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use nalgebra::{self, DVector};
use nalgebra_sparse::{
    ops::{serial::spmm_csr_dense, Op},
    CsrMatrix,
};

#[inline]
pub fn compute_gr_update(
    a: &CsrMatrix<f64>,
    residue: &DVector<f64>,
    x: &mut DVector<f64>,
    a_r: &mut DVector<f64>,
) {
    let num = residue.dot(residue);

    // a_r only used to store A*r result without allocating new vector
    spmm_csr_dense(0.0, &mut *a_r, 1.0, Op::NoOp(a), Op::NoOp(residue));
    let den = residue.dot(a_r);

    let alpha = num / den;
    x.axpy(alpha, residue, 1.0);
}