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1 files changed, 97 insertions, 0 deletions

diff --git a/racket/aoc2023/day-10/day-10.rkt b/racket/aoc2023/day-10/day-10.rkt
new file mode 100644
index 0000000..64d8727
--- /dev/null
+++ b/racket/aoc2023/day-10/day-10.rkt
@@ -0,0 +1,97 @@
+#lang racket
+
+(require advent-of-code
+         threading)
+
+(struct posn (r c) #:transparent)
+
+(define/match (add-posns _p1 _p2)
+  [((posn x1 y1) (posn x2 y2)) (posn (+ x1 x2) (+ y1 y2))])
+
+(define go-north (posn -1 0))
+(define go-south (posn 1 0))
+(define go-east (posn 0 1))
+(define go-west (posn 0 -1))
+
+(define initial-directions
+  (list (cons go-north '(#\| #\7 #\F))
+        (cons go-south '(#\| #\J #\L))
+        (cons go-east '(#\- #\J #\7))
+        (cons go-west '(#\- #\F #\L))))
+
+(define/match (pipe-neighbors _pipe)
+  [(#\|) (list go-north go-south)]
+  [(#\-) (list go-east go-west)]
+  [(#\L) (list go-north go-east)]
+  [(#\F) (list go-south go-east)]
+  [(#\7) (list go-south go-west)]
+  [(#\J) (list go-north go-west)])
+
+(define (make-pipe-grid in)
+  (for*/hash ([(row r) (in-indexed (string-split in "\n"))] 
+              [(ch c) (in-indexed (string->list row))])
+    (values (posn (add1 r) (add1 c)) ch)))
+
+(define (get-valid-S-neighbors S grid)
+  (for/list ([dir (in-list initial-directions)]
+             #:do [(match-define (cons d valid) dir)]
+             #:do [(define neighbor (add-posns d S))]
+             #:when (member (hash-ref grid neighbor 'none) valid))
+    neighbor))
+
+(define (to-next-pipe current previous grid [acc '()])
+  (cond
+    [(equal? (hash-ref grid current #f) #\S) acc]
+    [else
+     (define next
+       (for/first ([d (in-list (pipe-neighbors (hash-ref grid current)))]
+                   #:do [(define neighbor (add-posns d current))]
+                   #:unless (equal? neighbor previous))
+         neighbor))
+     (~> next (to-next-pipe _ current grid (cons current acc)))]))
+
+;; part 1
+(define input (fetch-aoc-input (find-session) 2023 10 #:cache #true))
+
+(define pipe-grid (make-pipe-grid input))
+
+(define S-posn
+  (for/first ([(k v) (in-hash pipe-grid)] #:when (equal? v #\S))
+    k))
+
+(define S-neighbors (get-valid-S-neighbors S-posn pipe-grid))
+
+(define pipe-loop (to-next-pipe (first S-neighbors) S-posn pipe-grid '()))
+
+(/ (add1 (length pipe-loop)) 2)
+
+;; part 2
+(define pipe-loop-set (~> (list->set pipe-loop) (set-add S-posn)))
+
+(define (trace-rays pt pipes grid)
+  (cond
+    [(set-member? pipes pt) #f]
+    [else (odd? (trace-ray pt pipes grid))]))
+
+(define (trace-ray pt pipes grid)
+  (define row (posn-r pt))
+  (for/fold ([acc 0] 
+             [corner #f] 
+             #:result acc)
+            ([col (in-naturals (posn-c pt))]
+             #:do [(define test-pt (posn row col))]
+             #:break (not (hash-has-key? grid test-pt))
+             #:when (set-member? pipes test-pt))
+    (define pipe (hash-ref grid test-pt))
+    (match* (corner pipe)
+      [(#f #\|) (values (add1 acc) #f)] ; vertical crossing
+      [(#f (or #\F #\L)) (values acc pipe)]
+      [(#\F #\J) (values (add1 acc) #f)] ; a  ┏━┛ shape counts as a vertical crossing
+      [(#\L #\7) (values (add1 acc) #f)]
+      [(#\F #\7) (values acc #f)] ; a  ┏━┓ shape doesn't count
+      [(#\L #\J) (values acc #f)]
+      [(_ _) (values acc corner)])))
+
+(~> pipe-grid 
+    hash-keys 
+    (count (λ~> (trace-rays pipe-loop-set pipe-grid)) _))