path: root/day17.lisp

(defun read-map (filespec)
  (with-open-file (stream filespec)
    (let ((lines (loop for line = (read-line stream nil)
                       while line
                       collect (make-array
                                (length line)
                                :initial-contents (loop for c across line
                                                        collect (- (char-code c)
                                                                   (char-code #\0)))))))
      (make-array (list (length lines) (length (first lines))) :initial-contents lines))))

(defun min-node (q)
  (let ((mk ())
        (mv -1))
    (maphash (lambda (k v) (when (or (= mv -1) (< v mv)) (setf mk k mv v))) q)
    mk))

(defun best-path (map min-moves max-moves)
  (let ((q (make-hash-table :test #'equalp))
        (v (make-hash-table :test #'equalp))
        (n (array-dimension map 0))
        (m (array-dimension map 1)))
    (labels ((is-final (node)
               (multiple-value-bind (i j) (values-list node)
                 (and (= i (1- n)) (= j (1- m)))))
             (is-possible (node)
               (multiple-value-bind (i j c) (values-list node)
                 (and (>= i 0) (>= j 0) (< i n) (< j m) (<= c max-moves))))
             (add-node (node val)
               (unless (or (not (is-possible node)) (gethash node v))
                 (multiple-value-bind (oval found-p) (gethash node q)
                   (multiple-value-bind (i j) (values-list node)
                     (let ((val (+ val (aref map i j))))
                       (setf (gethash node q) (if found-p (min val oval) val))))))))
      (setf (gethash '(0 1 1 #\R) q) (aref map 0 1))
      (setf (gethash '(1 0 1 #\D) q) (aref map 1 0))
      (loop for node = (min-node q)
            while node
            if (is-final node)
              return (gethash node q)
            do (multiple-value-bind (i j c d) (values-list node)
                 (let ((val (gethash node q)))
                   (remhash node q)
                   (setf (gethash node v) val)
                   (unless (or (equal d #\D) (and (not (equal d #\U)) (< c min-moves)))
                     (add-node (list (1- i) j (if (equal d #\U) (1+ c) 1) #\U) val))
                   (unless (or (equal d #\U) (and (not (equal d #\D)) (< c min-moves)))
                     (add-node (list (1+ i) j (if (equal d #\D) (1+ c) 1) #\D) val))
                   (unless (or (equal d #\R) (and (not (equal d #\L)) (< c min-moves)))
                     (add-node (list i (1- j) (if (equal d #\L) (1+ c) 1) #\L) val))
                   (unless (or (equal d #\L) (and (not (equal d #\R)) (< c min-moves)))
                     (add-node (list i (1+ j) (if (equal d #\R) (1+ c) 1) #\R) val))))))))

(defun solve1 (filespec)
  (best-path (read-map filespec) 0 3))

(defun solve2 (filespec)
  (best-path (read-map filespec) 4 10))

(print (solve1 "data/17/example.txt")) ;; 102
(print (solve1 "data/17/input.txt"))   ;; 1039
(print (solve2 "data/17/example.txt")) ;; 102
(print (solve2 "data/17/input.txt"))   ;; 1201