Table.sort - is it broken, or am I doing something wrong?

Question

sometimes, what should be the last element is not being put in the correct position (last) (all others are in the correct order) - no matter what I do, and I have to resort to this function, which works flawlessly:
function Table:sort( t, func )
    local tc = self:copy( t ) -- make shallow copy
    tc[#tc + 1] = "___dUmMy___" -- 3 underscores on each side.
    table.sort( tc, func )
    local index = 1
    for i = 1, #tc do
        if tc[i] ~= "___dUmMy___" then
            t[index] = tc[i]
            index = index + 1
        else
            -- skip it.       
        end
    end
    t[#tc] = nil -- shouldn't be necessary, but cheap insurance...
end
i.e. add an extra dummy element to the table, then sort it using the same sort function, then remove the dummy element.
Its like table.sort makes one iteration too few, unless the dummy is there to force it, or something...
This is driving me crazy!
Any ideas?

Answer 1

So, I figured it out.
Sometimes I want the exact opposite order than is (reliably) possible given the table sort algorithm.
If, in a table sort function, one replaces:
return order
with
return not order
(in a perfectly working sort function), it reverses the order of the sort but may not handle the last element properly.
The reason is this:
The table sort return value is determining whether to swap two values or not to swap them, it is *not* determining absolute ordering - if it were, then returning true would mean order one way, and returning false would mean order the opposite way.
Enough talk - test plugin: http://www.robcole.com/_temp/TestLrPlugins/Scratch_LrPlugin_0.0.zip
(Instructions: In plugin manager, click 'Local Test' button. Source code is in ExtendedManager.lua (tableSortTest function) and Framework/Data/Table.lua (reverseSortCopy method).
Rob