2 files changed, 953 insertions(+), 0 deletions(-)

A day13.lisp
A input/day13.txt

@@ 0,0 1,88 @@
+(defpackage #:advent2021.day13
+  (:use #:cl #:alexandria #:cl-ppcre #:advent2021.util)
+  (:export #:solve-part-1 #:solve-part-2))
+
+(in-package #:advent2021.day13)
+
+
+
+(with-puzzle-file (stream)
+  (defparameter +paper+
+    (loop :with paper := (make-array '(0 0)
+                                     :element-type 'bit
+                                     :adjustable t)
+          :for line := (read-line stream)
+          :until (string= line "")
+          :for (x y) := (mapcar #'parse-integer
+                           (split "," line))
+          :for (height width) := (array-dimensions paper)
+          :do (progn
+                (adjust-array paper (list (max height (1+ y))
+                                          (max width (1+ x))))
+                (setf (aref paper y x) 1))
+          :finally (return paper)))
+
+  (defparameter +folds+
+    (loop :for line := (read-line stream nil)
+          :while line
+          :for line* := (subseq line (length "fold along "))
+          :for (pos n) := (split "=" line*)
+          :collect (list (intern (string-upcase pos) :keyword)
+                         (parse-integer n)))))
+
+(defun fold-x (paper axis)
+  (loop :with (height width) := (array-dimensions paper)
+        :with new-paper := (make-array (list height axis)
+                                       :element-type 'bit)
+        :for y :below height
+        :do (loop :for x :below axis
+                  :for reflected-x := (+ axis (- axis x))
+
+                  :do (setf (aref new-paper y x)
+                            (logior (aref paper y x)
+                                    (if (< reflected-x width)
+                                        (aref paper y (+ axis (- axis x)))
+                                        0
+                                        ))))
+        :finally (return new-paper)))
+
+(defun fold-y (paper axis)
+  (loop :with (height width) := (array-dimensions paper)
+        :with new-paper := (make-array (list axis width)
+                                       :element-type 'bit)
+        :for y :below axis
+        :for reflected-y := (+ axis (- axis y))
+        :do (loop :for x :below width
+                  :do (setf (aref new-paper y x)
+                            (logior (aref paper y x)
+                                    (if (< reflected-y height)
+                                        (aref paper reflected-y x)
+                                        0))))
+        :finally (return new-paper)))
+
+(defun solve-part-1 ()
+  (loop :with paper := (fold-x +paper+ 655)
+        :with (height width) := (array-dimensions paper)
+        :for y :below height
+        :sum (loop :for x :below width
+                   :sum (aref paper y x))))
+
+(defun print-pape (paper)
+  (loop :with (height width) := (array-dimensions paper)
+        :for y :below height
+        :do (loop :for x :below width
+                  :for val := (aref paper y x)
+                  :do (write-char (if (= val 1)
+                                      #\X
+                                      #\Space)))
+        (terpri)))
+
+(defun solve-part-2 ()
+  (loop :with paper := +paper+
+        :for (basis axis) :in +folds+
+        :do (setf paper
+                  (ecase basis
+                    (:x (fold-x paper axis))
+                    (:y (fold-y paper axis))))
+        :finally (print paper)))
+

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+
+fold along x=655
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+fold along x=327
+fold along y=223
+fold along x=163
+fold along y=111
+fold along x=81
+fold along y=55
+fold along x=40
+fold along y=27
+fold along y=13
+fold along y=6