function y = concat(I)
%concat: concatenate bitstrings
%I is a vector of indices for two or more bitstrings
%Usage:
%  I = [ 3 5 1 2 ]
%  y = concat(I)'
%  print_without_spaces(y)
%  fprintf(1,'\n')
%creates in memory the bitstring
%y consisting of the left to right concatenation of the
%bitstrings from I,
% y = [ 0     0     1     0     0     1 ]
% and prints this string to the screen 
%  001001
% .

% documented in
% ftp://oz.ee.umn.edu/users/kieffer/seminar/notes2.ps
% Change log:
% 1999-06-30:DAV: This was very slow for large images, so I sped it up.
%   Now it's much faster.
% 1999-05-06:DAV: more documentation by David Cary
% ???:JCK: originally by John C. Kieffer
%   http://www.ee.umn.edu/users/kieffer/programs.html

[bitlength, value] = index_to_binary(I(:));
maxbits = max(bitlength(:));
% value = uint32(value); activates strange bug in bitget() in MATLAB Version 5.1.0.421 on PCWIN

      % build the results, one codeword per column,
      % one row per bit.
      list = zeros( maxbits, length(I) );
      % mostly from index_to_bitstring by Kieffer
      current_bit = 1;
      z = zeros( length(I), 1 ); % reserve space
      for i=maxbits:-1:1,
         % find which columns have valid bits.
         valid_bits = current_bit <= bitlength;
         % get next row of bits.
         z(:) = bitget(value,current_bit)+valid_bits; % profile says this is slow (5 s !)
         % For each cell of z,
         % 0 == invalid "empty".
         % 1 == valid "0" bit
         % 2 == valid "1" bit.
         list( i, : ) = z'; % insert into matrix
         current_bit = current_bit+1;
      end;
      
      % Strip out "empty" places (currently filled with "0").
      [dummyrow, dummycolumn, y] = find( list );
      y = logical(y - 1);
   


if(0)
   % working original program. Probably faster in C.
   % 38 s on DAV's machine.
   N=length(I);
   y=[];
   for i=1:N;
      A=index_to_bitstring(I(i));
      y=[y A];
   end
end;