Day 23.

src/23.rkt

@@ -0,0 +1,91 @@
+#lang plai
+
+(require data/queue
+         racket/set
+         "../lib.rkt"
+         "IntCode.rkt")
+
+(define input
+  (string->program (car (problem-input 23))))
+
+(define network
+  (build-vector 50 (λ (n) (resume-with-input (exec input) n))))
+
+(define packets
+  (build-vector 50 (λ (_) (make-queue))))
+
+(define part1
+  (let/cc k
+    (let loop ()
+      (for ([i (range 0 50)])
+        (let ([st (vector-ref network i)]
+              [input (vector-ref packets i)])
+          (type-case state st
+            [in (resume)
+                (if (queue-empty? input)
+                    (vector-set! network i (resume -1))
+                    (let* ([x (dequeue! input)]
+                           [y (dequeue! input)]
+                           [st (resume-with-input (resume x) y)])
+                      (vector-set! network i st)))]
+            [out (j resume)
+                 (let*-values ([(x st) (resume-with-output (resume))]
+                               [(y st) (resume-with-output st)])
+                   (when (= j 255) (k y))
+                   (enqueue! (vector-ref packets j) x)
+                   (enqueue! (vector-ref packets j) y)
+                   (vector-set! network i st))]
+            [halt (_) (error "Unexpected program state.")])))
+      (loop))))
+
+(set! network
+      (build-vector 50 (λ (n) (resume-with-input (exec input) n))))
+
+(collect-garbage 'major)
+
+(define (waiting? st)
+  (type-case state st
+    [in (_) #t]
+    [else #f]))
+
+(define part2
+  (let/cc k
+    (let ([prev-NATx 0]
+          [prev-NATy 0]
+          [NATx -1]
+          [NATy -1])
+      (let loop ()
+        (for ([i (range 0 50)])
+          (let ([st (vector-ref network i)]
+                [input (vector-ref packets i)])
+            (type-case state st
+              [in (resume)
+                  (if (queue-empty? input)
+                      (vector-set! network i (resume -1))
+                      (let* ([x (dequeue! input)]
+                             [y (dequeue! input)]
+                             [st (resume-with-input (resume x) y)])
+                        (vector-set! network i st)))]
+              [out (j resume)
+                   (let*-values ([(x st) (resume-with-output (resume))]
+                                 [(y st) (resume-with-output st)])
+                     (if (= j 255)
+                         (begin
+                           (set! NATx x)
+                           (set! NATy y))
+                         (begin
+                           (enqueue! (vector-ref packets j) x)
+                           (enqueue! (vector-ref packets j) y)))
+                     (vector-set! network i st))]
+              [halt (_) (error "Unexpected program state.")])))
+        (when (and (andmap (∘ waiting? (∂ vector-ref network)) (range 0 50))
+                   (andmap queue-empty? (vector->list packets)))
+          (when (= NATy prev-NATy) (k NATy))
+          (let ([addr0 (vector-ref packets 0)])
+            (set! prev-NATx NATx)
+            (set! prev-NATy NATy)
+            (enqueue! addr0 NATx)
+            (enqueue! addr0 NATy)))
+        (loop)))))
+
+(show-solution part1 part2)