Ensure successor edges follow topological sort

Bonsai.Core/Expressions/ExpressionBuilderGraphDescriptor.cs

@@ -1,4 +1,5 @@
-using System.Collections.ObjectModel;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Xml.Serialization;
 
 namespace Bonsai.Expressions
@@ -8,15 +9,32 @@ namespace Bonsai.Expressions
     /// </summary>
     public class ExpressionBuilderGraphDescriptor
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ExpressionBuilderGraphDescriptor"/> class.
+        /// </summary>
+        public ExpressionBuilderGraphDescriptor()
+        {
+            Nodes = new Collection<ExpressionBuilder>();
+            Edges = new Collection<ExpressionBuilderArgumentDescriptor>();
+        }
+
+        internal ExpressionBuilderGraphDescriptor(
+            IList<ExpressionBuilder> nodes,
+            IList<ExpressionBuilderArgumentDescriptor> edges)
+        {
+            Nodes = new Collection<ExpressionBuilder>(nodes);
+            Edges = new Collection<ExpressionBuilderArgumentDescriptor>(edges);
+        }
+
         /// <summary>
         /// Gets the collection of labels associated with each node in the expression builder graph.
         /// </summary>
-        public Collection<ExpressionBuilder> Nodes { get; } = new Collection<ExpressionBuilder>();
+        public Collection<ExpressionBuilder> Nodes { get; }
 
         /// <summary>
         /// Gets a collection of descriptors corresponding to each edge in the expression builder graph.
         /// </summary>
         [XmlArrayItem("Edge")]
-        public Collection<ExpressionBuilderArgumentDescriptor> Edges { get; } = new Collection<ExpressionBuilderArgumentDescriptor>();
+        public Collection<ExpressionBuilderArgumentDescriptor> Edges { get; }
     }
 }

Bonsai.Core/Expressions/ExpressionBuilderGraphExtensions.cs

@@ -1249,26 +1249,32 @@ public static ExpressionBuilderGraphDescriptor ToDescriptor(this ExpressionBuild
                 throw new ArgumentException("Cannot serialize a workflow with cyclical dependencies.", nameof(source));
             }
 
-            var nodes = buildOrder.SelectMany(component => component).ToArray();
-            var descriptor = new ExpressionBuilderGraphDescriptor();
+            int index = 0;
+            var nodeMap = buildOrder
+                .SelectMany(component => component)
+                .ToDictionary(node => node, node => index++);
 
-            foreach (var node in nodes)
-            {
-                descriptor.Nodes.Add(node.Value);
-            }
+            var nodes = new List<ExpressionBuilder>(nodeMap.Count);
+            nodes.AddRange(nodeMap.Keys.Select(node => node.Value));
 
-            var from = 0;
-            foreach (var node in nodes)
+            var edges = new List<ExpressionBuilderArgumentDescriptor>();
+            foreach (var entry in nodeMap)
             {
-                foreach (var successor in node.Successors)
+                var from = entry.Value;
+                foreach (var successor in entry.Key.Successors)
                 {
-                    var to = Array.IndexOf(nodes, successor.Target);
-                    descriptor.Edges.Add(new ExpressionBuilderArgumentDescriptor(from, to, successor.Label.Name));
+                    var to = nodeMap[successor.Target];
+                    edges.Add(new ExpressionBuilderArgumentDescriptor(from, to, successor.Label.Name));
                 }
-
-                from++;
             }
-            return descriptor;
+
+            edges.Sort((x, y) =>
+            {
+                var from = x.From.CompareTo(y.From);
+                if (from != 0) return from;
+                else return x.To.CompareTo(y.To);
+            });
+            return new ExpressionBuilderGraphDescriptor(nodes, edges);
         }
 
         /// <summary>