1 files changed, 72 insertions, 0 deletions
diff --git a/racket/aoc2023/day-03/day-03.rkt b/racket/aoc2023/day-03/day-03.rkt
new file mode 100644
index 0000000..60e81a6
--- /dev/null
+++ b/racket/aoc2023/day-03/day-03.rkt
@@ -0,0 +1,72 @@
+#lang racket
+
+(require advent-of-code
+         threading)
+
+(struct posn (x y) #:transparent)
+(struct part (n posns) #:transparent)
+
+(define (make-board port)
+  (for*/hash ([(row y) (in-indexed (port->lines port))]
+              [(col x) (in-indexed (string->list row))]
+              #:unless (equal? col #\.))
+    (define v
+      (cond
+        [(string->number (string col))]
+        [(equal? col #\*) 'gear]
+        [else 'other]))
+    (values (posn x y) v)))
+
+(define board (~> (open-aoc-input (find-session) 2023 3 #:cache #true) make-board))
+
+(define (posn<? a b)
+  (match-define (list (cons (posn a-x a-y) _) (cons (posn b-x b-y) _)) (list a b))
+  (if (= a-y b-y) (< a-x b-x) (< a-y b-y)))
+
+(define (find-cells f b)
+  (~> (for/hash ([(k v) (in-hash b)] #:when (f v))
+        (values k v))
+      hash->list
+      (sort posn<?)))
+
+(define (group-into-parts cells [acc '()])
+  (match* (cells acc)
+    [('() acc)
+     acc]
+    [((list* (cons (and pt (posn x y)) n) cs)
+      (list* (part n* (and pts (list* (posn x* y) rest-pts)))
+             rest-acc))
+     #:when (= (- x x*) 1)
+     (group-into-parts cs (cons (part (+ n (* n* 10)) (cons pt pts)) rest-acc))]
+    [((list* (cons pt n) cs) acc)
+     (group-into-parts cs (cons (part n (list pt)) acc))]))
+
+(define (neighbors p)
+  (for*/list ([dx '(-1 0 1)]
+              [dy '(-1 0 1)]
+              #:unless (and (= dx 0) (= dy 0)))
+    (posn (+ dx (posn-x p)) (+ dy (posn-y p)))))
+
+(define to-neighbors (λ~>> part-posns (append-map neighbors) remove-duplicates))
+(define (symbol-in-neighbors b pt acc)
+  (~>> pt
+       to-neighbors
+       (ormap (λ (p) (let ([lookup (hash-ref b p #f)])
+                       (or (equal? lookup 'gear) (equal? lookup 'other)))))
+       ((λ (bool) (if bool (+ acc (part-n pt)) acc)))))
+
+;; part 1
+(define parts (~>> board (find-cells integer?) group-into-parts))
+(foldl (curry symbol-in-neighbors board) 0 parts)
+
+;; part 2
+(define gears (~>> board (find-cells (curry equal? 'gear)) (map car)))
+(define parts-with-neighbors (map (λ (pt) (struct-copy part pt [posns (to-neighbors pt)])) parts))
+
+(define (find-parts-near-gear pts gear)
+  (filter-map (λ (pt) (and (findf (curry equal? gear) (part-posns pt)) (part-n pt))) pts))
+
+(~>> gears
+     (filter-map (λ~>> (find-parts-near-gear parts-with-neighbors)
+                       ((λ (ns) (if (= (length ns) 2) (* (first ns) (second ns)) #f)))))
+     (apply +))