#+title: Solution for p11

#+begin_src emacs-lisp
  (setq data '(125 17))
#+end_src

#+RESULTS:
| 125 | 17 |

Define the auxiliary functions
#+begin_src emacs-lisp
  (defun number-digits (x)
    (1+ (floor (log x 10))))

  (defun split (x)
    (let* ((n (number-digits x))
           (div (expt 10 (/ n 2))))
      (--map (funcall it x div) '(/ %))))

  (defun blink (x)
    (cond
     ((eq x 0) '(1))
     ((eq (% (number-digits x) 2) 0) (split x))
     (t (list (* 2024 x)))))
 #+end_src

 #+RESULTS:
 : blink

And now compute. For the first step a simple implementation suffices
 #+begin_src emacs-lisp
   (length (-last-item  (--iterate (-mapcat #'blink it) data (1+ 25))))
 #+end_src

 #+RESULTS:
 : 194482
 
Otherwise, we need to be smarter. We group Peebles in piles with the same engraving and blink only once for each group 
 #+begin_src emacs-lisp
   (setq freq-data (-frequencies data))

   (defun regroup (l)
     (-map (lambda (el) (cons el  (-reduce #'+ (-map #'cdr (--filter (eq el (car it)) l)))))  (-distinct (-map #'car l))))

   (--reduce-from (+ acc (cdr it)) 0 (-last-item (--iterate (regroup (-mapcat (lambda (a)
                                                          (--map (cons it (cdr a))
                                                                 (blink (car a)))) it))
                                      freq-data (1+ 75))))
 #+end_src

 #+RESULTS:
 : 232454623677743