2024 day one

1 files changed, 36 insertions, 0 deletions

diff --git a/2024/d01.lisp b/2024/d01.lisp
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+++ b/2024/d01.lisp
@@ -0,0 +1,36 @@
+(in-package :aoc/2024)
+
+(defparameter *input/d1*
+              (asdf:system-relative-pathname :advent "2024/d01.input"))
+
+(defun readlists (filename)
+  "Read the left and right columns of numbers into individual lists"
+  (let ((left-numbers '())
+        (right-numbers '()))
+    (with-open-file (stream filename :direction :input)
+      (loop for line = (read-line stream nil nil)
+            while line do
+              (let* ((tab-pos (search "   " line))
+                     (left (parse-integer (subseq line 0 tab-pos)))
+                     (right (parse-integer (subseq line (1+ tab-pos)))))
+                (push left left-numbers)
+                (push right right-numbers))))
+    (list
+     (nreverse left-numbers)
+     (nreverse right-numbers))))
+
+(let* ((input-list (readlists *input/d1*))
+       (left (sort (first input-list) #'<))
+       (right (sort (second input-list) #'<))
+       (diff (mapcar #'- left right))
+       (absdiff (mapcar #'abs diff))
+       (result (apply #'+ absdiff)))
+  (print result))
+; 2066446
+
+(let* ((input-list (readlists *input/d1*))
+       (left (first input-list))
+       (right (second input-list))
+       (result (apply #'+ (mapcar #'(lambda (x) (* x (count x right))) left))))
+  (print result))
+; 24931009