Theory Group

/* tree readtree(istream& ins, unsigned dimensions, const sobj&, const qobj&)
 * Reads a tree in string form from ins and builds it recursively
 * Returns NULL on error
 */
  
template<class sobj, class qobj,class siobj,class qiobj>
tree<siobj,qiobj>* do_readtree(istream& ins, unsigned dimensions,
                               LabelTable<sobj,siobj> &sTable, bool& fail)
{ 
  char c;
  sobj label;
  const char EMPTY_TREE = '~';
  tree<siobj,qiobj>* root = NULL;
  
  /* Read the node label */
  if (!(ins >> label) || label == EMPTY_TREE)
    return root;
    
  /* Instansiate a local tree using the newly-read label */
  if (!(root = new tree<siobj,qiobj>(sTable.obj_to_index(label))))
    return NULL;
    
  /* Read a left brace */
  if (!(ins >> c))
    { delete root; fail = true; return NULL; }
  
  /* Special case: Don't require that (~,~,~) appear for leaf nodes. */
  if (c != '(')
    { ins.putback(c); return root; }
  
  /* For children of each each dimensionality, starting with the first */
  for (unsigned dim = 0; dim < dimensions; dim++)
  {
    /* Recur to read a tree */
    root->set_link(
      dim, do_readtree<sobj,qobj,siobj,qiobj>(ins, dimensions, sTable, fail)
    );
      
    /* Read a comma */
    if (!(ins >> c))
      { delete root; fail = true; return NULL; }
    
    /* Special case: Treat omitted subtrees as empty. */
    if (dim >= dimensions - 1 || c != ',')
      { ins.putback(c); break; }
  } 
    
  /* Require a matching right brace */
  if (!(ins >> c) || c != ')')
    { delete root; fail = true; return NULL; }
    
  return root;
} 

template<class sobj, class qobj,class siobj,class qiobj>
tree<siobj,qiobj>* readtree(istream& ins, unsigned dimensions,
                            LabelTable<sobj,siobj> &sTable)
{
  bool fail = false;
 
  tree<siobj, qiobj> *root =
    do_readtree<sobj, qobj, siobj, qiobj>(ins, dimensions, sTable, fail);
  
  if (root && fail)
    { delete root; root = NULL; }

  return root;
}