Investigate performance enhancements

[JoelOtter]

Termloop's performance is acceptable for most applications, but that doesn't mean we shouldn't try and make it better. Let's use this issue to track ideas for improving performance. A couple of potential things to investigate:

• Use a pointer to an Event instead of an Event in Tick methods. This may save some extraneous copying.
• Process Tick events in parallel using a worker pool (can't do this for Draw as the order matters for rendering 'depth')

Feel free to add more in comments.

[aquilax]

Not creating new canvas on every Draw may help. But the main bottleneck seems to be RuneWidth

(pprof) top10
55s of 57.30s total (95.99%)
Dropped 126 nodes (cum <= 0.29s)
Showing top 10 nodes out of 29 (cum >= 0.79s)
      flat  flat%   sum%        cum   cum%
    48.07s 83.89% 83.89%     48.07s 83.89%  github.com/mattn/go-runewidth.(*Condition).RuneWidth
     2.36s  4.12% 88.01%     51.24s 89.42%  github.com/nsf/termbox-go.Flush
     1.75s  3.05% 91.06%     55.38s 96.65%  github.com/JoelOtter/termloop.(*Screen).Draw
     0.74s  1.29% 92.36%     48.81s 85.18%  github.com/mattn/go-runewidth.RuneWidth
     0.61s  1.06% 93.42%      0.61s  1.06%  github.com/JoelOtter/termloop.(*Screen).RenderCell
     0.55s  0.96% 94.38%      0.55s  0.96%  github.com/JoelOtter/termloop.(*BaseLevel).DrawBackground
     0.45s  0.79% 95.17%      0.45s  0.79%  runtime.futex
     0.24s  0.42% 95.58%      0.75s  1.31%  github.com/JoelOtter/termloop.(*Rectangle).Draw
     0.13s  0.23% 95.81%      0.87s  1.52%  runtime.makeslice
     0.10s  0.17% 95.99%      0.79s  1.38%  runtime.newarray

[aquilax]

Since Flush seems to be the bottleneck here are some ideas:

• Cache the result here https://github.com/nsf/termbox-go/blob/master/api.go#L168
• Don't redraw screen if nothing has changed
• Update only the changed region (not sure if this is possible with termbox);

[JoelOtter]

I'm going to close this for now as we've made pretty good strides. If anyone has any further ideas for performance improvements, please open a pull request or shout out in the Gitter chat.