;; The expedition comes across a peculiar patch of tall
;; trees all planted carefully in a grid. The Elves explain
;; that a previous expedition planted these trees as a
;; reforestation effort. Now, they're curious if this would
;; be a good location for a tree house.
;; First, determine whether there is enough tree cover here
;; to keep a tree house hidden. To do this, you need to
;; count the number of trees that are visible from outside
;; the grid when looking directly along a row or column.
;; The Elves have already launched a quadcopter to generate
;; a map with the height of each tree (your puzzle input).
;; For example:
;; 30373
;; 25512
;; 65332
;; 33549
;; 35390
;; Each tree is represented as a single digit whose value is
;; its height, where 0 is the shortest and 9 is the tallest.
;; A tree is visible if all of the other trees between it
;; and an edge of the grid are shorter than it. Only
;; consider trees in the same row or column; that is, only
;; look up, down, left, or right from any given tree.
;; All of the trees around the edge of the grid are visible
;; - since they are already on the edge, there are no trees
;; to block the view. In this example, that only leaves the
;; interior nine trees to consider:
;; The top-left 5 is visible from the left and top. (It
;; isn't visible from the right or bottom since other
;; trees of height 5 are in the way.)
;; The top-middle 5 is visible from the top and right.
;; The top-right 1 is not visible from any direction;
;; for it to be visible, there would need to only be
;; trees of height 0 between it and an edge.
;; The left-middle 5 is visible, but only from the right.
;; The center 3 is not visible from any direction; for
;; it to be visible, there would need to be only trees
;; of at most height 2 between it and an edge.
;; The right-middle 3 is visible from the right.
;; In the bottom row, the middle 5 is visible, but the 3
;; and 4 are not.
;; With 16 trees visible on the edge and another 5 visible
;; in the interior, a total of 21 trees are visible in this
;; arrangement.
;; Consider your map; how many trees are visible from
;; outside the grid?
(defun day8/parse-data (lst)
(loop for line in lst
collect (mapcar (lambda (ch)
(- (char-int ch)
(char-int #\0)))
(coerce line 'list))))
(defun day8/process-row (row mkcoordf)
(loop for subl on row
for idx from 0 below (length row)
when (day8/visible-from-end subl)
collect (funcall mkcoordf idx)))
(defun day8/process-grid (data mkcoordf1 mkcoordf2)
(loop for row in data
for idx from 0 below (length data)
collect (append (day8/process-row
row
(funcall mkcoordf1 idx))
(day8/process-row
(reverse row)
(funcall mkcoordf2 idx)))
into results
finally (return (apply #'append results))))
;; These are function constructors for addressing the
;; coordinate space from within code that does not know the
;; orientation of the table.
(defun tblr (width height)
"y = top to bottom, x = left to right"
(lambda (y)
(lambda (x)
(list x y))))
(defun tbrl (width height)
"y = top to bottom, x = right to left"
(lambda (y)
(lambda (x)
(list (- width x 1) y))))
(defun lrbt (width height)
"y = left to right, x = bottom to top"
(lambda (y)
(lambda (x)
(list y (- height x 1)))))
(defun lrtb (width height)
"y = left to right, x = top to bottom"
(lambda (y)
(lambda (x)
(list y x))))
;; Content with the amount of tree cover available, the
;; Elves just need to know the best spot to build their tree
;; house: they would like to be able to see a lot of trees.
;; To measure the viewing distance from a given tree, look
;; up, down, left, and right from that tree; stop if you
;; reach an edge or at the first tree that is the same
;; height or taller than the tree under consideration. (If a
;; tree is right on the edge, at least one of its viewing
;; distances will be zero.)
;; The Elves don't care about distant trees taller than
;; those found by the rules above; the proposed tree house
;; has large eaves to keep it dry, so they wouldn't be able
;; to see higher than the tree house anyway.
;; In the example above, consider the middle 5 in the second
;; row:
;; 30373
;; 25512
;; 65332
;; 33549
;; 35390
;; Looking up, its view is not blocked; it can see 1
;; tree (of height 3).
;; Looking left, its view is blocked immediately; it can
;; see only 1 tree (of height 5, right next to it).
;; Looking right, its view is not blocked; it can see 2
;; trees.
;; Looking down, its view is blocked eventually; it can
;; see 2 trees (one of height 3, then the tree of height
;; 5 that blocks its view).
;; A tree's scenic score is found by multiplying together
;; its viewing distance in each of the four directions. For
;; this tree, this is 4 (found by multiplying 1 * 1 * 2 *
;; 2).
;; However, you can do even better: consider the tree of
;; height 5 in the middle of the fourth row:
;; 30373
;; 25512
;; 65332
;; 33549
;; 35390
;; Looking up, its view is blocked at 2 trees (by
;; another tree with a height of 5).
;; Looking left, its view is not blocked; it can see 2
;; trees.
;; Looking down, its view is also not blocked; it can
;; see 1 tree.
;; Looking right, its view is blocked at 2 trees (by a
;; massive tree of height 9).
;; This tree's scenic score is 8 (2 * 2 * 1 * 2); this is
;; the ideal spot for the tree house.
;; Consider each tree on your map. What is the highest
;; scenic score possible for any tree?
