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;;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")))))
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