Yeah it's definitely similar to having parent nodes summarize their children. The motivation for the flat array structure was to have an entirely fixed-size array that's contiguous in memory to make it friendly for SIMD filtering. Having the data in the tree on the other hand could probably filter a little better since it can summarize at multiple levels, but my bet is that the flat array allows for a faster implementation for the case here with a few thousand intervals. On the one hand, the whole working set should probably fit in L1 cache for both the tree or a flat array, but on the other hand, a tree with pointers needs to do some pointer chasing. The tree could alternatively be represented in a flat array, but then elements need to be shuffled around. I don't know which is faster in practice for this problem, but my gut says the speed of a flat array + SIMD probably outweighs potential asymptotic improvements for the few thousand case

I don't think I've put the same amount of thought into it, but my gut feeling is:

1. If you're summarizing "a contiguous chunk exists from A to B", that would require a mutable rebalancing tree and the borders constantly shift and merge and subdivide.

2. If you're instead just summarizing "the range between constant indices X-Y is [1/0/both] right now", then that can be done with a fixed tree arrangement that can have an exact layout in memory.

A simple example of the latter would be 8 leaves of 01110000, 4 parents of [both, 1, 0, 0], 2 grandparents of [both, 0], etc. The 3-value aspects could also be encoded into two bitmasks, if "has a 1" and "has a 0".