fix(vanishing_poly): compute x^2 - h^2 when dim_h == 1 in evaluate_constraints

[phrwlk]

• Align constraint evaluation with native evaluation for dim_h == 1 by returning x^2 - h^2 instead of 0.
• Add a unit test constraints_test_dim1 in src/poly/domain/vanishing_poly.rs.
• Remove unused import ark_std::ops::Sub to silence the warning.

Test/build pass

[Pratyush]

I think you can just remove this if statement, no?

[phrwlk]

I think you can just remove this if statement, no?

Yes sir

[Pratyush]

Thank you, could you add a test for this setting?

[phrwlk]

Thank you, could you add a test for this setting?

Sorry, but the test is already there?

#[test]
fn constraints_test_dim1() {
    let mut rng = test_rng();
    let offset = Fr::rand(&mut rng);
    let cs = ConstraintSystem::new_ref();
    let x = Fr::rand(&mut rng);
    let x_var = FpVar::new_witness(ns!(cs, "x_var"), || Ok(x)).unwrap();
    let vp = VanishingPolynomial::new(offset, 1);
    let native = vp.evaluate(&x);
    let result_var = vp.evaluate_constraints(&x_var).unwrap();
    assert!(cs.is_satisfied().unwrap());
    assert_eq!(result_var.value().unwrap(), native);
}

[Pratyush]

Ah you're right, thanks!