[#1642] tree: Introduce Cursor type

* Use `Cursor` as parameter for `TreeSortedByFilename`

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

pkg/local_object_storage/pilorama/interface.go

@@ -37,7 +37,7 @@ type Forest interface {
 	TreeGetChildren(ctx context.Context, cid cidSDK.ID, treeID string, nodeID Node) ([]NodeInfo, error)
 	// TreeSortedByFilename returns children of the node with the specified ID. The nodes are sorted by the filename attribute..
 	// Should return ErrTreeNotFound if the tree is not found, and empty result if the node is not in the tree.
-	TreeSortedByFilename(ctx context.Context, cid cidSDK.ID, treeID string, nodeID MultiNode, last *string, count int) ([]MultiNodeInfo, *string, error)
+	TreeSortedByFilename(ctx context.Context, cid cidSDK.ID, treeID string, nodeID MultiNode, last *Cursor, count int) ([]MultiNodeInfo, *Cursor, error)
 	// TreeGetOpLog returns first log operation stored at or above the height.
 	// In case no such operation is found, empty Move and nil error should be returned.
 	TreeGetOpLog(ctx context.Context, cid cidSDK.ID, treeID string, height uint64) (Move, error)
@@ -79,6 +79,27 @@ const (
 	AttributeVersion  = "Version"
 )
 
+// Cursor keeps state between function calls for traversing nodes.
+// It stores the attributes associated with a previous call, allowing subsequent operations
+// to resume traversal from this point rather than starting from the beginning.
+type Cursor struct {
+	// Last traversed filename.
+	filename string
+}
+
+func NewCursor(filename string) *Cursor {
+	return &Cursor{
+		filename: filename,
+	}
+}
+
+func (c *Cursor) GetFilename() string {
+	if c == nil {
+		return ""
+	}
+	return c.filename
+}
+
 // CIDDescriptor contains container ID and information about the node position
 // in the list of container nodes.
 type CIDDescriptor struct {