1 files changed, 41 insertions, 0 deletions
diff --git a/racket/aoc2023/day-15/day-15.rkt b/racket/aoc2023/day-15/day-15.rkt
new file mode 100644
index 0000000..d049565
--- /dev/null
+++ b/racket/aoc2023/day-15/day-15.rkt
@@ -0,0 +1,41 @@
+#lang racket
+
+(require advent-of-code
+         threading)
+
+(define input
+  (~> (fetch-aoc-input (find-session) 2023 15 #:cache #true) string-trim (string-split ",")))
+
+(define (hash-algorithm str)
+  (for/fold ([acc 0]) ([c (in-string str)])
+    (~> c char->integer (+ acc) (* 17) (modulo _ 256))))
+
+;; part 1
+(for/sum ([code (in-list input)]) (hash-algorithm code))
+
+;; part 2
+(define (remove-lens boxes label)
+  (hash-update boxes
+               (hash-algorithm label)
+               (λ (lens-set) (remove label lens-set (λ (rem l) (equal? rem (car l)))))
+               '()))
+
+(define (insert-lens boxes label focal)
+  (define new-lens (cons label focal))
+  (hash-update boxes
+               (hash-algorithm label)
+               (λ (lens-set)
+                 (if (assoc label lens-set)
+                     (map (λ (pair) (if (equal? (car pair) label) new-lens pair)) lens-set)
+                     (append lens-set (list new-lens))))
+               (list new-lens)))
+
+(define (focusing-power boxes)
+  (for*/sum ([(box-number lenses) (in-hash boxes)] [(lens order) (in-indexed lenses)])
+    (* (add1 box-number) (add1 order) (cdr lens))))
+
+(for/fold ([boxes (hash)] #:result (focusing-power boxes)) ([code (in-list input)])
+  (match code
+    [(regexp #rx"(.*)=(.*)" (list _ label (app string->number focal)))
+     (insert-lens boxes label focal)]
+    [(regexp #rx"(.*)-" (list _ label)) (remove-lens boxes label)]))