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;;03:39
;;04:07 Solved
;;05:24 Painful refactorins
(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)
(mapcar #'- (cdr row) row))
(defun terminal-p (row)
(every #'= (cdr row) row))
(defun get-coefs (row pick &optional acc)
(if (terminal-p row)
(nreverse (cons (funcall pick row) acc))
(get-coefs (row-reducer row) pick (cons (funcall pick row) acc))))
(defun parse-line (line)
(mapcar #'parse-integer (uiop:split-string line)))
(defun forward (line)
(reduce #'+ (get-coefs line (lambda (a) (car (last a))))))
(defun backward (line)
(reduce #'- (get-coefs line #'car) :from-end t))
(defun solver (lines key)
(reduce #'+
(mapcar #'parse-line lines)
:key key))
(fiveam:test solutions
(fiveam:is (= 114 (solver (uiop:split-string eg-input :separator '(#\Newline))
#'forward)))
(fiveam:is (= 2105961943 (solver (uiop:read-file-lines "input")
#'forward)))
(fiveam:is (= 2 (solver (uiop:split-string eg-input :separator '(#\Newline))
#'backward)))
(fiveam:is (= 1019 (solver (uiop:read-file-lines "input")
#'backward))))
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