[Lilypond-auto] [LilyIssues-auto] [testlilyissues:issues] #5922 Rewrite Skyline code

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don't trim zero width buildings

https://codereview.appspot.com/547980044

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[issues:#5922] Rewrite Skyline code

Status: Started
Created: Sun Apr 19, 2020 09:42 PM UTC by Han-Wen Nienhuys
Last Updated: Fri Apr 24, 2020 03:50 PM UTC
Owner: Han-Wen Nienhuys

With this change, Carver MSDM goes from 38 seconds to 32 seconds, a
15% improvement.

• Do not use contiguous buildings. Instead, Y=-infinity is represented
  by simply omitting the building. This reduces memory requirements
  (for example, an empty skyline needs no buildings at all.), and
  obviates the Skyline::normalize() call which takes ~1 % of CPU.

• Buildings store Y coordinate of the left edge, rather than the
  intercept at x==0.0. This avoid excessive rounding problems when X
  is large, and lets us compute building.height_at(x[LEFT]) cheaply.

• Avoid using infinity and NaN. We only let the X coordinates at the
  be infinite (which is only necessary if a minimum height is
  specified)

• Aggressively prune zero width
  buildings. Adjust input/regression/skyline-point-extent to match
  this behavior

• Simplify the merge code, using less nested if statements.

• Add a unittest for the Skyline code

• Debug output is produced with Lookup::segments_to_line_stencil()

https://codereview.appspot.com/547980044

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