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C port of SD, a very efficient flash cards app
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Anthony Wang fce879b5c5
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README.md

SDC

This is a C port of SD, a very efficient (and a tiny bit overengineered) flash cards app. SDC is finished, which means that no new features will be added, only bug fixes.

Usage

Flash cards are stored in the cards table of a SQLite database. There are four columns: idx INTEGER PRIMARY KEY, weight INTEGER, key STRING, val STRING. The idx is a unique index for each card, starting at 0. The weight is how often the card should come up. The key and value are the front and reverse sides of the card.

To create a card deck, use sqlite3 cards "CREATE TABLE cards (idx INTEGER PRIMARY KEY, weight INTEGER, key STRING, val STRING). You may be able to get twice as fast performance by enabling WAL with PRAGMA journal_mode=WAL because WAL only writes the content once instead of twice.

Now build this project with gcc sd.c segmenttree.c -o sd -lsqlite3 -O2 -march=native and run ./sd to enjoy a fast flash cards experience! The program will display the key of a randomly selected card. Press any key to show the val of the card. Now press either y or n depending on whether you got the card correct, and the program adjusts that card's weight.

If you're wondering where the name came from, this is the C port of SD, which was named after a common type of flash card. 😀

Performance

SD is designed to be extremely efficient and supports decks with hundreds of millions of flash cards. If N is the number of cards, initializing the program requires O(N) time and O(N) memory. Selecting a random card and adjusting its weight requires O(log N) time. Internally SD uses segment trees to achieve this time complexity.

Some benchmark results using 10 card updates (without WAL):

C version: ./sd < test
  Time (mean ± σ):      57.4 ms ±   4.5 ms    [User: 6.9 ms, System: 2.4 ms]
  Range (min … max):    51.9 ms …  70.9 ms    41 runs
Go version: ./sd < test
  Time (mean ± σ):      92.7 ms ±   6.8 ms    [User: 8.4 ms, System: 4.5 ms]
  Range (min … max):    79.4 ms … 108.9 ms    33 runs

The C port is about 30% faster than the original Go code.

Tips and tricks

  • View card deck: sqlite3 cards .dump
  • Get total number of cards: sqlite3 cards "SELECT COUNT(*) FROM cards"
  • Get total number of cards with positive weight: sqlite3 cards "SELECT COUNT(*) FROM cards WHERE weight>0"
  • Search for string in keys: sqlite3 cards "SELECT * FROM cards WHERE key LIKE '%hello%'"
  • Add card to deck: sd-add.fish
  • Edit deck: sqlitevi.fish or sqlitebrowser