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## -*- texinfo -*-
## @deftypefn  {} {@var{s} =} speye (@var{m}, @var{n})
## @deftypefnx {} {@var{s} =} speye (@var{m})
## @deftypefnx {} {@var{s} =} speye ([@var{m}, @var{n}])
## Return a sparse identity matrix of size @var{m}x@var{n}.
##
## The implementation is significantly more efficient than
## @w{@code{sparse (eye (@var{m}))}} as the full matrix is not constructed.
##
## When called with a single argument, a square matrix of size
## @var{m}-by-@var{m} is created.  If called with a single vector argument,
## this argument is taken to be the size of the matrix to create.
## @seealso{sparse, spdiags, eye}
## @end deftypefn

function s = speye (m, n)

  if (nargin == 1)
    if (! isvector (m) || numel (m) > 2)
      print_usage ();
    endif

    if (isscalar (m))
      n = m;
    else
      n = m(2);
      m = m(1);
    endif
  else
    if (! (isscalar (m) && isscalar (n)))
      error ("speye: M and N must be scalar dimensions");
    endif
  endif

  ## Note: Matlab compatibility requires using 0 for negative dimensions.
  m = ifelse (m < 0, 0, m);
  n = ifelse (n < 0, 0, n);
  lo = min (m, n);
  s = sparse (1:lo, 1:lo, 1, m, n);

endfunction


%!assert (issparse (speye (4)))
%!assert (speye (4), sparse (1:4,1:4,1))
%!assert (speye (2,4), sparse (1:2,1:2,1,2,4))
%!assert (speye (4,2), sparse (1:2,1:2,1,4,2))
%!assert (speye ([4,2]), sparse (1:2,1:2,1,4,2))
%!assert (speye (2, -3), sparse (2, 0))

## Test input validation
%!error <Invalid call> speye (ones (2,2))
%!error <Invalid call> speye ([1, 2, 3])
%!error <M and N must be scalar dimensions> speye ([1, 2], 3)
%!error <M and N must be scalar dimensions> speye (1, [2, 3])