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(ql:quickload '(fiveam uiop arrows))
(defun coord-x-minor (width height)
(lambda (x y)
(when (and (< -1 x width)
(< -1 y height))
(+ x (* y width)))))
(defun forest (data)
(let ((width (length (car data)))
(height (length data)))
(values width height
(map 'vector
(lambda (x) (- (char-code x) 48))
(apply #'concatenate 'string data)))))
(defun line-of-sight (direction x y width height)
(let ((location (coord-x-minor width height)))
(case direction
(right (loop for l from (1+ x) below width collect (funcall location l y)))
(bottom (loop for l from (1+ y) below height collect (funcall location x l)))
(left (loop for l downfrom (1- x) to 0 collect (funcall location l y)))
(top (loop for l downfrom (1- y) to 0 collect (funcall location x l))))))
(defun solver-p1 (filename)
(multiple-value-bind (width height forest-arr) (forest (uiop:read-file-lines filename))
(let ((visibility-mask (make-array (* width height) :initial-element nil))
(location-x (coord-x-minor width height)))
(flet ((toggle-visibility (major minor location reverse)
(let ((range (if reverse
(loop for i downfrom (1- minor) to 0 collect i)
(loop for i below minor collect i))))
(dotimes (m major)
(loop
for n in range
for maxh = -1 then (max maxh tree-height)
for tree-height = (aref forest-arr (funcall location n m))
when (> tree-height maxh)
do (setf (aref visibility-mask (funcall location n m)) t)))))
(location-y (y x) (funcall location-x x y)))
(toggle-visibility height width location-x nil)
(toggle-visibility height width location-x t)
(toggle-visibility width height #'location-y nil)
(toggle-visibility width height #'location-y t))
(loop for v across visibility-mask counting v))))
(defun solver-p2 (filename)
(multiple-value-bind (width height forest-arr) (forest (uiop:read-file-lines filename))
(let ((score (make-array (* width height)))
(location-x (coord-x-minor width height)))
(flet ((score-direction (base dir)
(loop for l in dir
for tree-height = (aref forest-arr l)
count l until (>= tree-height base))))
(dotimes (y height)
(dotimes (x width)
(let ((base (aref forest-arr (funcall location-x x y))))
(arrows:->> '(right left top bottom)
(mapcar (lambda (direction)
(score-direction base (line-of-sight direction x y width height))))
(apply #'* )
(setf (aref score (funcall location-x x y)))))))
(loop for v across score maximize v)))))
(fiveam:test solutions
(fiveam:is (= 21 (solver-p1 "eg-in")))
(fiveam:is (= 1672 (solver-p1 "input")))
(fiveam:is (= 8 (solver-p2 "eg-in")))
(fiveam:is (= 327180 (solver-p2 "input"))))
(fiveam:run-all-tests)
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