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zigzag.m

## Copyright (C) 2006 Fredrik Bulow <fredrik.bulow@gmail.com>
##
## 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, see <http://www.gnu.org/licenses/>.
##

## -*- texinfo -*-
## @deftypefn {Function File} {} zigzag (@var{mtrx})
## Returns zigzag walk-off of the elements of @var{mtrx}.
## Essentially it walks the matrix in a Z-fashion.
##  
## mat = 
##   1   4   7
##   2   5   8
##   3   6   9
## then zigzag(mat) gives the output,
## [1   2   4   7   5   3   6   8   9], by walking as
## shown in the figure from pt 1 in that order of output.
## The argument @var{mtrx} should be a MxN matrix 
##
## An example of zagzig use:
## @example
## @group
## mat = reshape(1:9,3,3);
## zigzag(mat)
## ans =[1   2   4   7   5   3   6   8   9]
##
## @end group
## @end example
##
## @end deftypefn
## @seealso{zagzig}

## Author:   Fredrik Bulow <fredrik.bulow@gmail.com>

function rval = zigzag(mtrx)
  if nargin < 1
    error('usage: zigzag(matrix); see help zigzag');
  end
  n=size(mtrx);
  
  if(issquare(mtrx)) #Square matrix (quick case)

    ##We create a matrix of the same size as mtrx where odd elements are
    ##1, others 0.
    odd=kron(ones(ceil(n/2)),eye(2))((1:n(1)),(1:n(2)));

    ##We transpose even elements only.
    mtrx = mtrx.*odd + (mtrx.*(1-odd))';

    ##Now we mirror the matrix. The desired vector is now the
    ##concatenation of the diagonals.
    mtrx=mtrx(:,1+size(mtrx,2)-(1:size(mtrx,2)));

    ##Picking out the diagonals.
    rval  = ;
    for i = n(2)-1:-1:1-n(1)
      rval=[rval diag(mtrx,i)'];
    endfor

  else #Not square (Slow cases)
    mtrx=mtrx(:,1+size(mtrx,2)-(1:size(mtrx,2)));

    ##Picking out the diagonals and reversing odd ones manually.
    rval  = ;
    for i = n(2)-1:-1:1-n(1)
      new = diag(mtrx,i);
      if(floor(i/2)==i/2) ##Even?
      rval=;
      else                ##Odd!
      rval=;
      endif
    endfor
  endif
endfunction
%!
%!assert(zigzag(reshape(1:9,3,3)),[1   2   4   7   5   3   6   8   9])
%!

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