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(ql:quickload '(fiveam uiop))
(defun coord (width height)
(lambda (x y)
(when (and (< -1 x width)
(< -1 y height))
(+ x (* y width)))))
(let* ((data (uiop:read-file-lines "input"))
(width (length (car data)))
(height (length data))
(forest-arr
(apply #'vector
(mapcan (lambda (row)
(map 'list (lambda (x) (- (char-code x) 48)) row)) data)))
(visibility-mask (make-array (* width height) :initial-element nil))
(location (coord width height)))
(dotimes (y height)
;; left watch
(loop
for x from 0 below width
for maxh = -1 then (max maxh tree-height)
for tree-height = (aref forest-arr (funcall location x y))
when (> tree-height maxh)
do (setf (aref visibility-mask (funcall location x y)) t))
;; right watch
(loop
for x downfrom (1- width) to 0
for maxh = -1 then (max maxh tree-height)
for tree-height = (aref forest-arr (funcall location x y))
when (> tree-height maxh)
do (setf (aref visibility-mask (funcall location x y)) t)))
(dotimes (x width)
;; top watch
(loop
for y from 0 below height
for maxh = -1 then (max maxh tree-height)
for tree-height = (aref forest-arr (funcall location x y))
when (> tree-height maxh)
do (setf (aref visibility-mask (funcall location x y)) t))
;; bottom watch
(loop
for y downfrom (1- height) to 0
for maxh = -1 then (max maxh tree-height)
for tree-height = (aref forest-arr (funcall location x y))
when (> tree-height maxh)
do (setf (aref visibility-mask (funcall location x y)) t)))
(loop for v across visibility-mask counting v))
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