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(ql:quickload '(fiveam uiop))
(defun possible-directions (width height)
;; point on grid is p=x+y*width
(lambda (pos)
(let ((x (mod pos width))
(y (floor pos width)))
(delete nil
(list
(when (< -1 x (1- width)) (1+ pos)) ;; right move
(when (< -1 y (1- height)) (+ pos width)) ;; down move
(when (< 0 x width) (1- pos)) ;; left move
(when (< 0 y height) (- pos width))) ;; up move
:test #'eq))))
(defun elevations (data)
(map 'vector (lambda (chr)
(case chr
(#\S 0)
(#\E 27)
(t (- (char-code chr) 96))))
data))
(defun next-steps (place candidates land paths)
(let ((elevation (aref land place)))
(unless (= 27 elevation)
(mapcan (lambda (option)
(when (and (not (gethash option paths))
(>= (1+ elevation) (aref land option)))
(setf (gethash option paths) (cons option (gethash place paths)))
(list option)))
(funcall candidates place)))))
(defun shortest-path (starts candidates land paths)
(let ((next (mapcan (lambda (place) (next-steps place candidates land paths)) starts)))
(when next
(shortest-path next candidates land paths))))
(defun solver (input-file start-p)
(let* ((data (uiop:read-file-lines input-file))
(candidates (possible-directions (length (car data)) (length data)))
(land (elevations (apply #'concatenate 'string data)))
(paths (make-hash-table :test #'eq))
(finish (position 27 land :test #'eq))
(starts (loop for i from 0
for el across land
when (funcall start-p el) collect (progn (setf (gethash i paths) (list i)) i))))
(shortest-path starts candidates land paths)
(1- (length (gethash finish paths)))))
(fiveam:test solver
(fiveam:is (= 31 (solver "eg-in" (lambda (x) (= x 0)))))
(fiveam:is (= 29 (solver "eg-in" (lambda (x) (<= x 1)))))
(fiveam:is (= 361 (solver "input" (lambda (x) (= x 0)))))
(fiveam:is (= 354 (solver "input" (lambda (x) (<= x 1))))))
(fiveam:run-all-tests)
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