Function getWhetherNonZero

quest/src/core/utilities.cpp:647–677

 */

bool getWhetherNonZero(qcomp* diags, qindex dim, qreal eps) {
    assert_utilsGivenNonZeroEpsilon(eps);

    /// @todo
    /// consider multithreading or GPU-accelerating this
    /// when caller is big and e.g. has GPU memory

    for (qindex i=0; i<dim; i++) {

        // check each complex element has non-zero abs
        if (isApprox(std::abs(diags[i]), 0, eps))
            return false;

        // note that calc-expec functions which assume
        // hermiticity will only consult the real component,
        // so its magnitude alone should determine divergence.
        // But alas an elem = eps*i will pass the above validation
        // yet also be validly Hermitian (imag <= eps), and cause
        // real(elem)=0 to be accepted within the matrix. This
        // will cause a divergence or divison-by-zero error when
        // the matrix is raised to a negative exponent; as this
        // function was supposed to detect and prevent! Fixing
        // this thoroughly would necessitate creating another
        // matrix field, separating when .absIsApproxNonZero and
        // .realIsApproxNonZero. But this is revolting and we 
        // simply accept the above strange scenario as a
        // non-validated dge-case.
    }

    return true;
}

// non-zeroness of fixed-size matrices is always computed afresh
bool util_isApproxNonZero(DiagMatr1 m, qreal eps) { return getWhetherNonZero(m.elems, m.numElems, eps); }