function [PRN] = PRNgen_CA(prn_sequence, prn_num, shift)
% PRN C/A sequence generater 10 bit registers
% inputs
%    prn_sequence:  1 + x^3 + X^10 (-1,1) as column vector, 
%    prn_num: PRN number, from 1-10
%    shift: number of bits to shift
% output
%	PRN: C/A sequence (-1,1) as column vector
%
%  returns zero if error is detected

% David Benson
% 7/3/01

% Check for valid input 
if nargin ~= 3
   if nargin == 2
      shift = 0;
   else
      disp([ 'Error: wrong number of inputs'])
      PRN = 0;
      return
   end
end

n = 10;	% number of bits in register
switch prn_num
case 1
   tap1 = 2; tap2 = 6;
case 2
   tap1 = 3; tap2 = 7;
case 3
   tap1 = 4; tap2 = 8;
case 4 
   tap1 = 5; tap2 = 9;
case 5
   tap1 = 1; tap2 = 9;
case 6
   tap1 = 2; tap2 = 10;
case 7
   tap1 = 1; tap2 = 8;
case 8
   tap1 = 2; tap2 = 9;
case 9
   tap1 = 3; tap2 = 10;
case 10
   tap1 = 2; tap2 = 3;
otherwise
   PRN = 0;
   disp([' Error: not valid PRN number (1 - 10)'])
   return
end
if tap1 == tap2
   disp([' Error: Tap locations are the same'])
   PRN = 0;
   return
elseif ((tap1 > n) | (tap1 < 1) | (tap2 > n) | (tap2 < 1))
   disp([' Error: Tap locations out of range'])
   PRN = 0;
   return
end

% initilize variables
%register1 = ones(1,n);
register2 = ones(1,n);
epochcheck = zeros(1,n);
count = 0;

% run loop until all ones are found in the register
while sum(epochcheck) ~= n
   
   % add counter
   count = count + 1;
   
   % check XOR logic circuit on first register
   %o1 = xor(register1(3), register1(10));
   
   % check XOR logic circuit on second register
   o2 = xor(register2(2),xor(register2(3),xor(register2(6),xor(register2(8),xor(register2(9),register2(10))))));
  
   % Save final PRN sequence
   if prn_sequence(count) == 1
      reg1 = 0;
   else
      reg1 = 1;
   end
   
   PRN(count,:) = xor(reg1, xor(register2(tap1), register2(tap2)));
      
   % shift values
   for i = (n-1):-1:1
      %register1(i+1) = register1(i);
      register2(i+1) = register2(i);
   end
   %register1(1) = o1;
   register2(1) = o2;
   
   % epoch chech
   epochcheck = register2;
   
   % check for too long of a sequence
   if count == 2^n
      break
   end
end

% check if PRN sequence is max length
if count ~= (2^n - 1)
   disp([' Warning: PRN sequence is not max length'])
end

% check shift range
if shift >= count
   disp([' Error: shift out of range'])
   PRN = 0;
   return
end

% shift PRN sequence
if shift < 0
   shift = -shift;
   PRNtemp = PRN((shift+1):end);
   PRNtemp = [PRNtemp; PRN(1:shift)];
   PRN = PRNtemp;
elseif shift > 0
   shift = count - shift;
   PRNtemp = PRN((shift+1):end);
   PRNtemp = [PRNtemp; PRN(1:shift)];
   PRN = PRNtemp;
end

% reset high/low values to -1/1
for i = 1:length(PRN)
   if PRN(i) == 1
      PRNtemp(i,:) = -1;
   else
      PRNtemp(i,:) = 1;
   end
end
PRN = PRNtemp;