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(ql:quickload '(fiveam uiop))
(defun line-of-sight (direction p width height)
(case direction
(right (loop for l from (1+ p) below (* width (1+ (floor p width))) collect l))
(left (loop for l downfrom (1- p) to (* width (floor p width)) collect l))
(bottom (loop for l from (+ p width) below (* width height) by width collect l))
(top (loop for l downfrom (- p width) to 0 by width collect l))))
(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 solver-p1 (filename)
"Count how many trees are visible from the outside.
A tree is visible if all other trees between it and the edge of the grid are shorter."
(multiple-value-bind (width height forest) (forest (uiop:read-file-lines filename))
(loop for base across forest
and p from 0
count (some (lambda (direction)
(loop for l in (line-of-sight direction p width height)
for tree-height = (aref forest l)
always (< tree-height base)))
'(right left top bottom)))))
(defun solver-p2 (filename)
"A scenic score is the product of the viewing distance in each direction.
The viewing distance is the lenght until a tree of equal height appears."
(multiple-value-bind (width height forest) (forest (uiop:read-file-lines filename))
(loop for base across forest
and p from 0
maximize (apply #'*
(mapcar
(lambda (direction)
(loop for l in (line-of-sight direction p width height)
for tree-height = (aref forest l)
count l until (>= tree-height base)))
'(right left top bottom))))))
(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"))))
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