classdef D4Variable < sbp.OpSet
    properties
        m    % Number of grid points.
        h    % Step size
        x    % grid
        H    % Norm matrix
        HI   % H^-1
        D1   % SBP operator approximating first derivative
        D2   % SBP operator for second derivative
        D4   % SBP operator for fourth derivative
        Q    % Skew-symmetric matrix
        M    % Norm matrix, second derivative
        M4   % Norm matrix, fourth derivative
        e_l,  e_r  % Left and right boundary operator
        d1_l, d1_r % Left and right boundary first derivative
        d2_l, d2_r % Left and right boundary second derivative
        d3_l, d3_r % Left and right boundary third derivative
        borrowing % Struct with borrowing limits for different norm matrices
    end

    methods
        function obj = D4Variable(m, lim, order)
            x_l = lim{1};
            x_r = lim{2};
            L = x_r-x_l;
            obj.h = L/(m-1);
            obj.x = linspace(x_l, x_r,m)';

            if order == 2
                [H, HI, D1, D2, D4, e_l, e_r, M4, d2_l, d2_r, d3_l, d3_r, d1_l, d1_r] = ...
                    sbp.implementations.d4_variable_2(m, obj.h);
                obj.borrowing.N.S2 = 1.2500;
                obj.borrowing.N.S3 = 0.4000;
            elseif order == 4
                [H, HI, D1, D2, D4, e_l, e_r, M4, d2_l, d2_r, d3_l, d3_r, d1_l, d1_r] = ...
                    sbp.implementations.d4_variable_4(m, obj.h);
                obj.borrowing.N.S2 = 0.5055;
                obj.borrowing.N.S3 = 0.9290;
            elseif order == 6
                [H, HI, D1, D2, D4, e_l, e_r, M4, d2_l, d2_r, d3_l, d3_r, d1_l, d1_r] = ...
                    sbp.implementations.d4_variable_6(m, obj.h);
                obj.borrowing.N.S2 = 0.3259;
                obj.borrowing.N.S3 = 0.1580;
            else
                error('Invalid operator order.');
            end

            obj.m = m;

            obj.H    = H;
            obj.HI   = HI;
            obj.D1   = D1;
            obj.D2   = D2;
            obj.D4   = D4;
            obj.M4   = M4;
            obj.e_l  = e_l;
            obj.e_r  = e_r;
            obj.d1_l = d1_l;
            obj.d1_r = d1_r;
            obj.d2_l = d2_l;
            obj.d2_r = d2_r;
            obj.d3_l = d3_l;
            obj.d3_r = d3_r;
        end
    end
end