Re: [PATCH v2 5/7] xdl_change_compact(): introduce the concept of a change group

> The idea of xdl_change_compact() is fairly simple:
>
> * Proceed through groups of changed lines in the file to be compacted,
>   keeping track of the corresponding location in the "other" file.
>
> * If possible, slide the group up and down to try to give the most
>   aesthetically pleasing diff. Whenever it is slid, the current location
>   in the other file needs to be adjusted.
>
> But these simple concepts are obfuscated by a lot of index handling that
> is written in terse, subtle, and varied patterns. I found it very hard
> to convince myself that the function was correct.
>
> So introduce a "struct group" that represents a group of changed lines
> in a file. Add some functions that perform elementary operations on
> groups:
>
> * Initialize a group to the first group in a file
> * Move to the next or previous group in a file
> * Slide a group up or down
>
> Even though the resulting code is longer, I think it is easier to
> understand and review.