1 files changed, 144 insertions, 0 deletions
diff --git a/aoc2023-racket/day-20/day-20.rkt b/aoc2023-racket/day-20/day-20.rkt
new file mode 100644
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+++ b/aoc2023-racket/day-20/day-20.rkt
@@ -0,0 +1,144 @@
+#lang racket
+
+(require advent-of-code
+         threading
+         data/applicative
+         data/monad
+         megaparsack
+         megaparsack/text)
+
+(struct broadcaster ())
+(struct flipflop (state received))
+(struct conjunction (recieved))
+(struct cable (type dests))
+(struct nothing ())
+
+(define charlist->symbol (λ~>> (apply string) string->symbol))
+
+(define input (fetch-aoc-input (find-session) 2023 20 #:cache true))
+
+(define module/p
+  (do (or/p (do (char/p #\%)
+              [name <- (many+/p letter/p)]
+              (pure (cons (charlist->symbol name) (flipflop 'off '()))))
+            (do (char/p #\&)
+              [name <- (many+/p letter/p)]
+              (pure (cons (charlist->symbol name) (conjunction (hash)))))
+            (do [name <- (many+/p letter/p)]
+              (pure (cons (charlist->symbol name) (broadcaster)))))))
+
+(define cable/p
+  (do [mod <- module/p]
+    (string/p " -> ")
+    [names <- (many/p (many+/p letter/p) #:sep (string/p ", "))]
+    (pure (cable mod (map charlist->symbol names)))))
+
+(define cables (~> input (string-split "\n") (map (λ~>> (parse-string cable/p) parse-result!) _)))
+
+(define destinations
+  (for/hash ([cable (in-list cables)])
+    (values (car (cable-type cable)) (cable-dests cable))))
+
+(define (set-conjunction-initial-state c)
+  (cond
+    [(conjunction? (cdr c))
+     (~>> destinations
+          hash-keys
+          (filter (λ (k) (member (car c) (hash-ref destinations k))))
+          (map (λ (k) (cons k 'low)))
+          (make-immutable-hash)
+          conjunction
+          (cons (car c)))]
+    [else c]))
+
+(define (make-initial-conditions-hash cables)
+  (~>> cables
+       (map cable-type)
+       (map set-conjunction-initial-state)
+       make-immutable-hash))
+
+(define (receive mod from tone)
+  (match mod
+    [(flipflop state queue) (flipflop state (append queue (list tone)))]
+    [(conjunction received) (conjunction (hash-set received from tone))]
+    [(nothing) (nothing)]))
+
+; needed for part 2
+(define to-rx '(rk cd zf qx))
+(define sentry-tones (make-hash (for/list ([node to-rx]) (cons node 0))))
+
+(define (press-button-once current-state this-round)
+  (for/fold ([queue '(broadcaster)]
+             [all-cables-state current-state]
+             [high 0]
+             [low 0]
+             #:result (values all-cables-state high low))
+            ([_i (in-naturals)] #:break (empty? queue))
+    (match-define (list* hd tl) queue)
+    (define to (hash-ref destinations hd (nothing)))
+    (match (hash-ref all-cables-state hd)
+      [(broadcaster)
+       (define state*
+         (foldl (λ (r acc) (hash-update acc r (λ~> (receive hd 'low)) (nothing)))
+                all-cables-state
+                to))
+       (values (hash-ref destinations 'broadcaster) state* high (+ (length to) (add1 low)))]
+      [(flipflop 'off (list* 'low q))
+       (define state*
+         (~> all-cables-state
+             (foldl (λ (r acc)
+                      (when (member r to-rx)
+                        (println (~a r " received high tone at " this-round)))
+                      (hash-update acc r (λ~> (receive hd 'high)) (nothing)))
+                    _
+                    to)
+             (hash-set _ hd (flipflop 'on q))))
+       (values (append tl to) state* (+ (length to) high) low)]
+      [(flipflop 'on (list* 'low q))
+       (define state*
+         (~> all-cables-state
+             (foldl (λ (r acc) (hash-update acc r (λ~> (receive hd 'low)) (nothing))) _ to)
+             (hash-set _ hd (flipflop 'off q))))
+       (values (append tl to) state* high (+ (length to) low))]
+      [(flipflop on-or-off (list* 'high q))
+       (define state* (~> all-cables-state (hash-set _ hd (flipflop on-or-off q))))
+       (values tl state* high low)]
+      [(conjunction received)
+       #:when (or (empty? (hash-values received)) (member 'low (hash-values received)))
+
+       (when (member hd to-rx)
+         (hash-set! sentry-tones hd this-round))
+       (define state*
+         (foldl (λ (r acc)
+                  (hash-update acc r (λ~> (receive hd 'high)) (nothing)))
+                all-cables-state
+                to))
+       (values (append to tl) state* (+ (length to) high) low)]
+      [(conjunction _)
+       (define state*
+         (foldl (λ (r acc) (hash-update acc r (λ~> (receive hd 'low)) (nothing)))
+                all-cables-state
+                to))
+       (values (append tl to) state* high (+ (length to) low))]
+      [(nothing) (values tl all-cables-state high low)])))
+
+;; part 1
+(for/fold ([starting-state (make-initial-conditions-hash cables)]
+           [high 0]
+           [low 0]
+           #:result (* high low))
+          ([i (in-range 1000)])
+  (define-values (next-state this-high this-low) (press-button-once starting-state i))
+  (values next-state  (+ high this-high) (+ low this-low)))
+
+;; part 2
+;; rx receives a tone from gh, which receives four tones itself
+;; those tones arrive on regular synced cycles so it's just the LCM of those cycle lengths
+;; and since those cycle lengths are prime, it reduces to the product of the lengths
+;; this is a really hacky mutable solution, I'm sure there's better ways of flagging these cycles
+
+(for/fold ([starting-state (make-initial-conditions-hash cables)]
+           #:result (apply * (hash-values sentry-tones)))
+          ([i (in-range 1 5000)])
+  (define-values (next-state _high _low) (press-button-once starting-state i))
+  (values next-state))
+\ No newline at end of file