Move to a builder pattern for creating a BKTree.

src/Workspaces/Core/Portable/Utilities/BKTree.Builder.cs

+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Roslyn.Utilities
+{
+    internal partial class BKTree
+    {
+        private class Builder
+        {
+            private readonly char[][] values;
+
+            public Builder(IEnumerable<string> values)
+            {
+                this.values = values.Select(v => v.ToLower()).Distinct().Select(v => v.ToCharArray()).ToArray();
+            }
+
+            internal BKTree Create()
+            {
+                var nodes = new List<Node>();
+                foreach (var value in values)
+                {
+                    if (value.Length > 0)
+                    {
+                        Add(nodes, value);
+                    }
+                }
+
+                return new BKTree(nodes.ToArray());
+            }
+
+            private static void Add(List<Node> nodes, char[] lowerCaseCharacters)
+            {
+                if (nodes.Count == 0)
+                {
+                    nodes.Add(new Node(lowerCaseCharacters));
+                    return;
+                }
+
+                var currentNodeIndex = 0;
+                while (true)
+                {
+                    var currentNode = nodes[currentNodeIndex];
+
+                    var editDistance = EditDistance.GetEditDistance(currentNode.LowerCaseCharacters, lowerCaseCharacters);
+                    // This shoudl never happen.  We dedupe all items before proceeding to the 'Add' step.
+                    Debug.Assert(editDistance != 0);
+
+                    if (currentNode.AllChildren == null)
+                    {
+                        currentNode.AllChildren = new Dictionary<int, int>();
+                        nodes[currentNodeIndex] = currentNode;
+                        // Fall through. to actually add the child to this node.
+                    }
+                    else
+                    {
+                        int childNodeIndex;
+                        if (currentNode.AllChildren.TryGetValue(editDistance, out childNodeIndex))
+                        {
+                            // Edit distances collide.  Move to this child and add this word to it.
+                            currentNodeIndex = childNodeIndex;
+                            continue;
+                        }
+
+                        // Fall through. to actually add the child to this node.
+                    }
+
+                    currentNode.AllChildren.Add(editDistance, nodes.Count);
+                    nodes.Add(new Node(lowerCaseCharacters));
+                    return;
+                }
+            }
+        }
+    }
+}

src/Workspaces/Core/Portable/Utilities/BKTree.cs

@@ -10,7 +10,7 @@
 
 namespace Roslyn.Utilities
 {
-    internal class BKTree
+    internal partial class BKTree
     {
         // Root node is at index 0.
         private readonly Node[] nodes;
@@ -48,67 +48,7 @@ public static BKTree Create(params string[] values)
 
         public static BKTree Create(IEnumerable<string> values)
         {
-            return Create(values.Select(v => v.ToLower()).Distinct().Select(v => v.ToCharArray()).ToArray());
-        }
-
-        private static BKTree Create(char[][] values)
-        {
-            var nodes = new List<Node>();
-            foreach (var value in values)
-            {
-                if (value.Length > 0)
-                {
-                    Add(nodes, value);
-                }
-            }
-
-            return new BKTree(nodes.ToArray());
-        }
-
-        private static void Add(List<Node> nodes, char[] lowerCaseCharacters)
-        {
-            if (nodes.Count == 0)
-            {
-                nodes.Add(new Node(lowerCaseCharacters));
-                return;
-            }
-
-            var currentNodeIndex = 0;
-            while (true)
-            {
-                var currentNode = nodes[currentNodeIndex];
-
-                var editDistance = EditDistance.GetEditDistance(currentNode.LowerCaseCharacters, lowerCaseCharacters);
-                if (editDistance == 0)
-                {
-                    // Already in the graph.  Can happen because we added something that is the same
-                    // as an existing item, but only differs in case.
-                    return;
-                }
-
-                if (currentNode.AllChildren == null)
-                {
-                    currentNode.AllChildren = new Dictionary<int, int>();
-                    nodes[currentNodeIndex] = currentNode;
-                    // Fall through. to actually add the child to this node.
-                }
-                else
-                {
-                    int childNodeIndex;
-                    if (currentNode.AllChildren.TryGetValue(editDistance, out childNodeIndex))
-                    {
-                        // Edit distances collide.  Move to this child and add this word to it.
-                        currentNodeIndex = childNodeIndex;
-                        continue;
-                    }
-
-                    // Fall through. to actually add the child to this node.
-                }
-
-                currentNode.AllChildren.Add(editDistance, nodes.Count);
-                nodes.Add(new Node(lowerCaseCharacters));
-                return;
-            }
+            return new Builder(values).Create();
         }
 
         public IList<string> Find(string value, int? threshold = null)

src/Workspaces/Core/Portable/Workspaces.csproj

@@ -949,6 +949,7 @@
       <SubType>
       </SubType>
     </None>
+    <Compile Include="Utilities\BKTree.Builder.cs" />
     <PublicAPI Include="PublicAPI.Shipped.txt" />
     <PublicAPI Include="PublicAPI.Unshipped.txt" />
   </ItemGroup>