## Copyright (C) 2000, 2006, 2007 Paul Kienzle
##
## This file is part of Octave.
##
## Octave 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.
##
## Octave 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 Octave; see the file COPYING.  If not, see
## <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {Function File} {} intersect (@var{a}, @var{b})
## @deftypefnx {Function File} address@hidden, @var{ia}, @var{ib}] =} intersect (@var{a}, @var{b}, 'rows' )
##
## Return the elements in both @var{a} and @var{b}, sorted in ascending
## order. If @var{a} and @var{b} are both column vectors return a column
## vector, otherwise return a row vector.
##
## Return index vectors @var{ia} and @var{ib} such that @code{a(ia)==c} and
## @code{b(ib)==c}.
##
## If parameter 'rows' are also supplied, two parameters are expected in matrix form with equal number
## of columns. in this case, similar to unique( s, 'rows' ), rows of the matrices are handled as
## set entries and intersection among them is returned so.
##
## @end deftypefn
## @seealso{unique, union, setxor, setdiff, ismember}

function [c, ia, ib] = intersect (a, b, r)

  if (!(nargin == 2 || (nargin == 3 && strcmp (r, "rows"))))
 	print_usage();
  endif

  if (isempty (a) || isempty (b))
    c = ia = ib = [];
  else
    ## form a and b into sets
	
	if ( nargin == 3 ) % by rows

		if ( size(a,2) != size(b,2) )
			error("The two matrices must have equal number of columns!");
		endif

	    [a, ja] = unique (a,'rows');
    	[b, jb] = unique (b,'rows');

	    c = [ a ; b ];
    	[c, ic] = sortrows (c);               ## [a(:);b(:)](ic) == c

	    if (iscellstr (c))					## how to handle this case?
    	  ii = find ( strcmp (c(1:end-1), c(2:end) )); #### ???
	    else
    	  ii = find ( all( (c(1:end-1,:) == c(2:end,:)) , 2) )
	    endif
	    c  = c(ii,:);                       ## The answer
	else
	    [a, ja] = unique (a);
    	[b, jb] = unique (b);

	    c = [a(:); b(:)];
    	[c, ic] = sort (c);               ## [a(:);b(:)](ic) == c	

	    if (iscellstr (c))
    	  ii = find (strcmp (c(1:end-1), c(2:end)));
	    else
    	  ii = find (c(1:end-1) == c(2:end));
	    endif
	    c  = c(ii);                       ## The answer

    	if (size (b, 1) == 1 || size (a, 1) == 1)
      		c = c.';
    	endif
	endif

    ia = ja(ic(ii));                  ## a(ia) == c
    ib = jb(ic(ii+1) - length (a));   ## b(ib) == c

  endif

endfunction


%!# Test the routine for index vectors ia and ib
%!test
%! a = [3 2 4 5 7 6 5 1 0 13 13];
%! b = [3 5 12 1 1 7];
%! [c,ia,ib] = intersect(a,b);
%! assert( c,[1 3 5 7]);
%! assert(ia,[8 1 7 5]);
%! assert(ib,[5 1 2 6]);
%! assert(a(ia),c);
%! assert(b(ib),c);