omnitype 0.1.1

//! Constraint-based type inference engine.

use std::collections::HashMap;

use crate::error::Result;
use crate::types::{Type, TypeVar};

/// Represents a constraint between two types.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Constraint {
    /// Type equality constraint: T1 = T2
    Equal(Type, Type),

    /// Subtype constraint: T1 <: T2
    Subtype(Type, Type),

    /// Occurs check constraint (prevents infinite types)
    Occurs(TypeVar, Type),
}

/// A constraint solver for type inference.
pub struct ConstraintSolver {
    /// Type variable counter for generating fresh type variables
    next_var: u32,

    /// Set of constraints to solve
    constraints: Vec<Constraint>,

    /// Substitution mapping from type variables to types
    substitution: HashMap<TypeVar, Type>,
}

impl Default for ConstraintSolver {
    fn default() -> Self {
        Self::new()
    }
}

impl ConstraintSolver {
    /// Creates a new constraint solver.
    pub fn new() -> Self {
        Self { next_var: 0, constraints: Vec::new(), substitution: HashMap::new() }
    }

    /// Generates a fresh type variable.
    pub fn fresh_var(&mut self) -> TypeVar {
        let var = TypeVar(self.next_var);
        self.next_var += 1;
        var
    }

    /// Adds a new constraint to the solver.
    pub fn add_constraint(&mut self, constraint: Constraint) {
        self.constraints.push(constraint);
    }

    /// Solves the collected constraints and returns the substitution.
    pub fn solve(mut self) -> Result<HashMap<TypeVar, Type>> {
        while let Some(constraint) = self.constraints.pop() {
            self.solve_constraint(constraint)?;
        }
        Ok(self.substitution)
    }

    /// Solves a single constraint.
    fn solve_constraint(&mut self, constraint: Constraint) -> Result<()> {
        match constraint {
            Constraint::Equal(t1, t2) => self.unify(t1, t2)?,
            Constraint::Subtype(t1, t2) => self.subtype(t1, t2)?,
            Constraint::Occurs(var, ty) => self.occurs_check(var, &ty)?,
        }
        Ok(())
    }

    /// Unifies two types.
    fn unify(&mut self, _t1: Type, _t2: Type) -> Result<()> {
        // TODO: Implement unification algorithm
        Ok(())
    }

    /// Handles subtyping relationships.
    fn subtype(&mut self, _t1: Type, _t2: Type) -> Result<()> {
        // TODO: Implement subtyping rules
        Ok(())
    }

    /// Performs the occurs check to prevent infinite types.
    fn occurs_check(&self, _var: TypeVar, _ty: &Type) -> Result<()> {
        // TODO: Implement occurs check
        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_fresh_var() {
        let mut solver = ConstraintSolver::new();
        let var1 = solver.fresh_var();
        let var2 = solver.fresh_var();
        assert_ne!(var1, var2);
    }
}