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;; 11:45
;;
(ql:quickload '(fiveam arrows cl-heap))
(defun in-bounds (field position)
(destructuring-bind (maxrow maxcol) (array-dimensions field)
(destructuring-bind (row col) position
(and (< -1 row maxrow)
(< -1 col maxcol)))))
(defun advance (row col dir)
(cons dir
(ecase dir
(up (list (1- row) col))
(lf (list row (1- col)))
(dw (list (1+ row) col))
(rt (list row (1+ col))))))
(defun parse-input (filepath)
(let* ((input (uiop:read-file-lines filepath))
(rows (length input)))
(make-array (list rows (length (car input)))
:initial-contents
(mapcar (lambda (line)
(map 'vector (lambda (c)
(logand 15 (char-code c)))
line))
input))))
(defun backtrack (dir)
(ecase dir
(up 'dw)
(rt 'lf)
(dw 'up)
(lf 'rt)))
(defun next-moves (field tried walker)
(destructuring-bind (track dir row col) walker
(arrows:->>
(remove dir '(rt dw lf up) :key #'backtrack)
(mapcar (lambda (d)
(unless (and (eq d dir) (<= 2 track))
(destructuring-bind (ndir . np) (advance row col d)
(when (in-bounds field np)
(let* ((nt (if (eq ndir dir) (1+ track) 0))
(npath (list* nt ndir np)))
(unless (gethash npath tried)
(setf (gethash npath tried) t)
npath)))))))
(delete nil))))
(defun finish-p (field step)
(destructuring-bind (maxrow maxcol) (array-dimensions field)
(destructuring-bind (row col) (cddr step)
(and (= row (1- maxrow))
(= col (1- maxcol))))))
(defun vdequeue (q)
(cl-heap:pop-heap (slot-value q 'cl-heap:heap)))
(defun walk (options known field)
(loop for (pcost current) = (vdequeue options)
while current
until (finish-p field current)
do (dolist (next (next-moves field known current))
(cl-heap:enqueue options next (+ (apply #'aref field (cddr next)) pcost)))
finally (return pcost)))
(defun solver (filename)
(let* ((field (parse-input filename))
(options (make-instance 'cl-heap:priority-queue))
(known (make-hash-table :test #'equal))
(w (list -1 'rt 0 0)))
(cl-heap:enqueue options w 0)
(walk options known field)))
(fiveam:test solutions
(fiveam:is (= 102 (solver "eg-in")))
(fiveam:is (= 963 (solver "input"))))
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