Day 12

Added solution for day 12.

advent-of-code-2018.asd

@@ -19,4 +19,5 @@
                (:file "day8")
                (:file "day9")
                (:file "day10")
-               (:file "day11")))
+               (:file "day11")
+               (:file "day12")))

day12.lisp

@@ -0,0 +1,63 @@
+;;;; day12.lisp
+
+(in-package :advent-of-code-2018)
+
+(defun day12-state->id (state start)
+  (loop :for multiplier := 1 :then (* 2 multiplier)
+        :for index :from start :below (+ start 5)
+        :sum (* multiplier (aref state index))))
+
+(defun day12-parse-input ()
+  (labels ((char->state (char)
+             (cond
+             ((char= #\# char) 1)
+             ((char= #\. char) 0)
+             (t nil)))
+           (parse-rule (raw)
+             (when (char= #\# (char raw 9))
+               (day12-state->id (map 'simple-vector #'char->state (subseq raw 0 5)) 0))))
+    (let* ((input (read-puzzlefile "input12.txt"))
+           (initial-state (remove nil (map 'list #'char->state (first input))))
+           (raw-rules (rest (rest input)))
+           (rules (make-hash-table)))
+      (loop :for raw-rule :in raw-rules
+            :for rule := (parse-rule raw-rule)
+            :when rule :do (setf (gethash rule rules) t))
+      (list (make-array (length initial-state) :initial-contents initial-state)
+            rules))))
+
+(defun day12-pad-initial-state (istate padding)
+  (let ((state (make-array (+ padding (length istate) padding) :element-type 'bit :initial-element 0)))
+    (loop :for index :from padding :below (+ padding (length istate))
+          :for initial-val :across istate
+          :do (setf (aref state index) initial-val))
+    state))
+
+(defun day12-score-state (state initial-value)
+  (loop :for index :below (length state)
+        :for val := initial-value :then (incf initial-value)
+        :sum (* val (aref state index))))
+
+(defun day12-evolve (state rules)
+  (loop :with new-state := (make-array (length state) :element-type 'bit :initial-element 0)
+        :for index :below (- (length state) 5)
+        :when (gethash (day12-state->id state index) rules) :do (setf (aref new-state (+ 2 index)) 1)
+        :finally (return new-state)))
+
+(defun day12-do-generations (istate rules generations)
+  (loop 
+    :for state := (day12-pad-initial-state istate (+ 5 generations)) :then (day12-evolve state rules)
+    :repeat generations
+    :finally (return state)))
+
+(defun day12 ()
+  (destructuring-bind (istate rules) (day12-parse-input)
+    (let ((gen20 (day12-do-generations istate rules 20)))
+      (format t "The sum of pot numbers with plants at generation 20 is ~a.~%"
+              (day12-score-state gen20 -25))
+      (let* ((gen300 (day12-do-generations gen20 rules 280))
+             (gen301 (day12-evolve gen300 rules))
+             (score-301 (day12-score-state gen301 -310))
+             (delta-score (- score-301 (day12-score-state gen300 -310))))
+        (format t "The sum of pot numbers at generation 50000000000 is ~a.~%"
+                (+ score-301 (* delta-score (- 50000000000 301))))))))