Function sparsa 

fn sparsa(
    x: &mut [Complex64],
    y: &[f64],
    inside: &[bool],
    tau: f64,
    max_iter: usize,
    apply_core: &mut dyn FnMut(&mut [Complex64]),
    progress: impl FnMut(usize, usize),
) -> usize

SpaRSA 2.0 at STI’s options (module docs, step 4) for A x = op(M x), Aᵀ = A, with y zero outside inside. x holds the initial guess on entry and the solution on return. apply_core applies op in place. Returns the number of iterations (0 if τ ≥ max|A y|, when x is set to zero as SpaRSA does).