Re: [BUG-ish] diff compaction heuristic false positive

[Michael Haggerty <mhagger@alum.mit.edu>]

On 06/10/2016 10:41 AM, Jeff King wrote:
> On Fri, Jun 10, 2016 at 10:31:13AM +0200, Michael Haggerty wrote:
> 
>> I've often thought that indentation would be a good, fairly universal
>> signal for diff to use when deciding how to slide hunks around. Most
>> source code is indented in a way that shows its structure.
>>
>> I propose the following heuristic:
>>
>> * Prefer to start and end hunks following lines with the least
>>   indentation.
>>
>> * Define the "indentation" of a blank line to be the indentation of
>>   the previous non-blank line minus epsilon.
>>
>> * In the case of a tie, prefer to slide the hunk down as far as
>>   possible.
> 
> Hmm. That might help this case, but the original motivation for this
> heuristic was something like:
> 
>   ##
>   # foo
>   def foo
>     something
>   end
> 
>   ##
>   # bar
>   def bar
>     something_else
>   end
> 
> where we add the first function above the second. We end up with:
> 
> diff --git a/file.rb b/file.rb
> index 1f9b151..f991c76 100644
> --- a/file.rb
> +++ b/file.rb
> @@ -1,4 +1,10 @@
>  ##
> +# foo
> +def foo
> +  something
> +end
> +
> +##
>  # bar
>  def bar
>    something else
> 
> I.e., crediting the "##" to the wrong spot (or in C, the "/*"). I don't
> think indentation helps us there (sliding-up would, but like
> sliding-down, it just depends on the order of the hunks).
> 
> So I agree that adding indentation to the mix might help, but I don't
> think it can replace this heuristic.

Ummm, I think the indentation heuristic would work with that example,
too, as long as we consider there to be an imaginary line "0" of the
file (i.e., preceding the first real line) that has an indentation of -ε.

Michael