Fix goto_previous_sibling with padded first children

cli/src/tests/tree_test.rs

@@ -376,6 +376,36 @@ fn test_tree_cursor() {
     assert_eq!(copy.node().kind(), "struct_item");
 }
 
+#[test]
+fn test_tree_cursor_previous_sibling() {
+    let mut parser = Parser::new();
+    parser.set_language(get_language("rust")).unwrap();
+
+    let text = "
+    // Hi there
+    // This is fun!
+    // Another one!
+";
+    let tree = parser.parse(text, None).unwrap();
+
+    let mut cursor = tree.walk();
+    assert_eq!(cursor.node().kind(), "source_file");
+
+    assert!(cursor.goto_last_child());
+    assert_eq!(cursor.node().kind(), "line_comment");
+    assert_eq!(cursor.node().utf8_text(text.as_bytes()).unwrap(), "// Another one!");
+
+    assert!(cursor.goto_previous_sibling());
+    assert_eq!(cursor.node().kind(), "line_comment");
+    assert_eq!(cursor.node().utf8_text(text.as_bytes()).unwrap(), "// This is fun!");
+
+    assert!(cursor.goto_previous_sibling());
+    assert_eq!(cursor.node().kind(), "line_comment");
+    assert_eq!(cursor.node().utf8_text(text.as_bytes()).unwrap(), "// Hi there");
+
+    assert!(!cursor.goto_previous_sibling());
+}
+
 #[test]
 fn test_tree_cursor_fields() {
     let mut parser = Parser::new();

lib/src/tree_cursor.c

@@ -377,7 +377,11 @@ TreeCursorStep ts_tree_cursor_goto_previous_sibling_internal(TSTreeCursor *_self
   position = parent->position;
   uint32_t child_index = array_back(&self->stack)->child_index;
   const Subtree *children = ts_subtree_children((*(parent->subtree)));
-  for (uint32_t i = 0; i < child_index; ++i) {
+
+  // skip first child padding since its position should match the position of the parent
+  if (child_index > 0)
+    position = length_add(position, ts_subtree_size(children[0]));
+  for (uint32_t i = 1; i < child_index; ++i) {
     position = length_add(position, ts_subtree_total_size(children[i]));
   }
   if (child_index > 0)