GPU Aggregation (3/8): CPUAggregator

[Pessimistress]

For #7457

Change List

• Add CPUAggregator

[ibgreen]

Added some first impressions, will come back again for full review.

[ibgreen]

Looks solid!

[ibgreen]

Nit: If this makes good sense to you then fine. I was trying to understand in what sense we are "evaluating" something here. Since this returns a function, rather than evaluate directly, then maybe getVertexAccessFunction() would be a better name?

For non-rendering processing like aggregation, I personally feel that talking about rows (in a binary columnar table) rather than "vertexes" tends to be more helpful for understanding.

[Pessimistress]

VertexAccessor is provided by the user. For example,

const getBin: VertexAccessor = {
  sources: ['positions'],
  getValue: ({positions}, index, {binSize} => [
    Math.floor(positions[0] / binSize[0]),
    Math.floor(positions[1] / binSize[1])
  }
}

Directly maps to

in vec2 positions;
uniform vec2 binSize;

void getBin(out vec2 binId) {
  binId = floor(positions / binSize);
}

This function takes user-specified VertexAccessor, attributes and uniforms and returns a value getter at each point index, hence "evaluate".

[coveralls]

coverage: 89.646% (-0.02%) from 89.661%
when pulling 5e69351 on x/aggregation-3
into 92363b3 on master.

[coveralls]

coverage: 89.392% (-0.01%) from 89.405%
when pulling 29a12a9 on x/aggregation-3
into cccc99e on master.

[coveralls]

coverage: 89.393% (-0.01%) from 89.405%
when pulling 29a12a9 on x/aggregation-3
into cccc99e on master.

[coveralls]

coverage: 89.454% (+0.05%) from 89.405%
when pulling 59e0e2d on x/aggregation-3
into cccc99e on master.