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(require 'org-fc-core)

;;; Main Algorithm

(defun nth-tn (n)
  "The mathematical formula for the Nth triangular number."
  (/ (* n
	(1+ n))
     2))

(defun org-fc-algo-tn-next-parameters (ease box interval rating)
  "Calculate the next parameters of a card, based on the review RATING.
EASE, BOX and INTERVAL are the current parameters of the card."
  (let* ((incorrect-at-edge (and (eq rating 'incorrect)
				 (eq box 1)))
	 (incorrect (eq rating 'incorrect))
	 (correct t)
	 (next-ease (cond (incorrect-at-edge (- ease 0.01))
			  (incorrect (- ease 0.01))
			  (correct ease)))
	 (next-box (cond (incorrect-at-edge box)
			 (incorrect (1- box))
			 (correct 1+ box)))
	 (next-interval (cond (incorrect-at-edge 1)
			      (incorrect (nth-tn next-box))
			      (correct (nth-tn next-box)))))
    (list next-ease next-box next-interval)))

(defun org-fc-algo-tn-initial-review-data (position)
  "Initial TN review data for POSITION."
  (let ((box 1)
	(ease 1)
	(interval 1)
	(due (org-fc-timestamp-in 0)))
    (list position ease box interval due)))

;;; Footer

(provide 'org-fc-algo-tn)

;; org-fc-algo-tn.el ends here.