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Re: Gets vertical skylines from grob stencils (issue 5626052) Joe Neeman Tue Feb 21 13:02:23 2012

On Sun, Feb 19, 2012 at 1:22 PM, [EMAIL PROTECTED] <
[EMAIL PROTECTED]> wrote:

>
> On Feb 19, 2012, at 10:31 PM, Joe Neeman wrote:
>
> On Sun, Feb 19, 2012 at 11:59 AM, David Kastrup <[EMAIL PROTECTED]> wrote:
>
>> "[EMAIL PROTECTED]" <[EMAIL PROTECTED]> writes:
>>
>> > I've now optimized the crap out of this sucker and cached as much as I
>> > can cache.
>>
>> I'm not sure the caching is of much help.  What kind of information
>> would save recalculation?
>>
>> >  For example, I get the sense that the new key signature skyline
>> > function is not much faster than the old, which means that either the
>> > lookups of cached accidental skylines or the merging of these skylines
>> > to create the signature takes a lot of time.
>>
>> "Merging" sounds like O(n^2) unless one takes precautions.
>>
>
> No, it's O(n + m). (m is the length of the other skyline). Building a
> skyline from scratch is O(n log n). However, there may be room for more
> heuristics that speed things up (see non_overlapping_skyline, which
> resulted in a measurable speedup).
>
>
> If you get a chance to look over the entirety of the patch w/ your
> speed-up glasses on, I'd appreciate it.  I know it looks like a lot, but
> the gist of it is that there are two main vertical-skylines functions
> (ly:grob::vertical-skylines-from-stencil and
> ly:grob::vertical-skylines-from-element-stencils) and the other skyline
> functions are (supposed to be) speed ups
> (ly:key-signature::vertical-skylines is probably the heftiest one).  Lemme
> know if in any of these things you see places where I may be creating
> bottlenecks, where Scheme code may be slowing things down, where I'm
> passing around too-large data structures, needless recalculation (if it
> winds up being a lot of recalculation - I know I do some in beam.cc w/
> ly:beam::vertical-skylines, for example, but I'm not sure how much this
> slows things down).
>

Another possible speedup could come from the initial building of the
skylines using add_boxes: when the children of an axis-group mostly don't
have skylines, then we gather up all of their extent boxes, before turning
them into one skyline and merging it (this is O(n log n) in the number of
children). If the children do have skylines, we merge them one at a time
(this is O(n^2), but it didn't use to matter because there were very few
children that had their own skylines). To fix this, you could make a
constructor for skylines that takes a vector of skylines and merges them
recursively, in the same way that internal_build_skyline merges boxes
recursively.

Again, you should only do this if you have evidence that the skyline merges
caused by add_boxes are taking lots of time.

Cheers,
Joe
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