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# SDC
This is a C port of [SD](https://git.exozy.me/a/SD), a very efficient (and a tiny bit overengineered) flash cards app.
## 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. You can use the `sqlite3` CLI to create a card deck.
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](https://git.exozy.me/a/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](test.py). 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](https://en.wikipedia.org/wiki/Segment_tree) to achieve this time complexity.
Some benchmark results using 10 card updates:
```
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, and I'm currently working on further performance improvements using delayed database writes.
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