Seperate gnosis-algorithm from gnosis.el

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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
+
+;; Package-Requires: ((emacs "27.2") (compat "29.1.4.2"))
+
+;; 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-interval '(1 3)
+  "Gnosis initial interval.
+
+Interval by which a new question is displayed or when it's ef is at 1.3.
+
+First item: First interval
+Second item: Second interval."
+  :group 'gnosis
+  :type 'list)
+
+(defcustom gnosis-algorithm-ef '(0.3 0.3)
+  "Gnosis easiness factor.
+
+First item : Increase factor
+Second item: Decrease factor"
+  :group 'gnosis
+  :type 'list)
+
+(defcustom gnosis-algorithm-ff 0.5
+  "Gnosis forgetting factor.
+
+Used to calcuate new interval for failed questions."
+  :group 'gnosis
+  :type 'float)
+
+
+(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 (year month day)
+  "Find the difference between the current date and the given date.
+
+The structure of the given date is (YEAR MONTH DAY)."
+  (let ((current-date (gnosis-algorithm-date))
+	(given-date (encode-time 0 0 0 day month year)))
+    (- (time-to-days (current-time))
+       (time-to-days given-date))))
+
+(defun gnosis-algorithm-e-factor (ef quality)
+  "Calculate new e-factor given existing EF and binary QUALITY, 0 or 1."
+  (cond
+   ((not (numberp quality))
+    (error "Invalid argument passed to gnosis-algorithm-e-factor"))
+   ((= quality 0) ;; If the quality score is 0 (fail), decrease the ef by a small penalty
+    (max 1.3 (- ef (cadr gnosis-ef))))
+   ((= quality 1) ;; If the quality score is 1 (pass), increase the ef by a small reward
+    (+ ef (car gnosis-ef)))
+   (t (error "Invalid quality score passed to gnosis-algorithm-e-factor"))))
+
+(defun gnosis-algorithm-next-interval (last-interval n ef success ff)
+  "Calculate next interval.
+- LAST-INTERVAL : The number of days since the item was last reviewed.
+- N : Number of times the item has been reviewed.
+- EF : Easiness Factor.
+- SUCCESS : Success of the recall, ranges from 0 (unsuccessful) to 1
+  (successful).
+- FF: Failure factor
+
+Returns a tuple: (INTERVAL N EF) where,
+- INTERVAL : The number of days until the item should next be reviewed.
+- N : Incremented by 1.
+- EF : Modified based on the recall success for the item."
+  (cl-assert (and (>= success 0)
+		  (<= success 1)))
+  ;; Calculate the next easiness factor.
+  (let* ((next-ef (gnosis-algorithm-e-factor ef success))
+         ;; Calculate the next interval.
+         (interval
+          (cond
+	   ;; Show item same day on the first review
+	   ((= n 0) 0)
+           ;; Immediately next day if it's the first time review.
+           ((<= n 1) (car gnosis-interval))
+           ;; After 3 days if it's second review.
+           ((= n 2) (cadr gnosis-interval))
+           ;; Increase last interval by 1 if recall was successful. Keep last interval if unsuccessful.
+           (t (if (= success 1)
+                  (* ef last-interval)
+                (* ff last-interval))))))
+    (list (round interval) (1+ n) next-ef)))
+
+(provide 'gnosis-algorithm)
+;;; gnosis-algorithm.el ends here