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1 files changed, 60 insertions, 0 deletions

diff --git a/racket/aoc2020/day-11/day-11.rkt b/racket/aoc2020/day-11/day-11.rkt
new file mode 100644
index 0000000..e2fe052
--- /dev/null
+++ b/racket/aoc2020/day-11/day-11.rkt
@@ -0,0 +1,60 @@
+#lang racket
+
+(require advent-of-code)
+
+(define raw-grid (fetch-aoc-input (find-session) 2020 11))
+
+(define/match (parse _)
+  [(#\L) 'empty]
+  [(#\#) 'occupied]
+  [(#\.) 'floor])
+
+(define seat-grid
+  (for*/hash ([(row r) (in-indexed (in-list (string-split raw-grid)))]
+              [(col c) (in-indexed (in-string row))])
+    (values (cons r c) (parse col))))
+
+(define (next-seat-state seat state h rule [max-occupy 4])
+  (define neighbor-states (rule seat h))
+  (match* (state (count (curry eq? 'occupied) neighbor-states))
+    [('empty 0) 'occupied]
+    [('occupied n)
+     #:when (>= n max-occupy)
+     'empty]
+    [(_ _) state]))
+
+(define (stabilize h [i 1] #:rule rule #:max-occupy [max-occupy 4])
+  (define h*
+    (for/hash ([(seat state) (in-hash h)])
+      (cond
+        [(eq? state 'floor) (values seat state)]
+        [else (values seat (next-seat-state seat state h rule max-occupy))])))
+  (if (equal? h h*)
+      (count (curry equal? 'occupied) (hash-values h))
+      (stabilize h* (add1 i) #:rule rule #:max-occupy max-occupy)))
+
+;; part 1
+(define (find-nearest-neighbors p h)
+  (match-define (cons r c) p)
+  (for*/list ([r* (in-inclusive-range (sub1 r) (add1 r))]
+              [c* (in-inclusive-range (sub1 c) (add1 c))]
+              [p* (in-value (cons r* c*))]
+              #:unless (equal? p p*))
+    (hash-ref h p* 'out-of-bounds)))
+
+(stabilize seat-grid #:rule find-nearest-neighbors)
+
+;; part 2
+(define (find-visible-neighbors p h)
+  (match-define (cons r c) p)
+  (define directions
+    (for*/list ([dr '(-1 0 1)] [dc '(-1 0 1)] #:unless (= 0 dr dc))
+      (cons dr dc)))
+  (for/list ([dir (in-list directions)] #:do [(match-define (cons dr dc) dir)])
+    (for*/first ([i (in-naturals 1)]
+                 #:do [(define p* (cons (+ r (* i dr)) (+ c (* i dc))))
+                       (define state (hash-ref h p* 'out-of-bounds))]
+                 #:unless (equal? state 'floor))
+      state)))
+
+(stabilize seat-grid #:rule find-visible-neighbors #:max-occupy 5)