Re: [RFC/PATCH 1/3] teach --histogram to diff

> This is just half-a-review (bottom half of the file).
> [snip]
>> +                     result = histogram_diff(xpp, env,
>> +                             line1, lcs.begin1 - line1,
>> +                             line2, lcs.begin2 - line2);
>> +                     result = histogram_diff(xpp, env,
>> +                             lcs.end1 + 1, LINE_END(1) - lcs.end1,
>> +                             lcs.end2 + 1, LINE_END(2) - lcs.end2);
>
> The result from the first half before lcs is discarded?
>
>> +                     result *= -1;
>
> Again, what does this function (called recursively) return, and what does
> flipping the sign of it do?

>[snip]
>> +     reduce_common_start_end(xpp, env, &line1, &count1, &line2, &count2);
>
> What this does is logically not specific to histogram algorithm but can be
> applied to other backends, no? And I vaguely recall that Linus did try
> something like this once, found some issues with it when context is set to
> non zero, and stopped doing it (sorry, I do not have any more details).
>
> I am not suggesting you to remove this call or hoist the call to one level
> up to xdl_do_diff(), but I do have to wonder how much of the performance
> improvement you reported is due to this common head/tail reduction.