struct node { 
  enum { document_node, element_node, text_node } nodetype;
  struct nodedata *data;
  int n_children;
  struct node *child[0];
};

typedef int bool;

struct visitor {
  void *visitor_data;
  bool (*document_callback)(struct visitor *self, struct nodedata*);
  void (*document_end_callback)(struct visitor *self, struct nodedata*);
  bool (*element_callback)(struct visitor *self, struct nodedata*);
  void (*element_end_callback)(struct visitor *self, struct nodedata*);
  void (*text_callback)(struct visitor *self, struct nodedata*);
};

void visit_node(struct node *n, struct visitor *v);

static inline void visit_children(struct node *n, struct visitor *v)
{
  int ii;

  for (ii = 0; ii < n->n_children; ii++) {
    _visit_node(n->child[ii], v);
  }
}

static void _visit_node(struct node *n, struct visitor *v)
{
  switch (n->nodetype) {
  case element_node:
    if (v->element_callback(v, n->data)) visit_children(n, v);
    v->element_end_callback(v, n->data);
    break;
  case document_node:
    if (v->document_callback(v, n->data)) visit_children(n, v);
    v->document_end_callback(v, n->data);
    break;
  case text_node:
    v->text_callback(v, n->data);
    break;
  }
}

void visit_node(struct node *n, struct visitor *v)
{
  _visit_node(n, v);
}