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function [f]=ref_isfac(ff,L,a,M)
%-*- texinfo -*-
%@deftypefn {Function} ref_isfac
%@verbatim
%REF_ISFAC Reference inverse signal factorization
% Usage: f=ref_sfac(ff,a,M);
%
% Input parameters:
% ff : Factored signal
% a : Length of time shift.
% b : Length of frequency shift.
% Output parameters:
% f : Output signal.
%
%@end verbatim
%@strong{Url}: @url{http://ltfat.github.io/doc/reference/ref_isfac.html}
%@end deftypefn
% Copyright (C) 2005-2016 Peter L. Soendergaard <peter@sonderport.dk>.
% This file is part of LTFAT version 2.2.0
%
% This program is free software: you can redistribute it and/or modify
% it under the terms of the GNU General Public License as published by
% the Free Software Foundation, either version 3 of the License, or
% (at your option) any later version.
%
% This program is distributed in the hope that it will be useful,
% but WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
% GNU General Public License for more details.
%
% You should have received a copy of the GNU General Public License
% along with this program. If not, see <http://www.gnu.org/licenses/>.
% Calculate the parameters that was not specified
W=prod(size(ff))/L;
N=L/a;
M=L/b;
% The four factorization parameters.
[c,h_a,h_m]=gcd(a,M);
p=a/c;
q=M/c;
d=N/q;
permutation=zeros(q*b,1);
P=stridep(p,b);
% Create permutation
for l=0:q-1
for s=0:b-1
permutation(l*b+1+s)=mod(P(s+1)-1-h_m*l,b)*M+l*c+1;
end;
end;
f=ref_ifac(ff,W,c,d,p,q,permutation);
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