## Copyright (C) 2000, 2001, 2004, 2005, 2007, 2008, 2009 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.
##
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## 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} {[@var{b}, @var{c}] =} spdiags (@var{a})
## @deftypefnx {Function File} {@var{b} =} spdiags (@var{a}, @var{c})
## @deftypefnx {Function File} {@var{b} =} spdiags (@var{v}, @var{c}, @var{a})
## @deftypefnx {Function File} {@var{b} =} spdiags (@var{v}, @var{c}, @var{m}, @var{n})
## A generalization of the function @code{diag}.  Called with a single
## input argument, the non-zero diagonals @var{c} of @var{A} are extracted.
## With two arguments the diagonals to extract are given by the vector 
## @var{c}.
##
## The other two forms of @code{spdiags} modify the input matrix by
## replacing the diagonals.  They use the columns of @var{v} to replace
## the columns represented by the vector @var{c}.  If the sparse matrix
## @var{a} is defined then the diagonals of this matrix are replaced.
## Otherwise a matrix of @var{m} by @var{n} is created with the
## diagonals given by @var{v}.
##
## Negative values of @var{c} represent diagonals below the main
## diagonal, and positive values of @var{c} diagonals above the main
## diagonal.
##
## For example
##
## @example
## @group
## spdiags (reshape (1:12, 4, 3), [-1 0 1], 5, 4)
## @result{}    5 10  0  0
##       1  6 11  0
##       0  2  7 12
##       0  0  3  8
##       0  0  0  4
## @end group
## @end example
##
## @end deftypefn

function [A, c] = spdiags (v, c, m, n)

  if (nargin == 1 || nargin == 2)
    ## extract nonzero diagonals of v into A,c
    [nr, nc] = size (v);
    [i, j, v] = find (v);

    if (nargin == 1)
      ## c contains the active diagonals
      c = unique (j-i);
    endif
    ## FIXME: we can do this without a loop if we are clever
    offset = max (min (c, nc-nr), 0);
    A = zeros (min (nr, nc), length (c));
    for k = 1:length (c)
      idx = find (j-i == c(k));
      A(j(idx)-offset(k),k) = v(idx);
    endfor
  elseif (nargin == 3)
    ## Replace specific diagonals c of m with v,c
    [nr, nc] = size (m);
    B = spdiags (m, c);
    A = m - spdiags (B, c, nr, nc) + spdiags (v, c, nr, nc);
  else
    ## Create new matrix of size mxn using v,c
    [j, i, v] = find (v);
    offset = max (min (c(:), n-m), 0);
    j = j(:) + offset(i);
    i = j-c(:)(i);
    idx = i > 0 & i <= m & j > 0 & j <= n;
    A = sparse (i(idx), j(idx), v(idx), m, n);
  endif

endfunction
