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+## 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., 59 Temple Place, Suite 330, Boston, MA  02111-1307  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)
+
+  wfi = warn_fortran_indexing;
+  unwind_protect
+    warn_fortran_indexing = 0;
+    
+    if nargin == 1 || nargin == 2
+      ## extract nonzero diagonals of v into A,c
+      [i,j,v,nr,nc] = spfind(v);
+      if nargin == 1
+        c = unique(j-i);  # c contains the active diagonals
+      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);
+      end
+    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+=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
+    
+  unwind_protect_cleanup
+    warn_fortran_indexing = wfi;
+  end_unwind_protect
+
+endfunction