Modify BKTree.Lookup to pass the threshold to GetEditDistance when there will not be a need for the actual edit distance

src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Utilities/BKTree.cs

@@ -107,33 +107,48 @@ private void Lookup(
                 return;
             }
 
-            // We always want to compute the real edit distance (ignoring any thresholds).  This is
-            // because we need that edit distance to appropriately determine which edges to walk 
+            // We may need to compute the real edit distance (ignoring any thresholds) in the case
+            // where edges exist as we need that edit distance to appropriately determine which edges to walk 
             // in the tree.
             var characterSpan = currentNode.WordSpan;
+
+            // The GetEditDistance call below serves two purposes:
+            // 1) To determine whether currentNode should be added to the result
+            // 2) To determine whether children need to be searched
+            //
+            // If there are no edges, we don't need the information to determine case 2. So, as a
+            // performance optimization, in that case we send in a threshold to GetEditDistance
+            // that indicates only the work necessary to determine case 1 need be performed.
+            var edgesExist = currentNode.EdgeCount > 0;
             var editDistance = EditDistance.GetEditDistance(
                 _concatenatedLowerCaseWords.AsSpan(characterSpan.Start, characterSpan.Length),
-                queryCharacters.AsSpan(0, queryLength));
+                queryCharacters.AsSpan(0, queryLength),
+                edgesExist ? int.MaxValue : threshold);
 
+            // Case 1
             if (editDistance <= threshold)
             {
                 // Found a match.
                 result.Add(new string(_concatenatedLowerCaseWords, characterSpan.Start, characterSpan.Length));
             }
 
-            var min = editDistance - threshold;
-            var max = editDistance + threshold;
-
-            var startInclusive = currentNode.FirstEdgeIndex;
-            var endExclusive = startInclusive + currentNode.EdgeCount;
-            for (var i = startInclusive; i < endExclusive; i++)
+            // Case 2
+            if (edgesExist)
             {
-                var childEditDistance = _edges[i].EditDistance;
-                if (min <= childEditDistance && childEditDistance <= max)
+                var min = editDistance - threshold;
+                var max = editDistance + threshold;
+
+                var startInclusive = currentNode.FirstEdgeIndex;
+                var endExclusive = startInclusive + currentNode.EdgeCount;
+                for (var i = startInclusive; i < endExclusive; i++)
                 {
-                    Lookup(_nodes[_edges[i].ChildNodeIndex],
-                        queryCharacters, queryLength, threshold, ref result,
-                        recursionCount + 1);
+                    var childEditDistance = _edges[i].EditDistance;
+                    if (min <= childEditDistance && childEditDistance <= max)
+                    {
+                        Lookup(_nodes[_edges[i].ChildNodeIndex],
+                            queryCharacters, queryLength, threshold, ref result,
+                            recursionCount + 1);
+                    }
                 }
             }
         }