Read Only view of a RoaringBitmap

[Shikhar Singh]

Do we have a way to share a "read-only" view of a memory backed RoaringBitmap?

Expectations:

- all calls should be backed by an underlying bitmap 

- any calls that modify the bitmap throw a documented exception

Usecase:

I have a RoaringBitmap that needs to be move around throughout the codebase for operations that aren't expected to modify it.

Modifications should only happen by a holder object based on some internal logic.

A way to enforce this "read-only" behaviour by design and without any unnecessary object allocs/copies whenever the bitmap is asked for will make it easier to catch implementation bugs while keeping same performance.

Happy to contribute!

Regards,
Shikhar

[Shikhar Singh]

The way I am thinking is having a wrapper ReadOnlyRoaringBitmap class that delegates all read methods to underlying bitmap and causes an exception on any modify calls.

And exposing a new method in RoaringBitmap: asReadOnly() which returns a cached object (created lazily when first asked for)

I see we have something in the buffer based impl (Immutable* and Mutable*) with similar motivations. The above impl will be a lot simpler though.

[Daniel Lemire]

Just use an ImmutableBitmapDataProvider.

ImmutableBitmapDataProvider readonly = (ImmutableBitmapDataProvider) mybitmap;

And you are done.

[Shikhar]

Awesome, thank you so much!

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[Shikhar]

Doesn’t exactly meet the expectations I had

As the returned type won’t  be a RoaringBitmap  so can’t be used with other RoaringBitmaps without dirty cast code (which also kind of defeats the purpose). 

Think using ops like and, andNot of any other bitmaps where this bitmap needs to be given as input. Or even the static bit wise like ops that do things out-of-place

Please advice