--- /dev/null
+## Copyright (C) 2006 Muthiah Annamalai <muthiah.annamalai@uta.edu>
+##
+## 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/>.
+
+## Implement book keeping for a Pseudo-Random Binary Sequence ( PRBS )
+## also called as a Linear Feedback Shift Register.
+##
+## For the given PRBS in a intial state, compute the PRBS sequence length.
+## Length is period of output when the PRBS state is same as
+## the start state of PRBS.
+##
+## Example: If you had a PRBS shift register like the diagram
+## below with 4 registers we use representation by polynomial
+## of [ 1 2 3 4], and feedback connections between [ 1 3 4 ].
+## The output PRBS sequence is taken from the position 4.
+##
+## +---+ +----+ +---+ +---+
+## | D |----| D |---| D |---| D |
+## +---+ +----+ +---+ +---+
+## | | |
+## \ / /
+## [+]---------------+------+
+## 1 + 0.D + 1.D^2 + 1.D^3
+##
+## The code to implement this PRBS will be
+## prbs=prbs_generator([1 3 4],{[1 3 4]},[1 0 1 1]);
+## x = prbs_sequence(prbs) #gives 15
+##
+##
+## See Also: This function is to be used along with functions
+## prbs_generator.
+
+function [itrs,seq]=prbs_sequence(prbs)
+ if nargin != 1
+ print_usage;
+ endif
+ nstate=zeros(1,prbs.reglen);
+ itrs=0; seq = [];
+ inits = prbs.sregs;
+
+ ## For each iteration, shift the output bit. Then compute the xor pattern of connections.
+ ## Finally apply feedback the stuff. Insert the computed pattern.
+ while( true )
+ itrs = itrs + 1;
+
+ ## compute the feedback.
+ for itr2=1:prbs.conlen
+ val=0;
+ L=length(prbs.connections{itr2});
+ for itr3=2:L
+ val=bitxor(val,prbs.sregs(prbs.connections{itr2}(itr3)));
+ endfor
+ nstate(prbs.connections{itr2}(1))=val;
+ endfor
+
+ ## rotate the output discarding the last output.
+ seq = [seq, prbs.sregs(end)];
+ prbs.sregs=[0 prbs.sregs(1:prbs.reglen-1)];
+
+ ## insert the feedback.
+ for itr2=1:prbs.conlen
+ prbs.sregs(itr2)=nstate(itr2);
+ nstate(itr2)=0; # reset.
+ endfor
+
+ if(isequal(prbs.sregs,inits))
+ break
+ endif
+ endwhile
+end