# Limbo extension API
The `limbo_ext` crate simplifies the creation and registration of libraries meant to extend the functionality of `Limbo`, that can be loaded
like traditional `sqlite3` extensions, but are able to be written in much more ergonomic Rust.
---
## Currently supported features
- [ x ] **Scalar Functions**: Create scalar functions using the `scalar` macro.
- [ x ] **Aggregate Functions**: Define aggregate functions with `AggregateDerive` macro and `AggFunc` trait.
- [ x ] **Virtual tables**: Create a module for a virtual table with the `VTabModuleDerive` macro and `VTabCursor` trait.
- [] **VFS Modules**
---
## Installation
Add the crate to your `Cargo.toml`:
```toml
[features]
static = ["limbo_ext/static"]
[dependencies]
limbo_ext = { path = "path/to/limbo/extensions/core", features = ["static"] } # temporary until crate is published
# mimalloc is required if you intend on linking dynamically. It is imported for you by the register_extension
# macro, so no configuration is needed. But it must be added to your Cargo.toml
[target.'cfg(not(target_family = "wasm"))'.dependencies]
mimalloc = { version = "*", default-features = false }
# NOTE: Crate must be of type `cdylib` if you wish to link dynamically
[lib]
crate-type = ["cdylib", "lib"]
```
`cargo build` will output a shared library that can be loaded by the following options:
#### **CLI:**
`.load target/debug/libyour_crate_name`
#### **SQL:**
`SELECT load_extension('target/debug/libyour_crate_name')`
Extensions can be registered with the `register_extension!` macro:
```rust
register_extension!{
scalars: { double }, // name of your function, if different from attribute name
aggregates: { Percentile },
vtabs: { CsvVTable },
}
```
### Scalar Example:
```rust
use limbo_ext::{register_extension, Value, scalar};
/// Annotate each with the scalar macro, specifying the name you would like to call it with
/// and optionally, an alias.. e.g. SELECT double(4); or SELECT twice(4);
#[scalar(name = "double", alias = "twice")]
fn double(&self, args: &[Value]) -> Value {
if let Some(arg) = args.first() {
match arg.value_type() {
ValueType::Float => {
let val = arg.to_float().unwrap();
Value::from_float(val * 2.0)
}
ValueType::Integer => {
let val = arg.to_integer().unwrap();
Value::from_integer(val * 2)
}
}
} else {
Value::null()
}
}
```
### Aggregates Example:
```rust
use limbo_ext::{register_extension, AggregateDerive, AggFunc, Value};
/// annotate your struct with the AggregateDerive macro, and it must implement the below AggFunc trait
#[derive(AggregateDerive)]
struct Percentile;
impl AggFunc for Percentile {
/// The state to track during the steps
type State = (Vec<f64>, Option<f64>, Option<String>); // Tracks the values, Percentile, and errors
/// Define the name you wish to call your function by.
/// e.g. SELECT percentile(value, 40);
const NAME: &str = "percentile";
/// Define the number of expected arguments for your function.
const ARGS: i32 = 2;
/// Define a function called on each row/value in a relevant group/column
fn step(state: &mut Self::State, args: &[Value]) {
let (values, p_value, error) = state;
if let (Some(y), Some(p)) = (
args.first().and_then(Value::to_float),
args.get(1).and_then(Value::to_float),
) {
if !(0.0..=100.0).contains(&p) {
*error = Some("Percentile P must be between 0 and 100.".to_string());
return;
}
if let Some(existing_p) = *p_value {
if (existing_p - p).abs() >= 0.001 {
*error = Some("P values must remain consistent.".to_string());
return;
}
} else {
*p_value = Some(p);
}
values.push(y);
}
}
/// A function to finalize the state into a value to be returned as a result
/// or an error (if you chose to track an error state as well)
fn finalize(state: Self::State) -> Value {
let (mut values, p_value, error) = state;
if let Some(error) = error {
return Value::custom_error(error);
}
if values.is_empty() {
return Value::null();
}
values.sort_by(|a, b| a.partial_cmp(b).unwrap());
let n = values.len() as f64;
let p = p_value.unwrap();
let index = (p * (n - 1.0) / 100.0).floor() as usize;
Value::from_float(values[index])
}
}
```
### Virtual Table Example:
```rust
/// Example: A virtual table that operates on a CSV file as a database table.
/// This example assumes that the CSV file is located at "data.csv" in the current directory.
#[derive(Debug, VTabModuleDerive)]
struct CsvVTable;
impl VTabModule for CsvVTable {
type VCursor = CsvCursor;
/// Declare the name for your virtual table
const NAME: &'static str = "csv_data";
/// Declare the table schema and call `api.declare_virtual_table` with the schema sql.
fn connect(api: &ExtensionApi) -> ResultCode {
let sql = "CREATE TABLE csv_data(
name TEXT,
age TEXT,
city TEXT
)";
api.declare_virtual_table(Self::NAME, sql)
}
/// Open to return a new cursor: In this simple example, the CSV file is read completely into memory on connect.
fn open() -> Self::VCursor {
// Read CSV file contents from "data.csv"
let csv_content = fs::read_to_string("data.csv").unwrap_or_default();
// For simplicity, we'll ignore the header row.
let rows: Vec<Vec<String>> = csv_content
.lines()
.skip(1)
.map(|line| {
line.split(',')
.map(|s| s.trim().to_string())
.collect()
})
.collect();
CsvCursor { rows, index: 0 }
}
/// Filter through result columns. (not used in this simple example)
fn filter(_cursor: &mut Self::VCursor, _arg_count: i32, _args: &[Value]) -> ResultCode {
ResultCode::OK
}
/// Return the value for the column at the given index in the current row.
fn column(cursor: &Self::VCursor, idx: u32) -> Value {
cursor.column(idx)
}
/// Next advances the cursor to the next row.
fn next(cursor: &mut Self::VCursor) -> ResultCode {
if cursor.index < cursor.rows.len() - 1 {
cursor.index += 1;
ResultCode::OK
} else {
ResultCode::EOF
}
}
/// Return true if the cursor is at the end.
fn eof(cursor: &Self::VCursor) -> bool {
cursor.index >= cursor.rows.len()
}
}
/// The cursor for iterating over CSV rows.
#[derive(Debug)]
struct CsvCursor {
rows: Vec<Vec<String>>,
index: usize,
}
/// Implement the VTabCursor trait for your cursor type
impl VTabCursor for CsvCursor {
fn next(&mut self) -> ResultCode {
CsvCursor::next(self)
}
fn eof(&self) -> bool {
self.index >= self.rows.len()
}
fn column(&self, idx: u32) -> Value {
let row = &self.rows[self.index];
if (idx as usize) < row.len() {
Value::from_text(&row[idx as usize])
} else {
Value::null()
}
}
fn rowid(&self) -> i64 {
self.index as i64
}
}
```