day 10 part 2 in common lisp

common-lisp/day-10/solution.lisp

@@ -25,15 +25,26 @@
 
 (in-package :advent-of-code-day-10)
 
-(defun signal-strength-at-cycle (instructions cycle)
+(defun register-value-at-cycle (instructions cycle)
   (iter (for instruction in instructions)
         (for noop? = (not (consp instruction)))
         (for counter initially 1 then (+ counter (if noop? 1 2)))
         (for register initially 1 then (if noop? register (+ register (cdr instruction))))        
-        (for previous-counter previous counter)
+        (for previous-register-value previous register)                
-        (for previous-register previous register)                
         (when (> counter cycle)
-          (return (* cycle previous-register)))))
+          (return (if (first-iteration-p)
+                      1
+                      previous-register-value)))
+        (finally (return register))))
+
+(defun signal-strength-at-cycle (instructions cycle)
+  (* (register-value-at-cycle instructions cycle) cycle))
+
+(defun sprite-intersects? (pixel sprite)
+  (let ((sprite-start (- sprite 1))
+        (sprite-end (+ sprite 1)))
+    (and (>= pixel sprite-start)
+         (<= pixel sprite-end))))
 
 (defun parse-input (lines)
   (iter (for line in lines)
@@ -41,6 +52,25 @@
                      :noop
                      (cons :addx (parse-integer (cadr (str:split " " line))))))))
 
+(defun solution-part-2 (instructions)
+  (iter (with screen = (make-array '(6 40) :element-type 'character :initial-element #\.))
+        (for cycle from 1 to 240)
+        (for register = (register-value-at-cycle instructions cycle))        
+        (for x = (mod (- cycle 1) 40))
+        (for (values y nil) = (floor (- cycle 1) 40))        
+        (when (sprite-intersects? x register)
+          (setf (aref screen y x) #\#))
+        (finally (return screen))))
+
+(defun print-screen (screen stream)
+  (iter (for y from 0 to 5)
+        (iter (for x from 0 to 39)
+              (format stream "~a" (aref screen y x)))
+        (format stream "~%")))
+
+
+;; test cases
+
 (let ((input (parse-input (read-file-lines "example.txt"))))
   (assert (= 420 (signal-strength-at-cycle input 20)))
   (assert (= 1140 (signal-strength-at-cycle input 60)))
@@ -55,6 +85,20 @@
   (assert (= 14320 (iter (for cycle from 20 to 240 by 40)
                          (sum (signal-strength-at-cycle input cycle))))))
 
+
+;; this puzzle relies on visual confirmation to determine whether its correct or not
+;; so we just print the result to the terminal
+
+(let* ((input (parse-input (read-file-lines "example.txt")))
+       (screen (solution-part-2 input)))
+  (format t "~&~%screen for example input~%~%")
+  (print-screen screen t))
+
+(let* ((input (parse-input (read-file-lines "input.txt")))
+       (screen (solution-part-2 input)))
+  (format t "~&~%screen for normal input~%~%")
+  (print-screen screen t))
+
 ;; Local Variables:
 ;; mode: lisp
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