day 8 impl 3: the program analysis solution

8.rkt

@@ -26,17 +26,39 @@
   (cdr (execute-program input)))
 
 (define (part2 input)
-  (let/ec exit
-    (for ([ins (in-vector input)] [i (in-naturals)]
-          #:unless (string=? "acc" (insn-type ins)))
-      (define new-insn
-        (insn (match (insn-type ins) ["jmp" "nop"] ["nop" "jmp"])
-              (insn-arg ins)))
-      (define new-prg (vector-copy input))
-      (vector-set! new-prg i new-insn)
-      (match (execute-program new-prg)
-        [(cons 'loop _) (void)]
-        [(cons 'done acc) (exit acc)]))))
+  (define target-pc (vector-length input))
+  ;; create naive CFG (as list of edges)
+  (define model/list
+    (for/list ([i (in-naturals)] [ins (in-vector input)])
+      (match-define (insn type arg) ins)
+      (match type
+        [(or "nop" "acc") (list i (add1 i))]
+        ["jmp" (list i (+ i arg))])))
+  ;; detect connected components of unweighted CFG
+  (define ccs (map list->set (cc (undirected-graph model/list))))
+  ;; find which components we need to connect
+  (define to-cc (for/first ([cc (in-list ccs)] #:when (set-member? cc target-pc)) cc))
+  ;; execute the program, but bail as soon as there's a way to to-cc
+  (define (execute/graph G [v 0] [acc 0] [changed? #f])
+    (cond
+      ;; done!
+      [(= target-pc v) acc]
+      ;; not done, yet....
+      [else
+       ;; fetch instruction
+       (match-define (insn type arg) (vector-ref input v))
+       ;; determine the "alternate target" if this instruction were mutated
+       (define alt-target (match type
+                            ["nop" (+ v arg)]
+                            ["jmp" (add1 v)]
+                            ["acc" #f]))
+       ;; calculate new accumulator
+       (define new-acc (if (string=? type "acc") (+ acc arg) acc))
+       ;; if we haven't mutated an instruction yet, and alt-target goes into to-cc, use it
+       (if (and (not changed?) alt-target (set-member? to-cc alt-target))
+         (execute/graph G alt-target new-acc #t)
+         (execute/graph G (first (get-neighbors G v)) new-acc changed?))]))
+  (execute/graph (directed-graph model/list)))
 
 (module+ main
   (define input