Add 2024 day 9 part 1

1 files changed, 49 insertions, 0 deletions

diff --git a/2024/09/puzzle-1.el b/2024/09/puzzle-1.el
new file mode 100644
index 0000000..5ac1a38
--- /dev/null
+++ b/2024/09/puzzle-1.el
@@ -0,0 +1,49 @@
+(defsubst char-to-number (char)
+  (declare (pure t) (side-effect-free t))
+  (- char ?0))
+
+(defun first-nil (vector start)
+  (cl-loop for i from (1+ start) below (length vector)
+           when (null (aref vector i))
+           return i
+           finally return (length vector)))
+
+(defun last-number (vector start)
+  (cl-loop for i from (1- start) downto 0
+           when (aref vector i)
+           return i
+           finally return 0))
+
+(defun solve (input-file)
+  (let* ((input (with-temp-buffer
+                  (insert-file-contents-literally input-file)
+                  (buffer-string)))
+         (nums (mapcar #'char-to-number input))
+         (vector (make-vector (apply #'+ nums) nil))
+         (i 0) (j 0))
+
+    ;; Populate ‘vector’
+    (dolist (num nums)
+      (when (cl-evenp i)
+        (dotimes (k num)
+          (aset vector (+ j k) (/ i 2))))
+      (cl-incf i)
+      (cl-incf j num))
+
+    ;; Swap elements
+    (setq i (first-nil vector 0)
+          j (last-number vector (length vector)))
+    (while (< i j)
+      (let ((x (aref vector i))
+            (y (aref vector j)))
+        (aset vector i y)
+        (aset vector j x))
+      (setq i (first-nil vector i))
+      (setq j (last-number vector j)))
+
+    ;; Compute checksum
+    (cl-loop for i from 0 below (length vector)
+             for x = (aref vector i)
+             while x sum (* i x))))
+
+(message "%d" (solve "input"))
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