Diff-ing windows in vim

I am working on a script that has become fairly convoluted. I suspect there are several sections that have nearly identical code. Can I (and how can I) open the file in vim, with two (or more) windows on the buffer, and diff the contents of the windows on the same file? vimdiff seems to work only on two files. If I make a copy of the file and try to vimdiff the two versions开发者_如何学JAVA, the diff origin remains locked on the beginning of the file. Although I can unscroll-lock the windows, and move the windows to the parts of the file I want to compare the diffs do not show up. Any hints or tips? I could cut and paste the sections I want to compare to different files and then apply vimdiff but then I risk getting lost in what section came from where when I try to patch the separate files together, and I feel sure there must be a more straightforward, easier way.

What I usually do is diff to a copy

:%w %.alt
:vert diffsplit %.alt

And then happiliy rearrange the 'alt' version so that the pseudo-matching bits get aligned.

Note that (presumably) git contains spiffy merge/diff cow-powers that should be able to detect sub-file moved block changes.

Although I haven't (yet) actually put this into practice, I have a hunch that the very nice git plugin fugitive for vim might be able to leverage some of this horsepower to make this easier. Note: fully expect this to require scriptinh before being usable, but I still thought it would be nice to share this idea (perhaps you can share a script if you get to it first!)

As an alternative solution that I've been using occasionally and which works very nicely in my opinion is linediff.vim.

It allows you to use visual mode to select two bodies of text from arbitrary buffers (or the same for that matter) and run vimdiff on them. The beauty of it, is that when you edit and save the temporary diff buffers, you update the original buffers with the changes, without saving.

One of my use-cases is when I'm resolving merge issues related to script refactoring and reordering, where a function has been moved and perhaps also modified. In order to make sure you do not lose any of the modifications coming in from either ancestor, you diff the two versions of the function alone by visually selecting them and running the linediff command.