path: root/day15.lisp

(defun split-by-char (char string)
  (loop for i = 0 then (1+ j)
        as j = (position char string :start i)
        collect (subseq string i j)
        while j))

(defun read-seq (filespec)
  (with-open-file (stream filespec)
    (split-by-char #\, (read-line stream))))

(defun hash-algorithm (str)
  (let ((res 0))
    (loop for c across str do (setf res (mod (* 17 (+ res (char-code c))) 256))) res))

(defun solve1 (filespec)
  (reduce #'+ (mapcar #'hash-algorithm (read-seq filespec))))

(defun solve2 (filespec)
  (let ((seq (read-seq filespec))
        (boxes (make-array 256 :initial-element ())))
    (loop for str in seq
          as symbp = (loop for c across str
                           for i from 0
                           if (or (equal c #\-) (equal c #\=)) return i)
          as symb = (aref str symbp)
          as label = (subseq str 0 symbp)
          as hash = (hash-algorithm label)
          if (equal symb #\=)
            do (let* ((value (parse-integer (subseq str (1+ symbp))))
                      (box (aref boxes hash))
                      (lmember (assoc label box :test 'equal)))
                 (if lmember (setf (cdr lmember) value)
                     (setf (aref boxes hash) (acons label value box))))
          else
            do (setf (aref boxes hash)
                     (remove-if (lambda (a) (equal label (car a)))
                                (aref boxes hash) :count 1)))
    (loop for box across boxes
          for i from 1
          sum (loop for lens in (reverse box)
                    for j from 1
                    sum (* i j (cdr lens))))))

(print (solve1 "data/15/example.txt")) ;; 1320
(print (solve1 "data/15/input.txt"))   ;; 516804
(print (solve2 "data/15/example.txt")) ;; 145
(print (solve2 "data/15/input.txt"))   ;; 231844