commit 8d76db913773c02a383f070c9c57bb5b0a491cfc
from: Lukas Henkel <lh@entf.net>
date: Sat Dec 09 05:20:41 2023 UTC

Day 9 task 2

commit - d23cd36cd3c107662e3ba682530ec7350c7b9a71
commit + 8d76db913773c02a383f070c9c57bb5b0a491cfc
blob - fa4081ed4165f94c0b38d988f06ef7179295c57d
blob + 19d0ab12276a756d06ec7d08b240b542db2083ca
--- src/day-9.lisp
+++ src/day-9.lisp
@@ -3,18 +3,30 @@
   (:export #:day-9))
 (in-package #:aoc/day-9)
 
+(defun diff-sequence (numbers)
+  (loop with last = (first numbers)
+        for number in (rest numbers)
+        collect (- number last)
+        do (setf last number)))
+
 (defun find-next-value (numbers)
-  (let ((new-sequence (loop with last = (first numbers)
-                            for number in (rest numbers)
-                            collect (- number last)
-                            do (setf last number))))
+  (let ((new-sequence (diff-sequence numbers)))
     (if (every (curry #'= 0) new-sequence)
         (first numbers)
         (+ (lastcar numbers)
            (find-next-value new-sequence)))))
 
+(defun find-previous-value (numbers)
+  (let ((new-sequence (diff-sequence numbers)))
+    (if (every (curry #'= 0) new-sequence)
+        (first numbers)
+        (- (first numbers)
+           (find-previous-value new-sequence)))))
+
 (defun day-9 (input)
   (loop for line = (read-line input nil)
         while line
         for numbers = (read-number-list line)
-        sum (find-next-value numbers) fixnum))
+        sum (find-next-value numbers) into task-1 fixnum
+        sum (find-previous-value numbers) into task-2 fixnum
+        finally (return (values task-1 task-2))))