function [D_rv, D_flux, DvDt, solutionPenalties, residualPenalties] = constructDiffOps(scheme, g, order, schemeParams, opSet, BCs)
    %% DiffOps for solution vector
    [D, solutionPenalties] = constructTotalFluxDiffOp(scheme, g, order, schemeParams, opSet, BCs);
    D2 = constructSymmetricD2Operator(g, order, opSet);

    if isa(D, 'function_handle')
        D_rv = @(v,viscosity)(D(v) + D2(viscosity))*v;
        [D_flux, residualPenalties] = constructTotalFluxDiffOp(scheme, g, max(order-2,2), schemeParams, opSet, BCs);
        D_flux = @(v) -D_flux(v)*v;
        DvDt = @(v)D(v)*v;
    else
        D_rv = @(v,viscosity)(D + D2(viscosity))*v;
        [D_flux, residualPenalties] = constructTotalFluxDiffOp(scheme, g, max(order-2,2), schemeParams, opSet, BCs);
        D_flux = @(v) -D_flux*v;
        DvDt = @(v)D*v;
    end

    %% DiffOps for residual viscosity
    
    % DiffOp for flux in residual viscosity. Due to sign conventions of the implemnted schemes, we need to
    % change the sign.
    
    % DiffOp for time derivative in residual viscosity
    
end

function [D, penalties] = constructTotalFluxDiffOp(scheme, g, order, schemeParams, opSet, BCs)
    diffOp = scheme(g, order, schemeParams{:}, opSet);
    if isa(diffOp.D, 'function_handle')
        [D, penalties]  = addClosuresToDiffOpFunction(diffOp, BCs);
    else
        [D, penalties]  = addClosuresToDiffOp(diffOp, BCs);
    end
end

function [D, penalties] = addClosuresToDiffOp(diffOp, BCs)
    penalties = cell(size(BCs));
    D = diffOp.D;
    for i = 1:size(BCs,1)
        for j = 1:size(BCs,2)
            [closure, penalties{i,j}] = diffOp.boundary_condition(BCs{i,j}.boundary, BCs{i,j}.type);
             D = D + closure;
        end
    end
    
end

function [D, penalties] = addClosuresToDiffOpFunction(diffOp, BCs)
    penalties = cell(size(BCs));
    D = diffOp.D;
    for i = 1:size(BCs,1)
        for j = 1:size(BCs,2)
            [closure, penalties{i,j}] = diffOp.boundary_condition(BCs{i,j}.boundary, BCs{i,j}.type);
            D = @(v) D(v) + closure(v);
        end
    end
    
end

function D2 = constructSymmetricD2Operator(g, order, opSet)
    m = g.size();
    ops = cell(g.D(),1);
    I = cell(g.D(),1);
    for i = 1:g.D()
       lim = {g.x{i}(1), g.x{i}(end)};
       ops{i} = opSet(m(i), lim, order);
       I{i} = speye(m(i));
    end

    switch g.D()
        case 1
            e_r = ops{1}.e_r;
            e_l = ops{1}.e_l;
            Hi = ops{1}.HI;
            B = e_r*e_r' - e_l*e_l';
            if isequal(opSet,@sbp.D1Upwind)
                Dm = ops{1}.Dm;
                Dp = ops{1}.Dp;
                HiB = Hi*B;
                D2 = @(viscosity) Dm*spdiag(viscosity)*Dp-HiB*spdiag(viscosity)*Dp;
            else
                D2 = @(viscosity)ops{1}.D2(viscosity);
            end
        case 2
            % TODO: 
            % Currently only implemented for upwind operators.
            % Remove this part once the time-dependent D2 operator is implemented for other opSets
            % or if it is decided that it should only be supported for upwind operators.
            assert(isequal(opSet,@sbp.D1Upwind))
            e_e = kron(ops{1}.e_r,I{2});
            e_w = kron(ops{1}.e_l,I{2});
            Dm_x = kron(ops{1}.Dm,I{2});
            Dp_x  = kron(ops{1}.Dp,I{2});
            H_x  = kron(ops{1}.HI,I{2});
            B_x = e_e*e_e' - e_w*e_w';
            H_xB = H_x*B_x;
            D2_x = @(viscosity) (Dm_x*spdiag(viscosity)-H_xB*spdiag(viscosity))*Dp_x;

            e_n = kron(I{1},ops{2}.e_r);
            e_s = kron(I{1},ops{2}.e_l);
            Dm_y = kron(I{1},ops{2}.Dm);
            Dp_y  = kron(I{1},ops{2}.Dp);
            H_y = kron(I{1},ops{2}.HI);
            B_y = e_n*e_n' - e_s*e_s';
            H_yB = H_y*B_y;
            D2_y = @(viscosity) (Dm_y*spdiag(viscosity)-H_yB*spdiag(viscosity))*Dp_y;
            D2 = @(viscosity)D2_x(viscosity) + D2_y(viscosity);
        otherwise
            error('3D not yet implemented')
    end
    D2 = @(viscosity) D2(viscosity);
end