Initial work on JsonPathBuilder.

src/Hyperbee.Json/JsonElementPositionComparer.cs

@@ -11,6 +11,18 @@ internal class JsonElementPositionComparer : IEqualityComparer<JsonElement>
 
     static JsonElementPositionComparer()
     {
+        // We want a fast comparer that will tell us if two JsonElements point to the same exact
+        // backing data in the parent JsonDocument. JsonElement is a struct, and a value comparison
+        // for equality won't give us reliable results and would be expensive.
+        //
+        // The internal JsonElement constructor takes parent and idx arguments that are saves as fields.
+        // 
+        // idx: is an index used to get the position of the JsonElement in the backing data.
+        // parent: is the owning JsonDocument (could be null in an enumeration).
+        //
+        // These arguments are stored in private fields and are not exposed. While note ideal, we
+        // will access these fields through dynamic methods to use for our comparison.
+
         // Create DynamicMethod for _idx field
 
         var idxField = typeof( JsonElement ).GetField( "_idx", BindingFlags.NonPublic | BindingFlags.Instance );
@@ -26,7 +38,7 @@ static JsonElementPositionComparer()
 
         __getIdx = (Func<JsonElement, int>) getIdxDynamicMethod.CreateDelegate( typeof( Func<JsonElement, int> ) );
 
-        // Create DynamicMethod for _parent field
+        // Create DynamicMethod for _parent field 
 
         var parentField = typeof( JsonElement ).GetField( "_parent", BindingFlags.NonPublic | BindingFlags.Instance );
 
@@ -44,15 +56,24 @@ static JsonElementPositionComparer()
 
     public bool Equals( JsonElement x, JsonElement y )
     {
+        // check for quick out
+
         if ( x.ValueKind != y.ValueKind )
             return false;
 
+        // check parent documents
+
+        // BF: JsonElement ctor notes that parent may be null in some enumeration conditions.
+        // This check may not be reliable. If so, should be ok to remove the parent check.
+
         var xParent = __getParent( x );
         var yParent = __getParent( y );
 
-        if ( !ReferenceEquals( xParent, yParent ) )
+        if ( !ReferenceEquals( xParent, yParent ) ) 
             return false;
 
+        // check idx values
+
         return __getIdx( x ) == __getIdx( y );
     }
 
@@ -64,47 +85,3 @@ public int GetHashCode( JsonElement obj )
         return HashCode.Combine( parent, idx );
     }
 }
-
-public class JsonPathFinder
-{
-    public static string FindJsonPath( JsonElement rootElement, JsonElement targetElement )
-    {
-        var comparer = new JsonElementPositionComparer();
-
-        var stack = new Stack<(JsonElement element, string path)>();
-        stack.Push( (rootElement, string.Empty) );
-
-        while ( stack.Count > 0 )
-        {
-            var (currentElement, currentPath) = stack.Pop();
-
-            if ( comparer.Equals( currentElement, targetElement ) )
-                return currentPath;
-
-            switch ( currentElement.ValueKind )
-            {
-                case JsonValueKind.Object:
-                    foreach ( JsonProperty property in currentElement.EnumerateObject() )
-                    {
-                        var newPath = string.IsNullOrEmpty( currentPath ) ? property.Name : $"{currentPath}.{property.Name}";
-                        stack.Push( (property.Value, newPath) );
-                    }
-
-                    break;
-
-                case JsonValueKind.Array:
-                    var index = 0;
-                    foreach ( JsonElement element in currentElement.EnumerateArray() )
-                    {
-                        var newPath = $"{currentPath}[{index}]";
-                        stack.Push( (element, newPath) );
-                        index++;
-                    }
-
-                    break;
-            }
-        }
-
-        return null; // Target element not found in the JSON document
-    }
-}

src/Hyperbee.Json/JsonPathBuilder.cs

@@ -0,0 +1,59 @@
+using System.Text.Json;
+
+namespace Hyperbee.Json;
+
+public class JsonPathBuilder
+{
+    private readonly JsonElement _rootElement;
+
+    public JsonPathBuilder( JsonDocument rootDocument )
+    {
+        _rootElement = rootDocument.RootElement;
+    }
+
+    public JsonPathBuilder( JsonElement rootElement )
+    {
+        _rootElement = rootElement;
+    }
+
+    public string GetPath( JsonElement targetElement )
+    {
+        var comparer = new JsonElementPositionComparer();
+
+        var stack = new Stack<(JsonElement element, string path)>( 4 );
+        stack.Push( (_rootElement, string.Empty) );
+
+        while ( stack.Count > 0 )
+        {
+            var (currentElement, currentPath) = stack.Pop();
+
+            if ( comparer.Equals( currentElement, targetElement ) )
+                return currentPath;
+
+            switch ( currentElement.ValueKind )
+            {
+                case JsonValueKind.Object:
+                    foreach ( JsonProperty property in currentElement.EnumerateObject() )
+                    {
+                        var newPath = string.IsNullOrEmpty( currentPath ) ? property.Name : $"{currentPath}.{property.Name}";
+                        stack.Push( (property.Value, newPath) );
+                    }
+
+                    break;
+
+                case JsonValueKind.Array:
+                    var index = 0;
+                    foreach ( JsonElement element in currentElement.EnumerateArray() )
+                    {
+                        var newPath = $"{currentPath}[{index}]";
+                        stack.Push( (element, newPath) );
+                        index++;
+                    }
+
+                    break;
+            }
+        }
+
+        return null; // Target element not found in the JSON document
+    }
+}