# chase-rs

An efficient implementation of the chase algorithm in Rust for advanced reasoning engines.

## Overview

The chase algorithm is a fundamental technique in database theory and knowledge representation used for:

- Query answering under tuple-generating dependencies (TGDs)
- Computing universal models
- Ontology-based data access (OBDA)
- Datalog with existential rules

This implementation provides a **restricted chase** that ensures termination even with existential rules by tracking applied triggers.

## Features

- **Core Data Types**: Terms, Atoms, Rules, Instances
- **Existential Quantification**: Automatic generation of labeled nulls
- **Restricted Chase**: Termination guarantees via trigger tracking
- **Fluent API**: `RuleBuilder` for readable rule construction
- **Zero Dependencies**: Pure Rust with no external runtime dependencies

## Quick Start

```rust
use chase_rs::{chase, Atom, Instance, Term};
use chase_rs::chase::rule::RuleBuilder;

// Create initial facts
let instance: Instance = vec![
    Atom::new("Parent", vec![Term::constant("alice"), Term::constant("bob")]),
    Atom::new("Parent", vec![Term::constant("bob"), Term::constant("carol")]),
].into_iter().collect();

// Define rules
// Parent(X, Y) -> Ancestor(X, Y)
let rule1 = RuleBuilder::new()
    .when("Parent", vec![Term::var("X"), Term::var("Y")])
    .then("Ancestor", vec![Term::var("X"), Term::var("Y")])
    .build();

// Ancestor(X, Y), Parent(Y, Z) -> Ancestor(X, Z)
let rule2 = RuleBuilder::new()
    .when("Ancestor", vec![Term::var("X"), Term::var("Y")])
    .when("Parent", vec![Term::var("Y"), Term::var("Z")])
    .then("Ancestor", vec![Term::var("X"), Term::var("Z")])
    .build();

// Run the chase
let result = chase(instance, &[rule1, rule2]);

assert!(result.terminated);
println!("Derived {} facts", result.instance.len());
```

## Existential Rules

Rules with head-only variables (existential quantification) automatically generate fresh labeled nulls:

```rust
// Every person has an SSN: Person(X) -> HasSSN(X, Y)
let rule = RuleBuilder::new()
    .when("Person", vec![Term::var("X")])
    .then("HasSSN", vec![Term::var("X"), Term::var("Y")]) // Y is existential
    .build();
```

## Project Structure

```
src/
├── lib.rs              # Library root and re-exports
├── main.rs             # Binary entry point
└── chase/
    ├── mod.rs          # Module exports
    ├── term.rs         # Terms: Constants, Nulls, Variables
    ├── atom.rs         # Atoms: Predicate(term1, term2, ...)
    ├── instance.rs     # Database instance (set of facts)
    ├── rule.rs         # TGDs with RuleBuilder
    ├── substitution.rs # Variable bindings and unification
    └── engine.rs       # Core chase algorithm
tests/
├── integration_tests.rs
├── regression_tests.rs
└── property_tests.rs
```

## Building and Testing

```bash
# Run all tests
cargo test

# Run with optimizations
cargo build --release

# Check for lint warnings
cargo clippy
```

## License

This project is licensed under [BSD-3](LICENSE).