Don't reload the map in long matches with many players

In matches with many players the server would run out of indexes for
snapshot entities. This is internal to the server for delta updates.
Adjust values so they don't go negative and delta updates still work.

Based on change in Quake3e by Eugene C. in July 2016;
Fix: no more "Restarting server due to numSnapshotEntities wrapping"
ec-/Quake3e@ef47444

Author: zturtleman

code/server/sv_main.c

@@ -1105,12 +1105,6 @@ void SV_Frame( int msec ) {
 		Cbuf_AddText( va( "map %s\n", Cvar_VariableString( "mapname" ) ) );
 		return;
 	}
-	// this can happen considerably earlier when lots of clients play and the map doesn't change
-	if ( svs.nextSnapshotEntities >= 0x7FFFFFFE - svs.numSnapshotEntities ) {
-		SV_Shutdown( "Restarting server due to numSnapshotEntities wrapping" );
-		Cbuf_AddText( va( "map %s\n", Cvar_VariableString( "mapname" ) ) );
-		return;
-	}
 
 	if( sv.restartTime && sv.time >= sv.restartTime ) {
 		sv.restartTime = 0;

code/server/sv_snapshot.c

@@ -441,15 +441,16 @@ For viewing through other player's eyes, clent can be something other than clien
 */
 static void SV_BuildClientSnapshot( client_t *client ) {
 	vec3_t						org;
-	clientSnapshot_t			*frame;
+	clientSnapshot_t			*frame, *oldframe;
 	snapshotEntityNumbers_t		entityNumbers;
-	int							i;
+	int							i, j;
 	sharedEntity_t				*ent;
 	entityState_t				*state;
 	svEntity_t					*svEnt;
 	sharedEntity_t				*clent;
 	int							clientNum;
 	playerState_t				*ps;
+	int							offset;
 
 	// bump the counter used to prevent double adding
 	sv.snapshotCounter++;
@@ -504,6 +505,28 @@ static void SV_BuildClientSnapshot( client_t *client ) {
 		((int *)frame->areabits)[i] = ((int *)frame->areabits)[i] ^ -1;
 	}
 
+	// avoid wrapping snapshot entities index to negative while keeping the same modulo value
+	// and lower existing first_entity for detecting rolling off the buffer
+	if ( svs.nextSnapshotEntities >= 0x7FFFFFFE - entityNumbers.numSnapshotEntities ) {
+		Com_DPrintf( "Updating snapshot entities to avoid numSnapshotEntities wrapping.\n" );
+
+		offset = ( ( svs.nextSnapshotEntities / svs.numSnapshotEntities ) - 2 ) * svs.numSnapshotEntities;
+
+		for ( i = 0; i < sv_maxclients->integer; i++ ) {
+			oldframe = svs.clients[i].frames;
+
+			for ( j = 0; j < PACKET_BACKUP; j++, oldframe++ ) {
+				if ( oldframe->first_entity <= svs.nextSnapshotEntities - svs.numSnapshotEntities ) {
+					oldframe->first_entity %= svs.numSnapshotEntities;
+				} else {
+					oldframe->first_entity -= offset;
+				}
+			}
+		}
+
+		svs.nextSnapshotEntities -= offset;
+	}
+
 	// copy the entity states out
 	frame->num_entities = 0;
 	frame->first_entity = svs.nextSnapshotEntities;
@@ -512,7 +535,7 @@ static void SV_BuildClientSnapshot( client_t *client ) {
 		state = &svs.snapshotEntities[svs.nextSnapshotEntities % svs.numSnapshotEntities];
 		*state = ent->s;
 		svs.nextSnapshotEntities++;
-		// this should never hit, map should always be restarted first in SV_Frame
+		// this should never hit
 		if ( svs.nextSnapshotEntities >= 0x7FFFFFFE ) {
 			Com_Error(ERR_FATAL, "svs.nextSnapshotEntities wrapped");
 		}