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1 files changed, 39 insertions, 0 deletions

diff --git a/racket/aoc2021/day-03/day-03.rkt b/racket/aoc2021/day-03/day-03.rkt
new file mode 100644
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--- /dev/null
+++ b/racket/aoc2021/day-03/day-03.rkt
@@ -0,0 +1,39 @@
+#lang racket
+(require advent-of-code
+         threading)
+
+(define data
+  (~> (open-aoc-input (find-session) 2021 3 #:cache (string->path "./cache"))
+      port->lines
+      (map string->list _)))
+
+;; part 1
+(define most-common-bits
+  (for*/list ([row (in-list (apply map list data))] [len (in-value (length data))])
+    (if (> (count (λ (c) (char=? #\1 c)) row) (/ len 2)) #\1 #\0)))
+(define (bit-list->number lst)
+  (~> lst (apply string _) (string->number _ 2)))
+
+(define gamma (bit-list->number most-common-bits))
+(define epsilon (~> most-common-bits (map (λ (c) (if (char=? c #\1) #\0 #\1)) _) bit-list->number))
+
+(* gamma epsilon)
+
+;; part 2
+(define (rating-search data comparison)
+  (for/fold ([candidates data] #:result (bit-list->number (first candidates)))
+            ([bit (in-list most-common-bits)] [bit-index (in-range 0 (length most-common-bits))])
+    #:break (= 1 (length candidates))
+    (define keep-bit
+      (~> candidates
+          (apply map list _)
+          (list-ref _ bit-index)
+          (count (λ (c) (char=? #\1 c)) _)
+          (comparison _ (/ (length candidates) 2))
+          (if _ #\1 #\0)))
+    (filter (λ (row) (char=? keep-bit (list-ref row bit-index))) candidates)))
+
+(define oxygen-rating (rating-search data >=))
+(define scrubber-rating (rating-search data <))
+
+(* oxygen-rating scrubber-rating)