1 files changed, 46 insertions, 0 deletions
diff --git a/AoC2023/day09/solver.lisp b/AoC2023/day09/solver.lisp
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+++ b/AoC2023/day09/solver.lisp
@@ -0,0 +1,46 @@
+;;03:39
+(ql:quickload '(fiveam arrows transducers))
+(defparameter eg-input "0 3 6 9 12 15
+1 3 6 10 15 21
+10 13 16 21 30 45")
+
+(defun row-reducer (row)
+  (loop for (a b) on row by #'cdr
+        until (null b)
+        collect (- b a)))
+
+(defun terminal-p (row)
+  (every #'zerop row))
+
+(defun forward (row)
+  (if (terminal-p row)
+      0
+      (+  (forward (row-reducer row)) (car (last row)))))
+
+(defun backward (row)
+  (if (terminal-p row)
+      0
+      (-  (car row)  (backward (row-reducer row)))))
+
+(defun solver1 (lines)
+  (arrows:->>
+   lines
+   (mapcar (lambda (line)
+             (forward
+              (mapcar #'parse-integer (uiop:split-string line)))))
+   (apply #'+)))
+
+(defun solver2 (lines)
+  (arrows:->>
+   lines
+   (mapcar (lambda (line)
+             (backward
+              (mapcar #'parse-integer (uiop:split-string line)))))
+   (apply #'+)))
+
+(fiveam:test solutions
+  (fiveam:is (= 114 (solver1 (uiop:split-string eg-input :separator '(#\Newline)))))
+  (fiveam:is (= 2105961943 (solver1 (uiop:read-file-lines "input"))))
+
+  (fiveam:is (= 2 (solver2 (uiop:split-string eg-input :separator '(#\Newline)))))
+  (fiveam:is (= 1019 (solver2 (uiop:read-file-lines "input")))))