Account for the game being paused when tracking sound progress

When the game is paused in SSP world, speakers are not ticked. However,
System.nanoTime() continues to increase, which means the next tick
speakers believe there has been a big jump and so schedule a bunch of
extra audio.

To avoid this, we keep track of how long the game has been paused offset
nanoTime by that amount.

Fixes #994

src/main/java/dan200/computercraft/shared/peripheral/speaker/DfpwmState.java

@@ -7,6 +7,7 @@ package dan200.computercraft.shared.peripheral.speaker;
 
 import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.lua.LuaTable;
+import dan200.computercraft.shared.util.PauseAwareTimer;
 import net.minecraft.util.math.MathHelper;
 
 import javax.annotation.Nonnull;
@@ -36,7 +37,7 @@ class DfpwmState
     private boolean previousBit = false;
 
     private boolean unplayed = true;
-    private long clientEndTime = System.nanoTime();
+    private long clientEndTime = PauseAwareTimer.getTime();
     private float pendingVolume = 1.0f;
     private ByteBuffer pendingAudio;
 
@@ -107,7 +108,7 @@ class DfpwmState
 
     boolean isPlaying()
     {
-        return unplayed || clientEndTime >= System.nanoTime();
+        return unplayed || clientEndTime >= PauseAwareTimer.getTime();
     }
 
     float getVolume()

src/main/java/dan200/computercraft/shared/peripheral/speaker/SpeakerPeripheral.java

@@ -17,6 +17,7 @@ import dan200.computercraft.shared.network.client.SpeakerAudioClientMessage;
 import dan200.computercraft.shared.network.client.SpeakerMoveClientMessage;
 import dan200.computercraft.shared.network.client.SpeakerPlayClientMessage;
 import dan200.computercraft.shared.network.client.SpeakerStopClientMessage;
+import dan200.computercraft.shared.util.PauseAwareTimer;
 import net.minecraft.network.play.server.SPlaySoundPacket;
 import net.minecraft.server.MinecraftServer;
 import net.minecraft.state.properties.NoteBlockInstrument;
@@ -119,7 +120,7 @@ public abstract class SpeakerPeripheral implements IPeripheral
             return;
         }
 
-        long now = System.nanoTime();
+        long now = PauseAwareTimer.getTime();
         if( sound != null )
         {
             lastPlayTime = clock;

src/main/java/dan200/computercraft/shared/util/PauseAwareTimer.java

@@ -0,0 +1,53 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2021. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+package dan200.computercraft.shared.util;
+
+import net.minecraft.client.Minecraft;
+import net.minecraft.util.Util;
+import net.minecraftforge.api.distmarker.Dist;
+import net.minecraftforge.event.TickEvent;
+import net.minecraftforge.eventbus.api.SubscribeEvent;
+import net.minecraftforge.fml.common.Mod;
+
+/**
+ * A monotonically increasing clock which accounts for the game being paused.
+ */
+@Mod.EventBusSubscriber( Dist.CLIENT )
+public final class PauseAwareTimer
+{
+    private static boolean paused;
+    private static long pauseTime;
+    private static long pauseOffset;
+
+    private PauseAwareTimer()
+    {
+    }
+
+    public static long getTime()
+    {
+        return (paused ? pauseTime : Util.getNanos()) - pauseOffset;
+    }
+
+    @SubscribeEvent
+    public static void tick( TickEvent.RenderTickEvent event )
+    {
+        if( event.phase != TickEvent.Phase.START ) return;
+
+        boolean isPaused = Minecraft.getInstance().isPaused();
+        if( isPaused == paused ) return;
+
+        if( isPaused )
+        {
+            pauseTime = Util.getNanos();
+            paused = true;
+        }
+        else
+        {
+            pauseOffset += Util.getNanos() - pauseTime;
+            paused = false;
+        }
+    }
+}