2023/09 refactoring

- Create PREDICTION-SEQUENCES, with the sole responsibility of
  generating all the delta sequences for a given value history
- Add EXTRAPOLATE and EXTRAPOLATE-BACKWARD function to codify
  logic for part 1 and part 2 respectively

src/2023/day09.lisp

@@ -1,30 +1,36 @@
 (defpackage :aoc/2023/09 #.cl-user::*aoc-use*)
 (in-package :aoc/2023/09)
 
-'(&optional (strings (uiop:read-file-lines #P"src/2023/day09.txt")))
+
+(defun parse-input (&optional (strings (uiop:read-file-lines #P"src/2023/day09.txt")))
+  (mapcar #'extract-integers strings))
+
 
 (defun deltas (nums)
   (loop for n in nums by #'cdr for m in (cdr nums) by #'cdr
         collect (- m n)))
 
-(defun extrapolate (nums)
+(defun prediction-sequences (history)
+  (looping
+    (while (some (complement #'zerop) history)
+      (collect! history)
+      (setf history (deltas history)))))
+
+
+(defun extrapolate (history)
   (looping
-    (while (some (complement #'zerop) nums)
-      (sum! (car (last nums)))
-      (setf nums (deltas nums)))))
-; (mapcar #'extract-integers (uiop:read-file-lines #P"src/2023/day09.txt"))
-; (mapcar #'extrapolate *)
-; (reduce #'+ *)
-; 1930746032
-
-(defun extrapolate-2 (nums)
-  (bnd1 (dd (looping
-              (while (some (complement #'zerop) nums)
-                (collect! (first nums))
-                (setf nums (deltas nums)))))
-    (bnd1 (num 0)
-      (dolist (d (reverse dd) num)
-        (setf num (- d num))))))
-; (mapcar #'extract-integers (uiop:read-file-lines #P"src/2023/day09.txt"))
-; (mapcar #'extrapolate-2 *)
-; (reduce #'+ *)
+    (dolist (nums (prediction-sequences history))
+      (sum! (car (last nums))))))
+
+
+(defun extrapolate-backward (history)
+  (bnd1 (res 0)
+    (dolist (nums (reverse (prediction-sequences history)) res)
+      (setf res (- (first nums) res)))))
+
+
+(define-solution (2023 09) (input parse-input)
+  (values (reduce #'+ input :key #'extrapolate)
+          (reduce #'+ input :key #'extrapolate-backward)))
+
+(define-test (2023 09) (1930746032 1154))