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## -*- texinfo -*-
## @deftypefn  {} {@var{c} =} intersect (@var{a}, @var{b})
## @deftypefnx {} {@var{c} =} intersect (@var{a}, @var{b}, "rows")
## @deftypefnx {} {@var{c} =} intersect (@dots{}, "sorted")
## @deftypefnx {} {@var{c} =} intersect (@dots{}, "stable")
## @deftypefnx {} {@var{c} =} intersect (@dots{}, "legacy")
## @deftypefnx {} {[@var{c}, @var{ia}, @var{ib}] =} intersect (@dots{})
##
## Return the unique elements common to both @var{a} and @var{b}.
##
## If @var{a} and @var{b} are both row vectors then return a row vector;
## Otherwise, return a column vector.  The inputs may also be cell arrays of
## strings.
##
## If the optional input @qcode{"rows"} is given then return the common rows of
## @var{a} and @var{b}.  The inputs must be 2-D numeric matrices to use this
## option.
##
## The optional argument @qcode{"sorted"}/@qcode{"stable"} controls the order
## in which unique values appear in the output.  The default is
## @qcode{"sorted"} and values in the output are placed in ascending order.
## The alternative @qcode{"stable"} preserves the order found in the input.
##
## If requested, return column index vectors @var{ia} and @var{ib} such that
## @code{@var{c} = @var{a}(@var{ia})} and @code{@var{c} = @var{b}(@var{ib})}.
##
## Programming Note: The input flag @qcode{"legacy"} changes the algorithm
## to be compatible with @sc{matlab} releases prior to R2012b.
##
## @seealso{unique, union, setdiff, setxor, ismember}
## @end deftypefn

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

  if (nargin < 2 || nargin > 4)
    print_usage ();
  endif

  [a, b] = validsetargs ("intersect", a, b, varargin{:});

  ## Special case of empty matrices
  if (isempty (a) || isempty (b))
    ## Lots of type checking required for Matlab compatibility.
    if (isnumeric (a) && isnumeric (b))
      c = [];
    elseif (iscell (b))
      c = {};
    else
      c = "";
    endif
    ia = ib = [];
    return;
  endif

  by_rows = any (strcmp ("rows", varargin));
  optsorted = ! any (strcmp ("stable", varargin));
  optlegacy = any (strcmp ("legacy", varargin));

  if (optlegacy)
    isrowvec = ! iscolumn (a) || ! iscolumn (b);
  else
    isrowvec = isrow (a) && isrow (b);
  endif

  ## Form A and B into sets
  if (nargout > 1 || ! optsorted)
    [a, ia] = unique (a, varargin{:});
    ia = ia(:);
    [b, ib] = unique (b, varargin{:});
    ib = ib(:);
  else
    a = unique (a, varargin{:});
    b = unique (b, varargin{:});
  endif

  if (by_rows)
    c = [a; b];
    if (nargout > 1 || ! optsorted)
      [c, ic] = sortrows (c);
    else
      c = sortrows (c);
    endif
    match = find (all (c(1:end-1,:) == c(2:end,:), 2));
    if (optsorted)
      c = c(match, :);
    else
      c = [a; b];
      ## FIXME: Is there a way to avoid a call to sort?
      c = c(sort (ic(match)), :);
    endif
    len_a = rows (a);
  else
    c = [a(:); b(:)];
    if (nargout > 1 || ! optsorted)
      [c, ic] = sort (c);
    else
      c = sort (c);
    endif
    if (iscellstr (c))
      match = find (strcmp (c(1:end-1), c(2:end)));
    else
      match = find (c(1:end-1) == c(2:end));
    endif
    len_a = length (a);
    if (optsorted)
      c = c(match);
    else
      c = [a(:); b(:)];
      ## FIXME: Is there a way to avoid a call to sort?
      c = c(sort (ic(match)));
    endif

    ## Adjust output orientation for Matlab compatibility
    if (isrowvec)
      c = c.';
    endif
  endif

  if (nargout > 1)
    ia = ia(ic(match));            # a(ia) == c
    ib = ib(ic(match+1) - len_a);  # b(ib) == c
    if (! optsorted)
      ## FIXME: Is there a way to avoid a call to sort?
      ia = sort (ia);
      [~, idx] = min (ib);
      ib = [ib(idx:end); ib(1:idx-1)];
    endif
    if (optlegacy && isrowvec && ! by_rows)
      ia = ia.';
      ib = ib.';
    endif
  endif

endfunction


%!assert (intersect ([1 2 3 4], [9 8 4 2]), [2, 4])
%!assert (intersect ([1 2; 2 3; 4 5], [2 3; 3 4; 5 6], "rows"), [2 3])
%!assert (intersect ([1 NaN], [NaN NaN 5]), zeros (1,0))

%!test
%! a = [1 1 1 2 2 2];
%! b = [1 2 3 4 5 6];
%! c = intersect (a, b);
%! assert (c, [1,2]);

## Test multi-dimensional arrays
%!test
%! a = rand (3,3,3);
%! b = a;
%! b(1,1,1) = 2;
%! assert (intersect (a, b), sort (a(2:end)'));

## 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; 4; 5]);
%! assert (ib, [4; 1; 2; 6]);
%! assert (a(ia), c);
%! assert (b(ib), c);

## Test "rows" argument
%!test
%! a = [1,1,2;1,4,5;2,1,7];
%! b = [1,4,5;2,3,4;1,1,2;9,8,7];
%! [c,ia,ib] = intersect (a, b, "rows");
%! assert (c, [1,1,2;1,4,5]);
%! assert (ia, [1;2]);
%! assert (ib, [3;1]);
%! assert (a(ia,:), c);
%! assert (b(ib,:), c);

%!test
%! a = [1 2 3 4; 5 6 7 8; 9 10 11 12];
%! [b, ia, ib] = intersect (a, a, "rows");
%! assert (b, a);
%! assert (ia, [1:3]');
%! assert (ib, [1:3]');

## Test "stable" argument
%!test
%! a = [2 2 2 1 1 1];
%! b = [1 2 3 4 5 6];
%! c = intersect (a, b, "stable");
%! assert (c, [2,1]);

%!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, "stable");
%! assert (c, [3, 5, 7, 1]);
%! assert (ia, [1; 4; 5; 8]);
%! assert (ib, [1; 2; 6; 4]);
%! assert (a(ia), c);
%! assert (b(ib), c);

%!test
%! a = [1,4,5;1,1,2;2,1,7];
%! b = [1,4,5;2,3,4;1,1,2;9,8,7];
%! [c, ia, ib] = intersect (a, b, "rows", "stable");
%! assert (c, [1,4,5; 1,1,2]);
%! assert (ia, [1;2]);
%! assert (ib, [1;3]);
%! assert (a(ia,:), c);
%! assert (b(ib,:), c);

%!test
%! a = [1 2 3 4; 5 6 7 8; 9 10 11 12];
%! [b, ia, ib] = intersect (a, a, "rows", "stable");
%! assert (b, a);
%! assert (ia, [1:3]');
%! assert (ib, [1:3]');

## Test "legacy" argument
%!test
%! a = [7 1 7 7 4];
%! b = [7 0 4 4 0];
%! [c, ia, ib] = intersect (a, b);
%! assert (c, [4, 7]);
%! assert (ia, [5; 1]);
%! assert (ib, [3; 1]);
%! [c, ia, ib] = intersect (a, b, "legacy");
%! assert (c, [4, 7]);
%! assert (ia, [5, 4]);
%! assert (ib, [4, 1]);

%!test  # "legacy" + "rows"
%! A = [ 1 2; 3 4; 5 6; 3 4; 7 8 ];
%! B = [ 3 4; 7 8; 9 10 ];
%! [c, ia, ib] = intersect (A, B, "rows");
%! assert (c, [3, 4; 7, 8]);
%! assert (ia, [2; 5]);
%! assert (ib, [1; 2]);
%! [c, ia, ib] = intersect (A, B, "rows", "legacy");
%! assert (c, [3, 4; 7, 8]);
%! assert (ia, [4; 5]);
%! assert (ib, [1; 2]);

## Test orientation of output
%!shared a,b
%! a = 1:4;
%! b = 2:5;

%!assert (size (intersect (a, b)), [1, 3])
%!assert (size (intersect (a', b)), [3, 1])
%!assert (size (intersect (a, b')), [3, 1])
%!assert (size (intersect (a', b')), [3, 1])
%!assert (size (intersect (a, b, "legacy")), [1, 3])
%!assert (size (intersect (a', b, "legacy")), [1, 3])
%!assert (size (intersect (a, b', "legacy")), [1, 3])
%!assert (size (intersect (a', b', "legacy")), [3, 1])

## Test return type of empty intersections
%!assert (intersect (['a', 'b'], {}), {})
%!assert (intersect ([], {'a', 'b'}), {})
%!assert (intersect ([], {}), {})
%!assert (intersect ({'a', 'b'}, []), {})
%!assert (intersect ([], ['a', 'b']), "")
%!assert (intersect ({}, []), {})
%!assert (intersect (['a', 'b'], []), "")