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;;; org-fc-sm2.el --- SM2 Repetition Spacing Algorithm -*- lexical-binding: t; -*-
;; Copyright (C) 2020 Leon Rische
;; Author: Leon Rische <emacs@leonrische.me>
;; This program is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.
;; This program is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
;; GNU General Public License for more details.
;; You should have received a copy of the GNU General Public License
;; along with this program. If not, see <https://www.gnu.org/licenses/>.
;;; Commentary:
;;
;; This file implements a modified version of the SM2 review spacing algorithm,
;; similar to the one used by Anki.
;;
;;; Code:
;;;; Customization
(defcustom org-fc-sm2-changes
'((again . -0.3)
(hard . -0.15)
(good . 0.0)
(easy . 0.15))
"Changes to a cards ease depending on its rating."
:type 'list
:group 'org-fc)
(defcustom org-fc-sm2-fixed-intervals
'(0.0 0.01 1.0 6.0)
"Hard-coded intervals for the first few card boxes.
Values are in days."
:type 'list
:group 'org-fc)
(defcustom org-fc-sm2-ease-min 1.3 "Lower bound for a cards ease."
:type 'float
:group 'org-fc)
(defcustom org-fc-sm2-ease-initial 2.5 "Initial ease."
:type 'float
:group 'org-fc)
(defcustom org-fc-sm2-ease-max 5.0 "Upper bound for a cards ease."
:type 'float
:group 'org-fc)
(defcustom org-fc-sm2-fuzz-min 0.9
"Lower bound for random interval fuzz factor."
:type 'float
:group 'org-fc)
(defcustom org-fc-sm2-fuzz-max 1.1
"Upper bound for random interval fuzz factor."
:type 'float
:group 'org-fc)
(defun org-fc-sm2-fuzz (interval)
"Apply fuzz to INTERVAL.
INTERVAL is by a random factor between `org-fc-sm2-fuzz-min' and
`org-fc-sm2-fuzz-max'"
(*
interval
(+ org-fc-sm2-fuzz-min
(cl-random (- org-fc-sm2-fuzz-max org-fc-sm2-fuzz-min)))))
;;;; Parameter Calculation
(defun org-fc-sm2-next-box (box rating)
"Calculate the next box of a card in BOX, rated as RATING."
(cond
;; If a card is rated easy, skip the learning phase
((and (eq box 0) (eq rating 'easy)) 2)
;; If the review failed, go back to box 0
((eq rating 'again) 0)
;; Otherwise, move forward one box
(t (1+ box))))
(defun org-fc-sm2-next-ease (ease box rating)
"Calculate the next ease of a card, based on the review RATING.
EASE and BOX are the current parameters of the card."
(if (< box 2)
ease
(min
(max
(+ ease (alist-get rating org-fc-sm2-changes))
org-fc-sm2-ease-min)
org-fc-sm2-ease-max)))
(defun org-fc-sm2--next-interval (interval next-box next-ease)
"Calculate the next interval of a card.
INTERVAL is the current interval of the card, NEXT-BOX and
NEXT-EASE are the new parameters of the card."
(if (< next-box (length org-fc-sm2-fixed-intervals))
(nth next-box org-fc-sm2-fixed-intervals)
(org-fc-sm2-fuzz (* next-ease interval))))
(defun org-fc-sm2-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* ((next-ease (org-fc-sm2-next-ease ease box rating))
(next-box (org-fc-sm2-next-box box rating))
(next-interval (org-fc-sm2--next-interval interval next-box next-ease)))
(list next-ease next-box next-interval)))
;;;; Footer
(provide 'org-fc-sm2)
;;; org-fc-sm2.el ends here
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