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# Re: Revision walking, commit dates, slop

• Date: Wed, 22 May 2019 15:06:32 -0400
• From: Derrick Stolee <stolee@xxxxxxxxx>
• Subject: Re: Revision walking, commit dates, slop

```On 5/22/2019 2:29 PM, Jakub Narebski wrote:
> Derrick Stolee <stolee@xxxxxxxxx> writes:
>> On 5/20/2019 7:27 PM, Jakub Narebski wrote:
> Restating it yet again:
>
>    A.  corrected_date(C) = max(committer_date(C),
>                                max_P(committer_date(P) + offset(P)) + 1)
>
>    B.  offset(C) = max(corrected_date(C) - committer_date(C),
>                        max_P(offset(P)) + 1)

The problem with this definition is that it "defines" the corrected date, and
then _adjusts_ it by updating the offset. I consider

corrected_date(C) = committer_date(C) + offset(C)

to be part of the definition. You could restate the definition as follows:

corrected_date = max(committer_date(C) + max_P(offset(P)) + 1,
max_P(corrected_date(P)))

or, equivalently

corrected_date = max(committer_date(C) + max_P(offset(P)) + 1,
max_P(committer_date(P) + offset(P)))

This definition, in a single step, satisfies the conditions below:

>
>> The final definition needs two conditions on the offset of a commit C for
>> every parent P:
>>
>>  1. committer_date(C) + offset(C) > committer_date(P) + offset(P)
>>  2. offset(C) > offset(P)

Plus, the "+ 1" in the first step takes into account that "0" is a special offset
value in the commit-graph file format meaning "not computed".

> Well, we should check/test if performance benefits of "offset date"
> ("corrected date with rising offset") truly holds.

Yes, a full performance test will be required. I have full confidence that the
monotonic offset requirement will have only positive effect. That is, it will
not affect the case where committer-date was better than generation number, but will
help the cases where all the committer-dates are equal.

Thanks,
-Stolee

```