## Copyright (C) 2000 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 2, 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, write to the Free
## Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
## 02110-1301, USA.

## -*- texinfo -*-
## @deftypefn {Function File} {} intersect(@var{a}, @var{b})
## @deftypefnx {Function File} {[@var{c}, @var{ia}, @var{ib}] = } intersect (@var{a}, @var{b})
##
## 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}.
##
## @end deftypefn
## @seealso{unique, union, setxor, setdiff, ismember}

function [c, ia, ib] = intersect (a, b)
  if (nargin != 2)
    usage ("intersect (a, b)");
  endif

  if (isempty (a) || isempty (b))
    c = ia = ib = [];
  else
    ## form a and b into sets
    [a, ja] = unique (a);
    [b, jb] = unique (b);

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

    ii = find( c(1:end-1) == c(2:end) );  

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


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