performance: Use upstream optimizations

[jeswr]

In light of upstream changes we should avoid using the SimpleTransformIterator here to be able to take full advantage of them
@jacoscaz @rubensworks

[jeswr]

Moreover - I think it should look something like

import { AsyncIterator } from 'asynciterator';

export class NestedLoopJoin<D, T> implements AsyncIterator<T> {
  private source: AsyncIterator<T>;
  constructor(left: AsyncIterator<D>, private right: AsyncIterator<D>, join: (left: D, right: D) => T) {
    this.source = union(left.map(leftElem => right.clone().mapFilter(rightElem => join(leftElem, rightElem))));
  }
  read() {
    return this.source.read();
  }
  close() {
    this.source.close();
    this.right.close();
  }
  // Closing still needs to be handled so
}

export class DynamicNestedLoopJoin<D, T> implements AsyncIterator<T> {
  constructor(left: AsyncIterator<D>, right: (elem: D) => AsyncIterator<D>, join: (left: D, right: D) => T) {
    return union(left.map(leftElem => right(leftElem).mapFilter(rightElem => join(leftElem, rightElem))));
  }
}

where mapFilter (possibly better called maybeMap) differs from the current implementation of map in that it filters out null values,

[rubensworks]

Pinging @joachimvh as well, since he may have some insights on this as well.

[joachimvh]

Pinging @joachimvh as well, since he may have some insights on this as well.

I don't remember much about the code here, but they're simple classes so not that much you can do wrong I think. The code above looks fine at a first glance.

Returning from a constructor is interesting though, didn't know you could do that. Would that break an instanceof call? Is there any difference to extending UnionIterator and doing a super call?