implement nextSetBit

[richardstartin]

This implements similar functionality to java.util.BitSet.nextSetBit, except the next bit is returned as a long, since RoaringBitmaps can contain all 32 bits.

[coveralls]

Coverage increased (+0.09%) to 91.437% when pulling b17eaa7 on richardstartin:nextsetbit into 3089d7d on RoaringBitmap:master.

[lemire]

I'll merge and try to throw in some benchmarks.

[lemire]

(Great work btw.)

[lemire]

I changed the name "nextValue" because I think that's more in the spirit of the API.

The benchmark is in as of the last commit...

benchmark                          (density)  Mode  Cnt   Score   Error  Units
BitmapNextBenchmark.bitset_count         0.1  avgt    5   2.111 ± 0.022  us/op
BitmapNextBenchmark.bitset_count         0.2  avgt    5   1.881 ± 0.036  us/op
BitmapNextBenchmark.bitset_count         0.3  avgt    5   1.678 ± 0.014  us/op
BitmapNextBenchmark.bitset_count         0.4  avgt    5   1.636 ± 0.012  us/op
BitmapNextBenchmark.bitset_count         0.5  avgt    5   1.648 ± 0.036  us/op
BitmapNextBenchmark.roaring_count        0.1  avgt    5  13.601 ± 0.092  us/op
BitmapNextBenchmark.roaring_count        0.2  avgt    5  14.597 ± 0.104  us/op
BitmapNextBenchmark.roaring_count        0.3  avgt    5  13.999 ± 0.034  us/op
BitmapNextBenchmark.roaring_count        0.4  avgt    5  13.941 ± 0.043  us/op
BitmapNextBenchmark.roaring_count        0.5  avgt    5  13.898 ± 0.807  us/op

[richardstartin]

So it's multiplicatively slower because of container lookup overhead and binary searches. I didn't try to optimise this code at all though. Rather than implementing the bitset API verbatim it might have been interesting to get the next page of values after a given value, like a skip list.

[lemire]

@richardstartin

Right. It is an API designed to work well for BitSet. It is a terrible way to iterate over values in a Roaring bitmap.

[richardstartin]

@lemire agreed that it's a terrible iteration strategy to purposefully forget where you are each time you go back to the data structure, but it potentially has interesting use cases, either to retrieve an iterator starting at an offset, or to fill an array of values after an offset. This would be useful for stateless pagination.

[lemire]

@richardstartin

If you retrieve more than one value each time, thus amortizing the search cost, then it makes a lot of sense, I agree.