Use roaring bitmaps for posting list compression about groonga HOT 1 CLOSED

I tried this in https://github.com/groonga/groonga/tree/ii-roaring-bitmap and my conclusion is reject. Here are reasons:

• PForDelta is faster than roaring bitmaps
• PForDelta is smaller than roaring bitmaps
• Roaring bitmaps has faster intersection/union operations than PForDelta but we don't use them in our inverted index implementation. We need to use cursor operation for our search use-cases such as phrase search.

See also: An Experimental Study of Bitmap Compression vs. Inverted List Compression

Additionally, we don't use NewPForDelta nor OptPForDelta instead of PForDelta because the above paper shows that they are smaller than PForDelta but slower than PForDelta. We prefer to faster approach. So we may replace the current PForDelta implementation with SIMD based PForDelta implementation. SIMD based PForDelta implementation will be faster and the same size.

from groonga.