+
+void
+proc_push_transformations (struct dataset *ds)
+{
+ if (ds->n_stack >= ds->allocated_stack)
+ ds->stack = x2nrealloc (ds->stack, &ds->allocated_stack,
+ sizeof *ds->stack);
+ trns_chain_init (&ds->stack[ds->n_stack++]);
+}
+
+void
+proc_pop_transformations (struct dataset *ds, struct trns_chain *chain)
+{
+ assert (ds->n_stack > 0);
+ *chain = ds->stack[--ds->n_stack];
+}
+
+static enum trns_result
+store_case_num (void *var_, struct ccase **cc, casenumber case_num)
+{
+ struct variable *var = var_;
+
+ *cc = case_unshare (*cc);
+ *case_num_rw (*cc, var) = case_num;
+
+ return TRNS_CONTINUE;
+}
+
+/* Add a variable which we can sort by to get back the original order. */
+struct variable *
+add_permanent_ordering_transformation (struct dataset *ds)
+{
+ struct variable *temp_var = dict_create_var_assert (ds->dict, "$ORDER", 0);
+ struct variable *order_var
+ = (proc_in_temporary_transformations (ds)
+ ? dict_clone_var_in_place_assert (ds->permanent_dict, temp_var)
+ : temp_var);
+
+ static const struct trns_class trns_class = {
+ .name = "ordering",
+ .execute = store_case_num
+ };
+ const struct transformation t = { .class = &trns_class, .aux = order_var };
+ trns_chain_append (&ds->permanent_trns_chain, &t);
+
+ return temp_var;
+}