## Copyright (C) 2000-2001 Paul Kienzle
##
## This program 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 of the License, or
## (at your option) any later version.
##
## This program 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 this program; if not, write to the Free Software
## Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
## 02110-1301  USA

## -*- 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{spdiag}. 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} representive 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
    [i, j, v, nr, nc] = spfind (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