Fix pipe net conservation laws (#8540)

Content.Server/Atmos/EntitySystems/AtmosphereSystem.Gases.cs

@@ -108,6 +108,52 @@ namespace Content.Server.Atmos.EntitySystems
             NumericsHelpers.Add(receiver.Moles, giver.Moles);
         }
 
+        /// <summary>
+        ///     Divides a source gas mixture into several recipient mixtures, scaled by their relative volumes. Does not
+        ///     modify the source gas mixture. Used for pipe network splitting. Note that the total destination volume
+        ///     may be larger or smaller than the source mixture.
+        /// </summary>
+        public void DivideInto(GasMixture source, List<GasMixture> receivers)
+        {
+            var totalVolume = 0f;
+            foreach (var receiver in receivers)
+            {
+                if (!receiver.Immutable)
+                    totalVolume += receiver.Volume;
+            }
+
+            float? sourceHeatCapacity = null;
+            var buffer = new float[Atmospherics.AdjustedNumberOfGases];
+
+            foreach (var receiver in receivers)
+            {
+                if (receiver.Immutable)
+                    continue;
+
+                var fraction = receiver.Volume / totalVolume;
+
+                // Set temperature, if necessary.
+                if (MathF.Abs(receiver.Temperature - source.Temperature) > Atmospherics.MinimumTemperatureDeltaToConsider)
+                {
+                    // Often this divides a pipe net into new and completely empty pipe nets
+                    if (receiver.TotalMoles == 0) 
+                        receiver.Temperature = source.Temperature;
+                    else
+                    {
+                        sourceHeatCapacity ??= GetHeatCapacity(source);
+                        var receiverHeatCapacity = GetHeatCapacity(receiver);
+                        var combinedHeatCapacity = receiverHeatCapacity + sourceHeatCapacity.Value * fraction;
+                        if (combinedHeatCapacity > 0f)
+                            receiver.Temperature = (GetThermalEnergy(source, sourceHeatCapacity.Value * fraction) + GetThermalEnergy(receiver, receiverHeatCapacity)) / combinedHeatCapacity;
+                    }
+                }
+
+                // transfer moles
+                NumericsHelpers.Multiply(source.Moles, fraction, buffer);
+                NumericsHelpers.Add(receiver.Moles, buffer);
+            }
+        }
+
         /// <summary>
         ///     Shares gas between two gas mixtures. Part of LINDA.
         /// </summary>

Content.Server/NodeContainer/NodeGroups/PipeNet.cs

@@ -21,8 +21,6 @@ namespace Content.Server.NodeContainer.NodeGroups
     {
         [ViewVariables] public GasMixture Air { get; set; } = new() {Temperature = Atmospherics.T20C};
 
-        [ViewVariables] private readonly List<PipeNode> _pipes = new();
-
         [ViewVariables] private AtmosphereSystem? _atmosphereSystem;
 
         public GridId Grid => GridId;
@@ -47,7 +45,6 @@ namespace Content.Server.NodeContainer.NodeGroups
             foreach (var node in groupNodes)
             {
                 var pipeNode = (PipeNode) node;
-                _pipes.Add(pipeNode);
                 Air.Volume += pipeNode.Volume;
             }
         }
@@ -56,32 +53,27 @@ namespace Content.Server.NodeContainer.NodeGroups
         {
             base.RemoveNode(node);
 
-            var pipeNode = (PipeNode) node;
-            Air.Volume -= pipeNode.Volume;
-            // TODO: Bad O(n^2)
-            _pipes.Remove(pipeNode);
+            // if the node is simply being removed into a separate group, we do nothing, as gas redistribution will be
+            // handled by AfterRemake(). But if it is being deleted, we actually want to remove the gas stored in this node.
+            if (!node.Deleting || node is not PipeNode pipe)
+                return;
+
+            Air.Multiply(1f - pipe.Volume / Air.Volume);
+            Air.Volume -= pipe.Volume;
         }
 
         public override void AfterRemake(IEnumerable<IGrouping<INodeGroup?, Node>> newGroups)
         {
             RemoveFromGridAtmos();
 
-            var buffer = new GasMixture(Air.Volume) {Temperature = Air.Temperature};
-            var atmosphereSystem = EntitySystem.Get<AtmosphereSystem>();
-
+            var newAir = new List<GasMixture>(newGroups.Count());
             foreach (var newGroup in newGroups)
             {
-                if (newGroup.Key is not IPipeNet newPipeNet)
-                    continue;
-
-                var newAir = newPipeNet.Air;
-                var newVolume = newGroup.Cast<PipeNode>().Sum(n => n.Volume);
-
-                buffer.Clear();
-                atmosphereSystem.Merge(buffer, Air);
-                buffer.Multiply(MathF.Min(newVolume / Air.Volume, 1f));
-                atmosphereSystem.Merge(newAir, buffer);
+                if (newGroup.Key is IPipeNet newPipeNet)
+                    newAir.Add(newPipeNet.Air);
             }
+
+            _atmosphereSystem!.DivideInto(Air, newAir);
         }
 
         private void RemoveFromGridAtmos()

Content.Server/Rotatable/RotatableSystem.cs

@@ -30,7 +30,9 @@ namespace Content.Server.Rotatable
 
         private void AddRotateVerbs(EntityUid uid, RotatableComponent component, GetVerbsEvent<Verb> args)
         {
-            if (!args.CanAccess || !args.CanInteract)
+            if (!args.CanAccess
+                || !args.CanInteract
+                || Transform(uid).NoLocalRotation) // Good ol prototype inheritance, eh?
                 return;
 
             // Check if the object is anchored, and whether we are still allowed to rotate it.