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function zf = filtic(b,a,y,x)
//This function finds the initial conditions for the delays in the transposed direct-form II filter implementation
//Calling Sequence
//zf = filtic (b, a, y)
//zf = filtic (b, a, y, x)
//Parameters
//b: vector of real or complex numbers
//a: vector of real or complex numbers
//y: vector of real or complex numbers
//x: vector of real or complex numbers
//Description
//This function finds the initial conditions for the delays in the transposed direct-form II filter implementation.
//The vectors b and a represent the numerator and denominator coefficients of the filter's transfer function.
//Examples
//filtic([%i,1,-%i,5], [1,2,3*%i], [0.8*%i,7,9])
//ans =
// 0.00000 - 22.60000i
// 2.40000 + 0.00000i
// 0.00000 + 0.00000i
if (argn(2)>4 | argn(2)<3) | (argn(1)>1)
error("Wrong number of input agruments.")
end
if argn(2) < 4, x = []; end
nz = max(length(a)-1,length(b)-1);
zf=zeros(nz,1);
if length(a)<(nz+1)
a(length(a)+1:nz+1)=0;
end
if length(b)<(nz+1)
b(length(b)+1:nz+1)=0;
end
if length(x) < nz
x(length(x)+1:nz)=0;
end
if length(y) < nz
y(length(y)+1:nz)=0;
end
for i=nz:-1:1
for j=i:nz-1
zf(j) = b(j+1)*x(i) - a(j+1)*y(i)+zf(j+1);
end
zf(nz)=b(nz+1)*x(i)-a(nz+1)*y(i);
end
endfunction
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