How to implement "partial reduce"?

[d-bassarab]

"Partial reduce" is not the best name so let's look at the input:

'b1', 'b2', 'a1', 'b3', 'b4', 'a2', 'a3', 'b5', 'b6', 'b7'

The goal is to produce sub-arrays starting with 'a?' and having all 'b?' until next 'a?' comes along:

['a1', 'b3', 'b4]
['a2']
['a3', 'b5', 'b6', 'b7']

Note that initial 'b?'s where skipped because there was no preceding 'a?'.

Having limited rxjs experience the only solution I could come up with looks like this:

const { of } = require('rxjs');
const { concatMap, filter, reduce, share, takeWhile } = require('rxjs/operators');

const data$ = of('b1', 'b2', 'a1', 'b3', 'b4', 'a2', 'a3', 'b5', 'b6', 'b7').pipe(share());

data$
  .pipe(
    filter(v => v.indexOf('a') === 0),
    concatMap(a =>
      data$.pipe(
        takeWhile(v => v.indexOf('b') === 0),
        reduce((acc, v) => [...acc, v], [a])
      )
    )
  ).subscribe(console.log);

But it doesn't feel right because of sharing, double subscription and double processing of the stream. Is there a magical combination of operators that does the same better?

Thanks.