Java byte array to RoaringBitmap

[Shikhar Singh]

Is there a implemented way to convert a bitset represented as a byte array to a roaringbitmap?
Similar to: Bitset.valueOf(byte[] bytes)

I couldn't find any way to achieve this without additional memory copying OOTB.

As of now, I am replicating Bitset.valueOf(byte[] bytes) implementation. 
The reason is the bitmap is being persisted in Redis. I can't keep it in Roaring form as then it will be much more difficult to write atomically, so data is persisted kept as an uncompressed bitset. I guess such a pattern makes sense for many data storage too. (on-disk uncompressed, in-memory Roaring).

So I feel it makes sense to have such a helper method in RoaringBitmap code itself.

Happy to contribute code.

[Daniel Lemire]

As of now, I am replicating Bitset.valueOf(byte[] bytes) implementation.

Pull request invited. Please provide complete tests.

In C (in the CRoaring library), we have functions that allow you to go from a Roaring Bitmap to a standard bitset :

roaring_bitmap_t *r1 = roaring_bitmap_create(); 

for (uint32_t i = 100; i < 100000; i+= 1 + (i%5)) { roaring_bitmap_add(r1, i); } 

bitset_t * bitset = bitset_create(); 

bool success = roaring_bitmap_to_bitset(r1, bitset);

[Shikhar Singh]

Here you go: https://github.com/RoaringBitmap/RoaringBitmap/pull/650/files

Adds a very low memory method to create a  RoaringBitmap from a ByteBuffer (or a byte array), representing the uncompressed bitmap

Should be useful for performance sensitive cases as RoaringBitmap can be created from bytes directly read from the network, with no additional copying.

I have added tests and benchmarks. Do tell if I messed up somewhere.

[Daniel Lemire]

It is a good PR and I expect we will merge after some tuning.

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