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;;; gnosis-algorithm.el --- Spaced Repetition Algorithm for Gnosis -*- lexical-binding: t; -*-
;; Copyright (C) 2023 Thanos Apollo
;; Author: Thanos Apollo <[email protected]>
;; Keywords: extensions
;; URL: https://git.thanosapollo.org/gnosis
;; Version: 0.0.1
;; 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:
;; Work in progress
;;; Code:
(require 'cl-lib)
(require 'calendar)
(defcustom gnosis-algorithm-interval '(1 3)
"Gnosis initial interval for successful reviews.
First item: First interval,
Second item: Second interval."
:group 'gnosis
:type '(list integer))
(defcustom gnosis-algorithm-ef '(0.35 0.30 1.3)
"Gnosis easiness factor.
First item : Increase factor
Second item: Decrease factor
Third item : Starting total ef
Note: Starting total ef should not be above 3.0"
:group 'gnosis
:type '(list float))
(defcustom gnosis-algorithm-ff 0.5
"Gnosis forgetting factor.
Used to calcuate new interval for failed questions.
NOTE: Do not change this value above 1"
:group 'gnosis
:type 'float)
(defcustom gnosis-algorithm-ef-increase 0.1
"Increase ef increase value by this amount for every
`gnosis-algorithm-ef-frequency' number of successful reviews."
:group 'gnosis
:type 'float)
(defcustom gnosis-algorithm-ef-decrease 0.1
"Decrease ef decrease value by this amount for every
`gnosis-algorithm-ef-frequency' number of failed reviews."
:group 'gnosis
:type 'float)
(defcustom gnosis-algorithm-ef-frequency 3
"Frequency for updating ef increase and decrease values."
:group 'gnosis
:type 'integer)
(defun gnosis-algorithm-replace-at-index (index new-item list)
"Replace item at INDEX with NEW-ITEM in LIST."
(cl-loop for item in list
for i from 0
collect (if (= i index) new-item item)))
(defun gnosis-algorithm-round-items (list)
"Round all items in LIST to 2 decimal places."
(cl-loop for item in list
collect (/ (round (* item 100)) 100.0)))
(defun gnosis-algorithm-date (&optional offset)
"Return the current date in a list (year month day).
Optional integer OFFSET is a number of days from the current date."
(let* ((now (decode-time))
(now (list (decoded-time-month now)
(decoded-time-day now)
(decoded-time-year now))))
(let ((date (if (zerop (or offset 0))
now
(calendar-gregorian-from-absolute
(+ offset (calendar-absolute-from-gregorian now))))))
(list (nth 2 date) (nth 0 date) (nth 1 date)))))
(defun gnosis-algorithm-date-diff (date)
"Find the difference between the current date and the given DATE.
DATE format must be given as (year month day)."
(let ((given-date (encode-time 0 0 0 (caddr date) (cadr date) (car date))))
(- (time-to-days (current-time))
(time-to-days given-date))))
(cl-defun gnosis-algorithm-next-ef (&key ef success increase decrease frequency
c-successes c-failures)
"Returns the new EF, (increase-value decrease-value total-value)
Calculate the new e-factor given existing EF and SUCCESS, either t or nil.
Next EF is calculated as follows:
Upon a successful review, increase total value of ef by ef-increase
value.
Upon a failed review, decrease total ef by ef-decrease value (nth 1 ef).
For every FREQUENCY of C-SUCCESSES (consecutive successful reviews)
reviews, increase the ef increase value (first item) by INCREASE.
For every FREQUENCY of C-FAILURES reviews, decrease the ef decrease
value (second item) by DECREASE."
(let ((change-p (= (% (max 1 (if success c-successes c-failures)) frequency) 0))
(new-ef (if success (gnosis-algorithm-replace-at-index 2 (+ (nth 2 ef) (nth 0 ef)) ef)
(gnosis-algorithm-replace-at-index 2 (max 1.3 (- (nth 2 ef) (nth 1 ef))) ef))))
(cond ((and success change-p)
(setf new-ef (gnosis-algorithm-replace-at-index 0 (+ (nth 0 ef) increase) new-ef)))
((and (not success) change-p
(setf new-ef (gnosis-algorithm-replace-at-index 1 (+ (nth 1 ef) decrease) new-ef)))))
(gnosis-algorithm-round-items new-ef)))
(cl-defun gnosis-algorithm-next-interval (&key last-interval ef success successful-reviews
failure-factor initial-interval)
"Calculate next interval."
(cl-assert (< gnosis-algorithm-ff 1) "Value of `gnosis-algorithm-ff' must be lower than 1")
;; This should only occur in testing env or when the user has made breaking changes.
(cl-assert (> (nth 2 ef) 1) "Total ef value must be above 1")
(let* ((ef (nth 2 gnosis-algorithm-ef))
(interval (cond ((and (= successful-reviews 0) success)
(car initial-interval))
((and (= successful-reviews 1) success)
(cadr initial-interval))
(t (if success
(* ef last-interval)
(* failure-factor last-interval))))))
(gnosis-algorithm-date (round interval))))
(provide 'gnosis-algorithm)
;;; gnosis-algorithm.el ends here
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