit:
  naumenkogs:
    ACK 330d3aa1a2
  jonatack:
    Code review ACK 330d3aa1a2 plus rebase to master + debug build

Tree-SHA512: d13da6a927ff561eee8ac6b093bf3586dfe31d6c94173a5a6d8f3698e0ee224fb394d3635155d5141c165da59d2c2c37260122eb4f2e8bcda3e8a29b901d213e

src/net.cpp

@@ -192,8 +192,8 @@ CAddress GetLocalAddress(const CNetAddr *paddrPeer, ServiceFlags nLocalServices)
 static int GetnScore(const CService& addr)
 {
     LOCK(cs_mapLocalHost);
-    if (mapLocalHost.count(addr) == 0) return 0;
-    return mapLocalHost[addr].nScore;
+    const auto it = mapLocalHost.find(addr);
+    return (it != mapLocalHost.end()) ? it->second.nScore : 0;
 }
 
 // Is our peer's addrLocal potentially useful as an external IP source?
@@ -245,10 +245,10 @@ bool AddLocal(const CService& addr, int nScore)
 
     {
         LOCK(cs_mapLocalHost);
-        bool fAlready = mapLocalHost.count(addr) > 0;
-        LocalServiceInfo &info = mapLocalHost[addr];
-        if (!fAlready || nScore >= info.nScore) {
-            info.nScore = nScore + (fAlready ? 1 : 0);
+        const auto [it, is_newly_added] = mapLocalHost.emplace(addr, LocalServiceInfo());
+        LocalServiceInfo &info = it->second;
+        if (is_newly_added || nScore >= info.nScore) {
+            info.nScore = nScore + (is_newly_added ? 0 : 1);
             info.nPort = addr.GetPort();
         }
     }
@@ -290,12 +290,10 @@ bool IsReachable(const CNetAddr &addr)
 /** vote for a local address */
 bool SeenLocal(const CService& addr)
 {
-    {
-        LOCK(cs_mapLocalHost);
-        if (mapLocalHost.count(addr) == 0)
-            return false;
-        mapLocalHost[addr].nScore++;
-    }
+    LOCK(cs_mapLocalHost);
+    const auto it = mapLocalHost.find(addr);
+    if (it == mapLocalHost.end()) return false;
+    ++it->second.nScore;
     return true;
 }