oboeru - 覚える

A collection of simple tools for reviewing flashcards.

Inspired by oboeta.

Just use Anki

Anki was always a little bit excessive but it has only gotten worse as time goes on. Here are some key problems with it:

• Cards are stored in a binary database.

• Relies on tons of terrible software such as qtwebengine.

• Build system relies on java despite there not being a single line of java in Anki's code base.

• Newer versions are not portable because of the terrible build system.

• Anki's buttons for rating cards can have unintended consequences.

There is really no reason for flashcards to be this complicated.

Solution

oboeru solves this problem by not caring about how you store your cards. In fact all it does is maintain a ledger of card ids, their creation date, their due date, and the number of times the card has acted as a leech. Everything else is completely up to you.

What does oboeru do then?

oboeru reads supplied ledgers, finds cards up for review, shuffles them, and finally prints out the originating ledgers name and card id separated by a tab to stdout. Then it waits for commands provided via a named pipe. Depending on the response it will update the current card and print the next card to stdout or rewrite the ledgers and exit.

Algorithm

oboeru uses an algorithm inspired by the simplicity of the Leitner system but still drawing on some of the lessons of the SuperMemo SM-2 algorithm. Specifically a card can only be passed or failed and it's interval will always be changed by a fixed growth or shrink rate. Like in Anki, cards can become leeches from too many failures. The relevant code is as follows (some parts omitted):

static void
bump_card(Card *card, int status)
{
	diff = card->due - card->reviewed;
	switch (status) {
	case CARD_PASS:
		if (diff < MINIMUM_INCREASE)
			card->due += MINIMUM_INCREASE;
		else
			card->due += diff * GROWTH_RATE;
		break;
	case CARD_FAIL:
		if (diff > LEECH_AGE)
			card->leeches++;
		card->due += diff * SHRINK_RATE;
	}
	card->reviewed = time(NULL);
}

Take for example a card with an interval of 6 days, GROWTH_RATE = 1.8, and SHRINK_RATE = 0.66 (simplified for readability):

bump_card(card, CARD_PASS)
	card->due = +10.8 days
bump_card(card, CARD_FAIL)
	card->due = +4 days

Ledger Format

The format for storing the ledgers is 1 card per line with fields tab separated. An example follows:

# id	reviewed		due			leeches	extra
23	2021年07月09日16時18分	2021年08月09日05時01分	3	全て
0099	2021年07月09日14時18分	2021年08月11日14時19分	0	飲み込む

The time formatting is configurable via config.h. The extra field can be used for whatever, for example to check for duplicates. It can contain any character besides \0 and \n.

Installation

Modify config.h and config.mk to your liking and use the following to install (using root as needed):

make clean install

Example Usage

See omoidasu for one example of how to use these programs.