1 files changed, 56 insertions, 0 deletions
diff --git a/racket/aoc2022/day-08/day-08.rkt b/racket/aoc2022/day-08/day-08.rkt
new file mode 100644
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--- /dev/null
+++ b/racket/aoc2022/day-08/day-08.rkt
@@ -0,0 +1,56 @@
+#lang racket
+
+(require advent-of-code)
+
+(struct posn (r c) #:transparent)
+
+(define (make-tree-grid data)
+  (for*/hash ([(row r) (in-indexed data)] [(col c) (in-indexed (in-string row))])
+    (values (posn r c) (string->number (string col)))))
+
+(define tree-grid (make-tree-grid (in-lines (open-aoc-input (find-session) 2022 8))))
+
+;; part 1
+
+(define (can-see-to-outside? p height dr dc h)
+  (match-define (posn r c) p)
+  (not (for/first ([n (in-naturals 1)]
+                   #:do [(define p* (posn (+ r (* n dr)) (+ c (* n dc))))]
+                   #:break (not (hash-has-key? h p*))
+                   #:when (<= height (hash-ref h p*)))
+         #true)))
+
+(define (visible-in-any-direction? p height h)
+  (for*/or ([dr (in-list '(-1 0 1))] [dc (in-list '(-1 0 1))] #:when (= 1 (abs (+ dr dc))))
+    (can-see-to-outside? p height dr dc h)))
+
+(define (count-visible-trees trees)
+  (for/sum ([(tree-posn height) (in-hash trees)])
+           (cond
+             [(visible-in-any-direction? tree-posn height trees) 1]
+             [else 0])))
+
+(count-visible-trees tree-grid)
+
+;; part 2
+
+(define (scenic-score-in-direction p height dr dc h)
+  (match-define (posn r c) p)
+  (define score
+    (for/last ([n (in-naturals 1)]
+               #:do [(define p* (posn (+ r (* n dr)) (+ c (* n dc))))]
+               #:break (not (hash-has-key? h p*))
+               #:final (<= height (hash-ref h p*)))
+      n))
+  (if (not score) 0 score))
+
+(define (scenic-score p height h)
+  (for*/product ([dr (in-list '(-1 0 1))] [dc (in-list '(-1 0 1))] #:when (= 1 (abs (+ dr dc))))
+                (scenic-score-in-direction p height dr dc h)))
+
+(define (find-best-scenic-score trees)
+  (apply max
+         (for/list ([(tree-posn height) (in-hash trees)])
+           (scenic-score tree-posn height trees))))
+
+(find-best-scenic-score tree-grid)