1 files changed, 54 insertions, 0 deletions
diff --git a/racket/aoc2022/day-15/day-15.rkt b/racket/aoc2022/day-15/day-15.rkt
new file mode 100644
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--- /dev/null
+++ b/racket/aoc2022/day-15/day-15.rkt
@@ -0,0 +1,54 @@
+#lang racket
+
+(require advent-of-code
+         threading
+         fancy-app
+         algorithms
+         (prefix-in iset- data/integer-set))
+
+(struct beacon-record (sensor beacon))
+(struct posn (x y))
+
+(define beacon-records
+  (~> (fetch-aoc-input (find-session) 2022 15 #:cache #true)
+      (string-split "\n")
+      (map (λ~> (string-replace #px"[^\\d\\s-]" "")
+                string-split
+                (map string->number _)
+                (chunks-of 2)
+                (map (apply posn _) _)
+                (apply beacon-record _))
+           _)))
+
+(define (manhattan-distance-to-beacon record)
+  (match-define (beacon-record (posn sx sy) (posn bx by)) record)
+  (+ (abs (- sx bx)) (abs (- sy by))))
+
+(define (coverage-at-row record row)
+  (match-define (beacon-record (posn sx sy) _) record)
+  (define x-distance (- (manhattan-distance-to-beacon record) (abs (- row sy))))
+  (cond
+    [(negative? x-distance) (iset-make-range)]
+    [else (iset-make-range (- sx x-distance) (+ sx x-distance))]))
+
+(define (total-coverage-at-row records row)
+  (for/fold ([coverage (iset-make-range)]) ([r (in-list records)])
+    (iset-union coverage (coverage-at-row r row))))
+
+;; part 1
+(define (coverage-without-beacons records row)
+  (~> (total-coverage-at-row records row)
+      iset-count
+      (- (count (λ (b) (= row (posn-y b)))
+                (~> beacon-records (map beacon-record-beacon _) remove-duplicates)))))
+
+(coverage-without-beacons beacon-records 2000000)
+
+;; part 2
+(define (find-only-beacon beacon-records size)
+  (for/first ([y (in-range 0 size)]
+              #:do [(define xs (iset-complement (total-coverage-at-row beacon-records y) 0 size))]
+              #:when (= 1 (iset-count xs)))
+    (+ (* 4000000 (iset-get-integer xs)) y)))
+
+(find-only-beacon beacon-records 4000000)