Server: read the fade-in gain once per channel per frame

DecodeReceiveData() called CChannel::GetFadeInGain() twice per channel
pair, so every frame performed 2N^2-N int-to-float conversions and
divisions on N values that do not change within the frame: the fade-in
counters are only written under CServer::Mutex, which OnTimer() holds
for the whole decode phase.

Read each connected channel's fade-in gain into vecfFadeInGains in the
loop that builds vecChanIDsCurConChan, in the same order and under the
same lock, and multiply from that vector instead. The two factors are
applied in the same order as before.

Closes #3945

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: mcfnord

src/server.cpp

@@ -172,6 +172,7 @@ CServer::CServer ( const int          iNewMaxNumChan,
 
     // allocate worst case memory for the temporary vectors
     vecChanIDsCurConChan.Init ( iMaxNumChannels );
+    vecfFadeInGains.Init ( iMaxNumChannels );
     vecvecfGains.Init ( iMaxNumChannels );
     vecvecfPannings.Init ( iMaxNumChannels );
     vecvecsData.Init ( iMaxNumChannels );
@@ -678,6 +679,12 @@ void CServer::OnTimer()
                 // connected clients is less, only a subset of elements of this
                 // vector are actually used and the others are dummy elements)
                 vecChanIDsCurConChan[iNumClients] = i;
+
+                // read the fade-in gain of the channel once per frame for use in
+                // DecodeReceiveData() (the fade-in counters are only written under
+                // the server mutex, which is held here and for the whole decode phase)
+                vecfFadeInGains[iNumClients] = vecChannels[i].GetFadeInGain();
+
                 iNumClients++;
             }
         }
@@ -896,24 +903,24 @@ void CServer::DecodeReceiveData ( const int iChanCnt, const int iNumClients )
         CurOpusDecoder = nullptr;
     }
 
-    // get gains and pannings of all connected channels, compacted to the
-    // order of "vecChanIDsCurConChan".
-    // The second index of "vecvecfGains" does not represent
-    // the channel ID! Therefore we have to use
-    // "vecChanIDsCurConChan" to query the IDs of the currently
-    // connected channels
+    // get gains and pannings of all connected channels, compacted to the order of
+    // "vecChanIDsCurConChan" (the second index of "vecvecfGains" does not represent the
+    // channel ID but the position in "vecChanIDsCurConChan")
     vecChannels[iCurChanID].GetGainsAndPannings ( vecChanIDsCurConChan, iNumClients, vecvecfGains[iChanCnt], vecvecfPannings[iChanCnt] );
 
+    // apply the audio fade-in: each connected channel fades in with its own fade-in gain,
+    // and the fade-in gain of the current channel is applied to all other channels as well
+    // to avoid the client volumes being at 100% when joining a server (#628); the fade-in
+    // gains were read once per frame into "vecfFadeInGains" in the same order
+    const float fCurChanFadeInGain = vecfFadeInGains[iChanCnt];
+
     for ( int j = 0; j < iNumClients; j++ )
     {
-        // consider audio fade-in
-        vecvecfGains[iChanCnt][j] *= vecChannels[vecChanIDsCurConChan[j]].GetFadeInGain();
+        vecvecfGains[iChanCnt][j] *= vecfFadeInGains[j];
 
-        // use the fade in of the current channel for all other connected clients
-        // as well to avoid the client volumes are at 100% when joining a server (#628)
         if ( j != iChanCnt )
         {
-            vecvecfGains[iChanCnt][j] *= vecChannels[iCurChanID].GetFadeInGain();
+            vecvecfGains[iChanCnt][j] *= fCurChanFadeInGain;
         }
     }
 

src/server.h

@@ -276,6 +276,7 @@ class CServer : public QObject, public CServerSlots<MAX_NUM_CHANNELS>
 
     CVector<QString> vstrChatColors;
     CVector<int>     vecChanIDsCurConChan;
+    CVector<float>   vecfFadeInGains;
 
     CVector<CVector<float>>   vecvecfGains;
     CVector<CVector<float>>   vecvecfPannings;