Re: [PATCH 8/8] diff: improve positioning of add/delete blocks in diffs

From: Michael Haggerty <mhagger@alum.mit.edu>
Date: Aug 3, 2016, 23:30 UTC
Message-ID: <f84cc951-4eb0-153c-7984-fc0993d215a7@alum.mit.edu>
In-Reply-To: <CAGZ79kYs9dnRzyAV_MXXUDbRCLuxQeETHKRpVsczQMCMHkQUPw@mail.gmail.com>

On 08/04/2016 12:51 AM, Stefan Beller wrote:

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> On Wed, Aug 3, 2016 at 3:41 PM, Michael Haggerty <mhagger@alum.mit.edu> wrote:
>> On 08/04/2016 12:30 AM, Stefan Beller wrote:
>>> On Wed, Aug 3, 2016 at 3:00 PM, Michael Haggerty <mhagger@alum.mit.edu> wrote:
>>>
>>>> +        return 10 * score - bonus;
>>>
>>> Would it make sense to define-ify the 10 as well
>>> as this is the only hardcoded number in the
>>> scoring function?
>>
>> I started answering this question by explaining why it is not important
>> to *optimize* the number 10 (namely because scores are only ever
>> compared against other scores, so an overall scaling factor makes no
>> difference). The factor 10 only has to be large enough to provide enough
>> dynamic range for the other (adjustable) parameters.
> 
> But it only scales the score, not the bonus.

Yes, that's how it provides the overall scale of the score. If it multiplied both values, then it would be completely pointless.

This is an important point for the optimization, but less so for the implementation of the heuristic here. I was dynamically optimizing the ten other variables, and everything that goes into the bonus includes one of those factors. If I had also let this factor of 10 vary, then the net behavior of the algorithm would be completely unchanged if I would, say, double all eleven parameters. This is bad for optimization, because (1) it increases the search space unnecessarily, and (2) it means that whole lines in the parameter space give identical behavior, making the algorithm waste time searching along those lines for a minimum.

Show 6 quoted lines

> So another way to write it
> would be
> 
>     score - bonus/10;
> 
> assuming the values of score and bonus are large enough.

Score is the number of columns that some line is indented, so it can be 0 or 1 or any other positive integer. It is not "large enough", which is why the "10" can't be "1".

If the calculations were done in floating point, then the factor could be "1", because then the other factors could be made less than one.

> In some prior conversation you said you take the indent and add an epsilon
> for some special conditions to make one indent better than the other.
> 
> Epsilons are usually very small compared to the rest of the equation,

I should have mentioned that this heuristic is quite a bit more advanced than my original proposal to use "indent" plus some "epsilon" factors. The old discussion about epsilons is not relevant here except maybe as an inspiration and starting point for this version.

> but if I look at the boni definitions ranging from -63..50 they are scaled up
> so much that the bonus may become larger than '1' unit of 'score', i.e.
> it is not an epsilon any more. Or to put it another way:
> If we were to s/10/100/ the results would be worse.

If you would change s/10/100/ and simultaneously multiply the other constants by 10, the end results would be unchanged.

> Rather the 10 describes the ratio of "advanced magic" to pure indentation
> based scoring in my understanding.

No, it's basically just a number against which the other constants are compared. E.g., if another bonus wants to balance out against exactly one space of indentation, its constant needs to be 10. If it wants to balance out against exactly 5 spaces, its constant needs to be 50. Etc.

Michael