Parser

(defstruct node
  name weight children parent)

(alexa:define-string-lexer aoc-lexer
  ()
  ("[a-z]+" (return (values 'id (intern $@))))
  ("\\(([0-9]+)\\)" (return (values 'weight (and $1 (parse-integer $1)))))
  ("->" (return '->))
  ("," (return '\,))
  (" |\\n" (values)))

(yacc:define-parser *aoc-parser*
  (:start-symbol nodes)
  (:terminals (id weight -> \,))
  (nodes
   (node (lambda (n &aux (m (make-hash-table)))
           (setf (gethash (node-name n) m) n)
           m))
   (node nodes (lambda (n ns)
                 (setf (gethash (node-name n) ns) n)
                 ns)))
  (node
   (id weight children-opt
       (s:op (make-node :name _ :weight _ :children _))))

  (children-opt
   ()
   (-> children (s:op _2)))
  (children
   (id #'list)
   (id \, children (s:op (cons _1 _3)))))