* Org Flashcards
Spaced-repetition system for use with Emacs org-mode.

This package should be considered *work in progress*.  I use it on a
daily basis but the API regularly changes in breaking ways.

I still need to write proper setup instructions.
For now, feel free to look around for pieces of code that might be
useful to you.

#+CAPTION: Review Demo
[[file:images/review.png]]

** Introduction
In the most abstract sense, this package deals with

1. Attaching timestamped review information to headlines
2. Querying all headings where reviews are due
3. Reviewing due *positions* of headings

As mentioned in step 3, a heading can have multiple *positions*,
e.g. to implement cloze-deletions where multiple items are reviewed
independently from each other.

In the reviewing step, display functions can be registered by card
type. This allows easy addition of user-defined card types without
having to think about storing and updating review data.

Review functions are called with point on the headline of the card
that should be reviewed and get passed a single argument,
the position to be reviewed.

They are expected to return either ~'quit~ to end the review or one of
~'again~, ~'hard~, ~'good~, ~'ease~, to rate the card.

While the primary application is learning information using spaced
repetition, at the end, the API should be flexible enough to implement
other kinds of repeating tasks where it is necessary to store data in
addition to the next date.

One example would be storing one exercise per heading, using the
positions to store one or more sets and logging the number of
repetitions done on each "review".

** Updating the Card Format
I hope the current card / log format is flexible enough to accommodate
upcoming changes.

In case a update to the org sources is needed, I'll add a changelog
entry with updating instructions.
** Prior Art
There are a few other packages for implementing a SRS based on org-mode.

The biggest difference between this package and the ones I've found so
far:

1. Use of =awk= for quickly finding cards due for review
2. Support for multiple *positions* in a card

Below, I've listed a few packages that are actively maintained and
implement a lot of useful functionality.

- [[https://gitlab.com/phillord/org-drill/][phillord/org-drill]]
- [[https://github.com/abo-abo/pamparam][abo-abo/pamparam]]

Thanks to the maintainers and all contributors for their work on these
packages!

*** TODO Mention supermemo, anki, memosyne
** Performance
All user-facing commands (especially during review) should be as fast
as possible (<300ms).

Using the =awk= indexer, searching 2500 org files (~200k lines in
total) for due flashcards takes around ~500ms on my laptop (Thinkpad
L470, SSD).

Using the lisp indexer based on ~org-map-entries~,
searching a single 6500 line file with 333 flashcards takes ~1000ms,
indexing the same file with =awk= takes around ~50ms.
** Design Goals
*** Easy Implementation of Custom Card Types
** Design Choices
*** All Relevant Data Kept Org Files
For easy version control
*** Multiple Cards per Org-Mode Heading
*** Review Directly on Org Files
Reviewing cards is done directly on their org source file
(instead of storing pre-processed/generated cards in a separate folder
or in a database).
*** Timestamps Format
The org-mode default timestamp format does not include timezone
information and only hours and minutes of the time are stored.

Because it includes the abbreviate name of the day (~%a~),
timestamps can't be compared to each other using string comparison.

They also include spaces which makes it hard to parse the timestamps
in the review data table in awk..

To avoid these issues, all timestamps added or used by org-fc are
ISO8601 formatted (e.g. =2020-01-15T11:58:12=) using *UTC0* as the
timezone.
*** Use of [[https://github.com/abo-abo/hydra][hydra]]
To keep this package as small as possible, [[https://github.com/abo-abo/hydra][abo-abo/hydra]] is used
for showing statistics and hotkeys during review.

The main benefit of this is that it saves us from writing a lot of
(hard) window / split / buffer management code.

As a consequence, if you want to add your own hotkeys to one of the
hydras used by this package, you'll have to write a new one instead of
being able to redefine keys / add keys in an existing one.
** Getting Started
Before using this package, a few variables have to be set:

- ~org-fc-source-path~ :: should be set to the absolute path of the
- ~org-fc-directories~ :: list of directories to search for flashcards
  cloned repository
- ~org-fc-review-history-file~ :: where to store the review history

Here is how that could look like using [[https://github.com/jwiegley/use-package/][use-package]]:

#+begin_src emacs-lisp
  (use-package org-fc
    :load-path "~/src/org-fc"
    :config
    (setq org-fc-source-path "~/src/org-fc/")
    (setq org-fc-directories '("~/org"))
    (setq org-fc-review-history-file "~/org/fc_reviews.tsv"))
#+end_src
*** Default Hydra
[[file:org-fc-hydra.el]] defines a hydra for accessing commonly used
org-fc commands and for marking headlines as flashcards.

It can be loaded and bound to a hotkey like this:

#+begin_src emacs-lisp
  (require 'org-fc-hydra)
  (global-set-key (kbd "C-c f") 'org-fc-hydra/body)
#+end_src
*** TODO Demo File
A file demonstrating all card types is included.
~M-x org-fc-demo~ starts a review of this file.

Note that the review data of the cards in this file *is not updated*.
** Marking Headlines as Cards
A *card* is an org-mode headline with a =:fc:= tag attached to it.
Each card can have multiple *positions* reviewed independently from
each other, e.g. one for each hole of a cloze card.

Review data (ease, interval in days, box, due date) is stored in a table
in a drawer inside the card.

#+begin_src org
  :REVIEW_DATA:
  | position | ease | box | interval | due                    |
  |----------+------+-----+----------+------------------------|
  |        2 | 2.65 |   6 |   107.13 |    2020-04-07T01:01:00 |
  |        1 | 2.65 |   6 |   128.19 |    2020-04-29T06:44:00 |
  |        0 | 2.95 |   6 |   131.57 |    2020-04-30T18:03:00 |
  :END:
#+end_src

Review results are appended to a csv file to avoid cluttering the org
files.

Each card needs at least two properties, an *unique* ~:ID:~ and a
~:FC_TYPE:~.  In addition to that, the date a card was created
(i.e. the headline was marked as a flashcard) is stored to allow
making statistics for how many cards were created in the last day /
week / month.

#+begin_src org
  :PROPERTIES:
  :ID:       4ffe66a7-7b5c-4811-bd3e-02b5c0862f55
  :FC_TYPE:  normal
  :FC_CREATED: 2019-10-11T14:08:32
  :END:
#+end_src

Card types (should) implement a ~org-fc-type-...-init~ command that
initializes these properties and sets up the review data drawer

All timestamps created and used by org-flashcards use ISO8601 format
with second precision and without a timezone (timezone UTC0).

This prevents flashcard due dates from showing up in the org-agenda
and allows filtering for due cards by string-comparing a timestamp
with one of the current time.
** Review
Reviewing cards is done by opening the file the card is in,
using a special narrowing function to hide other headings
and drawers.

With ~(point)~ on the headline to be reviewed,
the setup function for this card type is called
(e.g. to hide the cloze holes of the card).

Then the flip function for the card type is called,
usually opening a *hydra* showing available hotkeys.

Once the card is flipped, another hydra for rating the card is shown.

A review session can be started using ~org-fc-review-all~
to review all cards that are due, or using ~org-fc-review-buffer~ to
review only cards in the current buffer.

The current review session can be ended / reset using
~org-fc-review-quit~.

Ideally, don't use any other hotkeys while in a review session.
This exits the review hydra without ending the current review session
making it necessary to do so manually (~org-fc-review-quit~).

*** Display of Cards during Review
TODO: Add image

Headlines are presented for review by hiding the all top level
headings before and after the one the heading to be reviewed is
located in.

This is done through the function ~org-fc-org-narrow-tree~.
~org-fc-show-all~ can be used to remove all overlays (i.e. reset the
display of the buffer).

All parent headings are shown but their body text (~section~) is
hidden.

If the file has a ~#+TITLE:~ keyword this is shown, too.

To hide the title during review (e.g. for a "Definition" flashcard),
add a ~:notitle:~ tag to the heading.

To hide the heading text of the current card during review, add a
~:noheading:~ tag.
*** Implementation of Card Review
Review is implemented by storing due cards in a global variable.  The
buffer the card is displayed in never leaves =org-mode=, [[https://github.com/abo-abo/hydra][abo-abo/hydra]]
is used to show review statistics (number of cards remaining, percent
again/hard/good/easy) and prompt for user actions.

1. jump to the file + id of the current card
2. set it up for review (i.e. hiding parts of the buffer)
3. open a hydra prompting to flip the card
4. flip the card or quit the review session
5. open a hydra prompting for a rating
6. rate the card or quit the review session
7. set the current card to the next card due
8. continue at 1.

If an error occurs during review, ~org-fc-review-quit~ can be used to
reset the current buffer and the review state.
** (Un)suspending Cards
Cards can be suspended (excluded from review) by adding a =suspended=
tag, either by hand or using the ~org-fc-suspend-card~ command.

All cards in the current buffer can be suspended using the
~org-fc-suspend-buffer~ command.

The reason for using a per-headline tag instead of a file keyword is
that this way cards stay suspended when moved to another buffer.

Cards can be un-suspended using the ~org-fc-unsuspend-card~ and
~org-fc-unsuspend-buffer~ commands.

If the card being unsuspended was not due for review yet,
or was due less than 10% of its interval ago, its review data is not
reset. If it was due by more than that, its review data is reset to
the initial values.
** Statistics
~org-fc-dashboard~ shows a buffer with statistics for review performance
and cards / card types.
*** TODO Replace with R scripts run on the review history / card index
*** Review History
The review history is stored in a tsv file, to avoid cluttering org
files. This makes it easy to calculate review statistics.

At first, I used an org drawer to store the review history but that
added to much overhead to the files (in one instance 6.5k lines of
review history for a file of 9.5k lines in total).

Columns:
1. Date in ISO8601 format, second precision
2. Filename
3. Card ID
4. Position
5. Ease (before review)
6. Box (before review)
7. Interval (before review)
8. Rating

More advanced review algorithms might need to use the review history
of a card. In this case, the card ID + position should be used to look
up the review history, as the filename can change when moving cards
from file to file.
** Card Types
*** Normal Cards
During review, the heading is shown with its "Back" subheading
collapsed, when flipping the card, the back heading is shown,
then the user is asked to rate the review performance.

Positions: =front=
*** Text-Input Cards
On review, the user is asked to type in a string which is then
compared to the one stored in the ~:ANSWER:~ property of the card.

Positions: =front=
*** Double Cards
Similar to normal cards, but reviewed both in the "Front -> Back"
direction and in the "Back -> Front" direction.

Positions: =front=, =back=
*** Cloze Cards
The cards text contains one or more *holes*.  During review, one hole
is hidden while the text of (some) remaining ones is shown.

Flipping the card reveals the text of the hidden hole,
using ~org-fc-type-cloze-hole-face~ to highlight it.

Card titles can contain holes, too.

Positions: =0=, =1=, ...

Cloze cards can have a number of sub-types.

**** TODO Document type-specific properties
**** TODO Implement & document type-changing functions
**** Deletion ~'deletion~
Only one hole is hidden.
**** Enumerations ~'enumeration~
All holes *behind* the currently review one are hidden, too.

Useful for memorizing lists where the order of items is important.
**** Context ~'context~
Holes ~org-fc-type-cloze-context~ (default 1) around the currently
reviewed one are shown.

Useful for memorizing longer lists where the order of items is important.
**** Hole Syntax
Deletions can have the following forms

- ~{{text}}~
- ~{{text}@id}~
- ~{{text}{hint}}~
- ~{{text}{hint}@id}~

~text~ should not contain any "}",
unless it is part of a ~$latex$~ block.
In this case, ~latex~ should not contain any "$".

Holes *inside* latex blocks are not handled correctly at the moment.
As a workaround, create multiple smaller latex blocks and wrap each in
a hole.
*** TODO Listening Cards
When reviewing the card, an audio file is played.
Flipping the card, a transcription / translation is revealed.

Useful for learning to understand sentences spoken in a foreign
language.
*** Compact Cards
For cards without a "Back" heading, the headline text is considered as
the front, the main text as the back.

This is useful for cards with a short front text, e.g. when learning
definitions of words.
*** Defining Own Card Types
To define a custom card type,
you need to implement three functions:

- ~(...-init)~ to initialize a heading as a flashcard of this type,
  setting up the cards properties & review data.
  Should be marked as ~(interactive)~.
- ~(...-setup position)~ to setup ~position~ of the card for review
- ~(...-flip)~ to flip the card
- ~(...-update)~ to update the review data of the card, e.g. if a new
  hole is added to a cloze card

All of these are called with ~(point)~ on the cards heading.

Take a look at the =org-fc-type-<name>.el= files to see how these
functions could be implemented.
** TODO Custom Review Spacing Algorithms                          :longterm:
The interfaces defined by this package should be flexible enough to
allow implementing custom review spacing algorithms.

This is not possible at the moment because the awk scripts and the
functions for reading / updating the review data drawer make strong
assumptions about the format of the review data.

A good implementation of this should allow using different spacing
algorithms based on a ~:FC_SPACING:~ property in the card.
** TODO Sharing Decks                                             :longterm:
It should be possible to share sets of cards by removing the review
data and syncing them with git.

At least one of the existing emacs flashcard packages implements this
functionality.
** Incremental Reading
- [[https://github.com/alphapapa/org-web-tools]]
*** TODO Supermemo link
** Internals
If your not interested in implementing your own card types or
contributing to this package, you can skip this section.

*** Components
**** =org-fc.el=
Main file.
**** =org-fc-dashboard.el=
Dashboard displaying card / position / review statistics.
**** =org-fc-review.el=
Functions related to reviewing cards, updating the review data drawer
and logging review results.
**** =org-fc-sm2.el=
Implementation of the [[https://www.supermemo.com/en/archives1990-2015/english/ol/sm2][SM2]] review spacing algorithm,
modified to behave like the algorithm used by [[https://apps.ankiweb.net/docs/manual.html#what-algorithm][Anki]].

It uses four ratings (again, hard, good, easy) instead of the six used
in the supermemo variant.

The first few reviews are done in fixed intervals
(0.01 days / approx 15 minutes, 1 day, 6 days).

After these intervals, reviews are scheduled by multiplying the cards
current interval with its ease (initially 2.5, bound to be >= 1.3 and
<= 5.0), then multiplying a random factor ~1 to avoid "chunking" of
flashcards due for review.

All of these parameters can be configured using the variables defined
in =org-fc-sm2.el=.
**** =org-fc-awk.el=
Functions for interacting with the awk indexer / filter / stats scripts.
**** =org-fc-overlay.el=
Functions for hiding / revealing parts of org-mode buffers.
**** =org-fc-type-<name>.el=
Implementations of flashcard types, for more details, see the "Card
Types" section of this document.
**** TODO Document core api of each file
*** Coding Style
Components are split into multiple smaller files,
with each function prefixed by the files base-name.

Public functions are named ~basename-functionname~,
internal helper functions are named ~basename--functionname~.
*** Testing
Unit-testing is done using ~org-fc-assert-...~ macros
defined in =org-fc-assert.el=.

These assertions are placed right after the function definition
and run when the file is loaded. If an assertion fails,
an ~'org-fc-assertion-error~ is raised.

**** TODO Integration Testing
Integration testing is done by providing an input org file, a set of
operations to be performed on it and an org file with the expected
output.

Tests are run by copying the input file to a temporary file, executing
the operations on it, then comparing it to the expected output.

Files for this live in the =fixtures/= folder.
*** dash.el
The code in this package uses [[https://github.com/magnars/dash.el#threading-macros][threading macros]] and list functions
(often in their anaphoric form) from [[https://github.com/magnars/dash.el][magnars/dash.el]].

Make sure to read that documentation before going reading / working on
the source code.
*** =awk=
~find~ is used to generate a list of =.org= files in
~org-fc-directories~, these are then passed to =awk= scripts
to generate lists of cards and card-positions.

Only files starting with ~[a-Z0-9_]~ and a ~.org~ extension are
indexed to exclude temp / hidden files.
This can be customized with the ~org-fc-find-name~ variable.

[[https://www.gnu.org/software/gawk/][gawk]] is a programming language for processing / parsing text.

Assuming the input org files are well formatted, they can be
efficiently parsed using regexeps and a small number of state
variables.

=awk= scripts in this package come in three types:

1. Indexing, for generating lists of cards / positions
3. Filtering, e.g. for selecting only unsuspended cards due now
2. Aggregation, for generating statistics from these lists

- =awk/indexer_cards.awk= :: list all card headings
- =awk/indexer_positions.awk= :: list all card positions
- =awk/filter_due.awk= :: select only unsuspended cards due right now
- =awk/stats_cards.awk= :: stats over cards
- =awk/stats_positions.awk= :: stats over positions
- =awk/stats_reviews.awk= :: stats over the reviews tsv file

These scripts use the =gawk= version of =awk= which should be
available on any modern Linux / UNIX distribution.

Configurable tags and properties can be passed to the indexer scripts as
variables. If a tag or property is not passed to the script,
a default value is used.

*** Format
Output is generated in *tab separated* form and *does not* include a
header with column names. For the indexing scripts, the first two
columns are the filename and the ID of the heading.

The ~org-fc-tsv-parse~ function can be used to parse a tsv
string into a plist, given a list of headers with optional type
specifications.

=0= (false) and =1= (true) are used for boolean values (e.g. for the
"suspended" column).

Dates are converted to ISO-8601 format, no timezone, minute-precision
(e.g. =2019-10-09T16:49=).

Unlike the format used by org mode, timestamps in ISO-8601 format can
be compared lexicographically.

Processing script output *tab separated* key-value pairs with no header.