sub HDI_WIDTH    {0x1}
sub HDI_FORMAT   {0x4}
sub HDF_SORTUP   {0x400}
sub HDF_SORTDOWN {0x200}
sub HDM_FIRST    {0x1200}
sub HDM_GETITEM  {&HDM_FIRST+11}
sub HDM_SETITEM  {&HDM_FIRST+12}

sub HDF_LEFT     {0x0}
sub HDF_RIGHT    {0x1}
sub HDF_CENTER   {0x2}
sub HDF_STRING   {0x4000}


 # loop over columns, setting the arrow on the clicked column
 # and removing it from the others

for (0 .. scalar $myListView->GetColumnOrderArray()){
  my $arrowstate = ($_ == $columnclicked) # $columnclicked courtesy of ColumnClick event handler
                   ? $state               # $state can be 'asc' or 'desc' (or something alike)
                   : 'unset';             #        and 'unset'

  setSortArrow($myListView, $_, $arrowstate);
}




sub setSortArrow {

  my ($listview, $column, $state) = @_;

  # request information on the format and the width of the column header
  my $hditem = pack("L11", 
                    HDI_FORMAT | HDI_WIDTH,    # mask
                    0,                         # cxy
                    0,                         # pszText
                    0,                         # hbm
                    0,                         # cchTextMax
                    0,                         # fmt
                    0,                         # lParam
                    0,                         # iImage
                    0,                         # iOrder
                    0,                         # type
                    0                          # pvFilter
                   );

  my $ptr = pack('P44', $hditem);

  my $hwnd = $listview->GetHeader();
  my $r = Win32::GUI::SendMessage($hwnd, HDM_GETITEM, $column, unpack('L!', $ptr));

  return unless $r;

  my @members = unpack('L11', $hditem);  # cxy and fmt now updated by call to HDM_GETITEM

  my $width = $members[1];
  my $cxy   = 0;
  my $fmt   = sprintf("%X", $members[5]);
  my $justf = (($fmt | HDF_RIGHT) eq $fmt) ? HDF_RIGHT 
                                           : ((($fmt | HDF_CENTER) eq $fmt) ? HDF_CENTER
                                                                            : HDF_LEFT);
  my $flags = HDF_STRING | $justf;    # keep text and its justification

  unless($state eq 'unset'){
    $flags |= ($state eq 'asc') ? HDF_SORTUP : HDF_SORTDOWN;

    # text on narrow columns will be trunacted with ellipses: adjust width a little
    # FIXME: "80" is a little arbitrary and repeated sorting will keep very narrow 
    #        columns growing until they are 80 pixels wide

    if((($fmt & HDF_SORTDOWN & HDF_SORTUP) > 0) && ($width < 80)){
      $cxy = $width + 5;
    }
  }

  $members[0] = HDI_FORMAT | ($cxy ? HDI_WIDTH : 0x0);
  $members[1] = $cxy;
  $members[5] = $flags;

  $hditem = pack("L11", @members);
  $ptr = pack('P44', $hditem);

  $r = Win32::GUI::SendMessage($hwnd, HDM_SETITEM, $column, unpack('L!', $ptr));

}