%Let  x  be a bitstring and suppose  x  is the i-th string in
%the list of all bitstrings 0, 1, 00, 01, 10, 11, 000, ...
% Then, bitstring_to_index(x) = i.  The integer  i  shall be
%called the index of x.
%
% Usage:
%   i =  bitstring_to_index( [ 0 0 ] )
% returns i = 3.
%   t = concat( i )'
% recovers the original bits [ 0 0 ].
% (Perhaps concat() should be renamed index_to_bitstring).
%
% WARNING: because we are using IEEE754 doubles,
% we only have 53 bits (of mantissa),
% the integer 2^53+1
% is the smallest positive integer that
% cannot be exactly represented:
%   (2^53+1)-(2^53)
% is rounded off to zero.
% This means that a "index"
% can represent bitstrings of up to 52 bits.
% (53 bit bitstrings will always round the last
% bit off to zero).
%
% See also CONCAT, PRINT_BITSTRINGS, BINARY_TO_INDEX, BITMAX.

% by John C. Kieffer
% http://www.ee.umn.edu/users/kieffer/programs.html
% documented in
% ftp://oz.ee.umn.edu/users/kieffer/seminar/notes1.ps
% more documentation by David Cary 1999-05-06

function y=bitstring_to_index(x)
N=length(x);
if(53 <= N )
   error('Sorry, this is too big for me to handle.')
end;

S=1;
for i=1:N;
   S=2*S+x(i);
end
y=S-1;