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function result = iscatastrophic(trellis)
// This function determines if a convolutional code is catastrophic or not
// Calling Sequence
// RESULT = ISCATASTROPHIC(TRELLIS)
// Description
// RESULT = ISCATASTROPHIC(TRELLIS) returns 1 if the specified
// trellis corresponds to a catastrophic convolutional code, else 0.
// Examples
// eg_1.numInputSymbols = 4;
// eg_1.numOutputSymbols = 4;
// eg_1.numStates = 3;
// eg_1.nextStates = [0 1 2 1;0 1 2 1; 0 1 2 1];
// eg_1.outputs = [0 0 1 1;1 1 2 1; 1 0 1 1];
// res_t_eg_1=istrellis(eg_1)
// res_c_eg_1=iscatastrophic(eg_1)
// if (res_c_eg_1) then
// disp('Example 1 is catastrophic')
// else
// disp('Example 1 is not catastrophic')
// end
// eg_2.numInputSymbols = 2;
// eg_2.numOutputSymbols = 4;
// eg_2.numStates = 2;
// eg_2.nextStates = [0 0; 1 1 ]
// eg_2.outputs = [0 0; 1 1];
// res_t_eg_2=istrellis(eg_2)
// res_c_eg_2=iscatastrophic(eg_2)
// if (res_c_eg_2) then
// disp('Example 2 is catastrophic')
// else
// disp('Example 2 is not catastrophic')
// end
// Authors
// Pola Lakshmi Priyanka, IIT Bombay
//*************************************************************************************************************************************//
// Check number of input arguments
[out_a,inp_a]=argn(0)
if inp_a~=1 then
error('comm:iscatastrophic: Invalid number of input arguments')
end
if out_a>1 then
error('comm:iscatastrophic: Invalid number of output arguments')
end
// Check if the input is a valid trellis
if ~istrellis(trellis),
error('comm:iscatastrophic:Input should be a valid trellis structure.');
end
result = 0;
// Find indices of zeros in trellis structure
[r_idx,c_idx] = find(trellis.outputs==0);
//Find Connectivity matrix and check if it is catastrophic
A = zeros(trellis.numStates,trellis.numStates);
for k = 2:length(r_idx)
A(r_idx(k),trellis.nextStates(r_idx(k),c_idx(k))+1)=1;
end
test = A;
for i = 1:trellis.numStates
for j = 1:trellis.numStates
if test(j,j)==1
result = 1
end
end
if result==1
break
else
test = test*A;
end
end
endfunction
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