Re: A stable sort

[Dirk Laurie <dirk.laurie@...>]

2013/5/8 Eike Decker <zet23t@googlemail.com>:

>> I only wrote this quickly and haven't given it intensive testing, but this
>> sort function is pretty simple, short, stable and fast

And wrong.

         local pos = a
         while a <= e1 and b <= e2 do
            local va,vb = arr[a], arr[b]
            if a <= e1 and fn(va,vb) then
               arr[pos] = va
               a = a + 1
            else
               arr[pos] = vb
               b = b + 1
            end
            pos = pos + 1
         end

Say on the first iteration you take the "else" branch. It stores `vb`
in `arr[pos]` which is `arr[a]`. So `arr[a]` is now clobbered. It
still sits in `va` but `va` is not used again.

You can postpone the inevitable by:

         local pos, va, vb = a, arr[a], arr[b]
         while a <= e1 and b <= e2 do
            if a <= e1 and fn(va,vb) then
               arr[pos] = va
               a = a + 1
               va = arr[a]
            else
               arr[pos] = vb
               b = b + 1
               vb = arr[b]
            end
            pos = pos + 1
         end

Now you need to take the "else" branch twice in a row before
something goes wrong.

There are in-place merge sorts around but they are not this simple.