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1 files changed, 40 insertions, 0 deletions

diff --git a/racket/aoc2023/day-04/day-04.rkt b/racket/aoc2023/day-04/day-04.rkt
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+++ b/racket/aoc2023/day-04/day-04.rkt
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+#lang racket
+
+(require advent-of-code
+         data/applicative
+         data/either
+         data/monad
+         megaparsack
+         megaparsack/text
+         threading)
+
+(struct card (n wins))
+
+(define card/p
+  (do (string/p "Card")
+    (many/p space/p)
+    [n <- integer/p]
+    (string/p ":")
+    (many/p space/p)
+    [winners <- (many-until/p integer/p #:sep (many/p space/p) #:end (try/p (string/p " | ")))]
+    (many/p space/p)
+    [has <- (many+/p integer/p #:sep (many/p space/p))]
+    (pure (card n (set-count (set-intersect (first winners) has))))))
+
+(define raw-cards (~> (open-aoc-input (find-session) 2023 4 #:cache #true) port->lines))
+
+;; part 1
+(for/sum ([raw-card (in-list raw-cards)]
+          #:do [(match-define (success (card _ wins)) (parse-string card/p raw-card))]
+          #:unless (= wins 0))
+  (expt 2 (sub1 wins)))
+
+;; part 2
+(for/fold ([counts (for/hash ([n (in-inclusive-range 1 (length raw-cards))])
+                     (values n 1))]
+           #:result (apply + (hash-values counts)))
+          ([raw-card (in-list raw-cards)]
+           #:do [(match-define (success (card n wins)) (parse-string card/p raw-card))])
+  (define bonus-range (inclusive-range (+ n 1) (+ n wins)))
+  (define won-cards (hash-ref counts n))
+  (foldl (λ (n acc) (hash-update acc n (curry + won-cards))) counts bonus-range))