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net: Improve -newsync to use a PID controller.

The previous -newsync code wasn't too bad but was basically a hacky
version of a PID controller. So replace this with an actual PID
controller with tuning parameters manually tuned based on an evening
of experimentation. So far this seems to be a significant improvement
over the old -newsync code.

--- a/src/net_client.c

+++ b/src/net_client.c

@@ -150,6 +150,12 @@

 static boolean need_to_acknowledge;

 static unsigned int gamedata_recv_time;

+// The latency (time between when we sent our command and we got all

+// the other players' commands from the server) for the last tic we

+// received. We include this latency in tics we send to the server so

+// that they can adjust to us.

+static int last_latency;

+

 // Hash checksums of our wad directory and dehacked data.

 sha1_digest_t net_local_wad_sha1sum;

@@ -159,10 +165,6 @@

 unsigned int net_local_is_freedoom;

-// Average time between sending our ticcmd and receiving from the server

-

-static fixed_t average_latency;

-

 #define NET_CL_ExpandTicNum(b) NET_ExpandTicNum(recvwindow_start, (b))

 // Called when we become disconnected from the server

@@ -172,20 +174,15 @@

     D_ReceiveTic(NULL, NULL);

 }

-// Expand a net_full_ticcmd_t, applying the diffs in cmd->cmds as

-// patches against recvwindow_cmd_base.  Place the results into

-// the d_net.c structures (netcmds/nettics) and save the new ticcmd

-// back into recvwindow_cmd_base.

-

-static void NET_CL_ExpandFullTiccmd(net_full_ticcmd_t *cmd, unsigned int seq,

-                                    ticcmd_t *ticcmds)

+// Called when a packet is received from the server containing game

+// data. This updates the clock synchronization variable (offsetms)

+// using a PID filter that keeps client clocks in sync.

+static void UpdateClockSync(unsigned int seq,

+                            unsigned int remote_latency)

 {

-    int latency;

-    fixed_t adjustment;

-    int i;

+    static int last_error, cumul_error;

+    int latency, error;

-    // Update average_latency

-

     if (seq == send_queue[seq % BACKUPTICS].seq)

     {

         latency = I_GetTimeMS() - send_queue[seq % BACKUPTICS].time;

@@ -199,43 +196,38 @@

     }

     else

     {

-        latency = -1;

+        return;

     }

-    if (latency >= 0)

-    {

-        if (seq <= 20)

-        {

-            average_latency = latency * FRACUNIT;

-        }

-        else

-        {

-            // Low level filter

+    // PID filter. These are manually trained parameters.

+#define KP 0.1

+#define KI 0.01

+#define KD 0.02

-            average_latency = (fixed_t)((average_latency * 0.9)

-                            + (latency * FRACUNIT * 0.1));

-        }

-    }

+    // How does our latency compare to the worst other player?

+    error = latency - remote_latency;

+    cumul_error += error;

-    //printf("latency: %i\tremote:%i\n", average_latency / FRACUNIT, 

-    //                                   cmd->latency);

+    offsetms = KP * (FRACUNIT * error)

+             - KI * (FRACUNIT * cumul_error)

+             + (KD * FRACUNIT) * (last_error - error);

-    // Possibly adjust offsetms in d_net.c, try to make players all have

-    // the same lag.  Don't adjust in the first few tics of play, as 

-    // we don't have an accurate value for average_latency yet.

+    last_error = error;

+    last_latency = latency;

-    if (seq > TICRATE)

-    {

-        adjustment = (cmd->latency * FRACUNIT) - average_latency;

+    //printf("%i,%i,%i\n", latency, remote_latency, offsetms);

+}

-        // Only adjust very slightly; the cumulative effect over 

-        // multiple tics will sort it out.

+// Expand a net_full_ticcmd_t, applying the diffs in cmd->cmds as

+// patches against recvwindow_cmd_base.  Place the results into

+// the d_net.c structures (netcmds/nettics) and save the new ticcmd

+// back into recvwindow_cmd_base.

-        adjustment = adjustment / 100;

+static void NET_CL_ExpandFullTiccmd(net_full_ticcmd_t *cmd, unsigned int seq,

+                                    ticcmd_t *ticcmds)

+{

+    int i;

-        offsetms += adjustment;

-    }

-

     // Expand tic diffs for all players

     for (i=0; i<NET_MAXPLAYERS; ++i)

@@ -375,7 +367,7 @@

         sendobj = &send_queue[i % BACKUPTICS];

-        NET_WriteInt16(packet, average_latency / FRACUNIT);

+        NET_WriteInt16(packet, last_latency);

         NET_WriteTiccmdDiff(packet, &sendobj->cmd, settings.lowres_turn);

     }

@@ -715,11 +707,21 @@

         }

         // Store in the receive window

-        

+

         recvobj = &recvwindow[index];

         recvobj->active = true;

         recvobj->cmd = cmd;

+

+        // If a packet is lost or arrives out of order, we might get

+        // the tic in the next packet instead (because of extratic).

+        // If that's the case then the latency for receiving that tic

+        // now will be bogus. So we only use the last tic in the packet

+        // to trigger a clock sync update.

+        if (i == num_tics - 1)

+        {

+            UpdateClockSync(seq + i, cmd.latency);

+        }

     }

     // Has this been received out of sequence, ie. have we not received