def cg(matvec, b, x0, disp=False, tol=1e-5, maxiter=None):
    hist = {'norm_r': []}
    if disp:
        print('%9s' % 'norm_r')

    def write_hist(hist, norm_r):
        hist['norm_r'].append(norm_r)

    def callback(x):
        r = matvec(x) - b
        norm_r = norm(r, np.inf)
        write_hist(hist, norm_r)
        if disp:
            print('%9.8f' % norm_r)

    callback(x0)
    xk = x0
    rk = matvec(xk) - b
    dk = -rk
    cnt_iter = 0

    while norm(rk, np.inf) > tol:
        alpha_k = rk.dot(rk) / dk.dot(matvec(dk))
        xk = xk + alpha_k * dk
        rk_new = rk + alpha_k * matvec(dk)
        beta_k = rk_new.dot(rk_new) / rk.dot(rk)
        rk = rk_new
        dk = -rk + beta_k * dk
        cnt_iter += 1
        callback(xk)
        if maxiter is not None and cnt_iter > maxiter:
            break

    return xk, hist