Add tests for bidirectional cursor

cli/src/tests/tree_test.rs

@@ -306,7 +306,7 @@ fn test_tree_cursor() {
         .parse(
             "
                 struct Stuff {
-                    a: A;
+                    a: A,
                     b: Option<B>,
                 }
             ",
@@ -331,6 +331,49 @@ fn test_tree_cursor() {
     assert!(cursor.goto_next_sibling());
     assert_eq!(cursor.node().kind(), "field_declaration_list");
     assert_eq!(cursor.node().is_named(), true);
+
+    assert!(cursor.goto_last_child());
+    assert_eq!(cursor.node().kind(), "}");
+    assert_eq!(cursor.node().is_named(), false);
+    assert_eq!(cursor.node().start_position(), Point { row: 4, column: 16 });
+
+    assert!(cursor.goto_previous_sibling());
+    assert_eq!(cursor.node().kind(), ",");
+    assert_eq!(cursor.node().is_named(), false);
+    assert_eq!(cursor.node().start_position(), Point { row: 3, column: 32 });
+
+    assert!(cursor.goto_previous_sibling());
+    assert_eq!(cursor.node().kind(), "field_declaration");
+    assert_eq!(cursor.node().is_named(), true);
+    assert_eq!(cursor.node().start_position(), Point { row: 3, column: 20 });
+
+    assert!(cursor.goto_previous_sibling());
+    assert_eq!(cursor.node().kind(), ",");
+    assert_eq!(cursor.node().is_named(), false);
+    assert_eq!(cursor.node().start_position(), Point { row: 2, column: 24 });
+
+    assert!(cursor.goto_previous_sibling());
+    assert_eq!(cursor.node().kind(), "field_declaration");
+    assert_eq!(cursor.node().is_named(), true);
+    assert_eq!(cursor.node().start_position(), Point { row: 2, column: 20 });
+
+    assert!(cursor.goto_previous_sibling());
+    assert_eq!(cursor.node().kind(), "{");
+    assert_eq!(cursor.node().is_named(), false);
+    assert_eq!(cursor.node().start_position(), Point { row: 1, column: 29 });
+
+    let mut copy = tree.walk();
+    copy.reset_to(cursor);
+
+    assert_eq!(copy.node().kind(), "{");
+    assert_eq!(copy.node().is_named(), false);
+
+    assert!(copy.goto_parent());
+    assert_eq!(copy.node().kind(), "field_declaration_list");
+    assert_eq!(copy.node().is_named(), true);
+
+    assert!(copy.goto_parent());
+    assert_eq!(copy.node().kind(), "struct_item");
 }
 
 #[test]

lib/src/tree_cursor.c

@@ -97,6 +97,16 @@ static inline bool ts_tree_cursor_child_iterator_next(
   return true;
 }
 
+static inline Length length_sub_zero(Length a, Length b) {
+  // length_sub doesn't account for 0 row subtraction, i.e. only columns
+  // should be subtracted, but changing point_sub breaks other tests
+  Length result = length_sub(a, b);
+  if (b.extent.row == 0) {
+    result.extent.column -= b.extent.column;
+  }
+  return result;
+}
+
 static inline bool ts_tree_cursor_child_iterator_previous(
   CursorChildIterator *self,
   TreeCursorEntry *result,
@@ -119,13 +129,14 @@ static inline bool ts_tree_cursor_child_iterator_previous(
     self->structural_child_index--;
   }
 
-  self->position = length_sub(self->position, ts_subtree_padding(*child));
+  self->position = length_sub_zero(self->position, ts_subtree_padding(*child));
   self->child_index--;
 
   // unsigned can underflow so compare it to child_count
   if (self->child_index < self->parent.ptr->child_count) {
     Subtree previous_child = ts_subtree_children(self->parent)[self->child_index];
-    self->position = length_sub(self->position, ts_subtree_size(previous_child));
+    Length size = ts_subtree_size(previous_child);
+    self->position = length_sub_zero(self->position, size);
   }
 
   return true;