function ABCD = stuffABCD(a,g,b,c,form)%ABCD = stuffABCD(a,g,b,c,form='CRFB')%Compute the ABCD matrix for the specified structure.%See realizeNTF.m for a list of supported structures.%mapABCD is the inverse function.% Code common to all structures.if nargin<5    if nargin<4		fprintf(1,'%s error: Insufficient arguments (%.0f).\n',mfilename,nargin)		return    else		form = 'CRFB';    endendorder = length(a);odd = rem(order,2); even = ~odd;ABCD = zeros(order+1,order+2);if length(b)==1    b = [b zeros(1,order)];endswitch formcase 'CRFB'    %C=(0 0...c_n)    % This is done as part of the construction of A, below    %B1 = (b_1 b_2... b_n), D=(b_(n+1) 0)    ABCD(:,order+1) = b';    %B2 = -(a_1 a_2... a_n)    ABCD(1:order,order+2) = -a';    diagonal = 1:(order+2):order*(order+1);    ABCD(diagonal) = ones(1,order);    subdiag = diagonal((1+even):2:order)+1;    ABCD(subdiag)= c(1+even:2:order);    supdiag = subdiag((1+odd):length(subdiag))-2;    ABCD(supdiag) = -g;    dly = (2+odd):2:order;	% row numbers of delaying integrators    ABCD(dly,:) = ABCD(dly,:) + diag(c(dly-1))*ABCD(dly-1,:);case 'CRFF'    %B1 = (b_1 b_2... b_n), D=(b_(n+1) 0)    ABCD(:,order+1) = b';    %B2 = -(c_1 0... 0)    ABCD(1,order+2) = -c(1);    diagonal = 1:(order+2):order*(order+1);	% # of elements = order    ABCD(diagonal) = ones(1,order);    subdiag = diagonal(1:2:order-1)+1;    ABCD(subdiag)= c(2:2:order);    if even		multg = 1:2:order;	% rows to have g*(following row) subtracted.		ABCD(multg,:) = ABCD(multg,:) - diag(g)*ABCD(multg+1,:);    elseif order >= 3		supdiag = diagonal(3:2:order)-1;		ABCD(supdiag) = -g;    end	    multc = 3:2:order;		% rows to have c*(preceding row) added.	    ABCD(multc,:) = ABCD(multc,:) + diag(c(multc))*ABCD(multc-1,:);	    ABCD(order+1,1:2:order) = a(1:2:order);    for i = 2:2:order		ABCD(order+1,:) = ABCD(order+1,:) + a(i)*ABCD(i,:);    endcase 'CIFB'    %C=(0 0...c_n)    % This is done as part of the construction of A, below    %B1 = (b_1 b_2... b_n), D=(b_(n+1) 0)    ABCD(:,order+1) = b';    %B2 = -(a_1 a_2... a_n)    ABCD(1:order,order+2) = -a';    diagonal = 1:(order+2):order*(order+1);    ABCD(diagonal) = ones(1,order);    subdiag = diagonal(1:order)+1;    ABCD(subdiag)= c;    supdiag = diagonal((2+odd):2:order)-1;    ABCD(supdiag) = -g;case 'CIFF'    %B1 = (b_1 b_2... b_n), D=(b_(n+1) 0)    ABCD(:,order+1) = b';    %B2 = -(c_1 0... 0)    ABCD(1,order+2) = -c(1);    diagonal = 1:(order+2):order*(order+1);    ABCD(diagonal) = ones(1,order);    subdiag = diagonal(1:order-1)+1;    ABCD(subdiag)= c(2:end);    %C = (a_1 a_2... a_n)    ABCD(order+1,1:order) = a(1:order);    supdiag = diagonal((2+odd):2:order)-1;    ABCD(supdiag) = -g;case 'CRFBD'    %C=(0 0...c_n)    ABCD(order+1,order) = c(order);    %B1 = (b_1 b_2... b_n), D=(b_n+1 0)    ABCD(:,order+1) = b';    %B2 = -(a_1 a_2... a_n)    ABCD(1:order,order+2) = -a';    diagonal = 1:(order+2):order*(order+1);    ABCD(diagonal) = ones(1,order);    dly = (1+odd):2:order;	% row numbers of delaying integrators    subdiag = diagonal(dly)+1;    ABCD(subdiag)= c(dly);    supdiag = subdiag((1+odd):length(subdiag))-2;    ABCD(dly,:) = ABCD(dly,:) - diag(g)*ABCD(dly+1,:);    if order>2	coupl = 2+even:2:order;	ABCD(coupl,:) = ABCD(coupl,:) + diag(c(coupl-1))*ABCD(coupl-1,:);    endcase 'CRFFD'    diagonal = 1:(order+2):order*(order+1);    subdiag = diagonal(1:order-1)+1;    supdiag = diagonal(2:order)-1;    %B1 = (b_1 b_2... b_n), D=(b_(n+1) 0)    ABCD(:,order+1) = b';    %B2 = -(c_1 0... 0)    ABCD(1,order+2) = -c(1);    ABCD(diagonal) = ones(1,order);    multc = 2:2:order;		% rows to have c*(preceding row) added.    if order>2	ABCD(subdiag(2:2:end)) = c(3:2:end);    end    if even	ABCD(supdiag(1:2:end)) = -g;    else	% subtract g*(following row) from the multc rows	ABCD(multc,:) = ABCD(multc,:) - diag(g)*ABCD(multc+1,:);    end    ABCD(multc,:) = ABCD(multc,:) + diag(c(multc))*ABCD(multc-1,:);    % C    ABCD(order+1,2:2:order) = a(2:2:order);    for i = 1:2:order	ABCD(order+1,:) = ABCD(order+1,:) + a(i)*ABCD(i,:);    end    % The above gives y(n+1); need to add a delay to get y(n).    % Do this by augmenting the states. Note: this means that    % the apparent order of the NTF is one higher than it acually is.    [A B C D] = partitionABCD(ABCD,2);    A = [A zeros(order,1); C 0];    B = [B; D];    C = [zeros(1,order) 1];    D = [0 0];    ABCD = [A B; C D];otherwise    fprintf(1,'%s error: Form %s is not yet supported.\n',mfilename,form)end