Day 21

Added solution for day 21. This only works for my registers so probably won't do for other inputs.

advent-of-code-2018.asd

@@ -27,7 +27,8 @@
                (:file "day17")
                (:file "day18")
                (:file "day19")
+               (:file "day20")
+               (:file "day21")
                (:file "day23")
-               (:file "day25")
-               (:file "day20")))
+               (:file "day25")))
 

day19.lisp

@@ -1,3 +1,4 @@
+
 ;;;; day19.lisp
 
 (in-package :advent-of-code-2018)

day21.lisp

@@ -0,0 +1,24 @@
+;;;; day21.lisp
+
+(in-package :advent-of-code-2018)
+
+;;; works for my input
+;;; how to find the correct register? would need more puzzle inputs probably
+(defun day21-next (state)
+  (loop
+    :while (day19-execute! state)
+    :until (= 28 (pstate-ip state))
+    :finally (return (aref (pstate-registers state) 1))))
+
+;;; Part 2 slow
+;;; further reassembling the code by hand would probably allow for optimization
+(defun day21 ()
+  (let* ((state (day19-initialize-program (day19-parse-input (puzzlefile 21))))
+         (first-value (day21-next state)))
+    (format t "A value of ~a ends the program fastest.~%" first-value)
+    (loop :with seen := (make-hash-table)
+          :for prev-value := nil :then next-value
+          :for next-value := first-value :then (day21-next state)
+          :until (gethash next-value seen nil)
+          :do (setf (gethash next-value seen) t)
+          :finally (format t "A value of ~a ends the program the latest.~%" prev-value))))