(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"))))