fix: Correct the fillup_index
#938
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`fillup_index` used to be our arbitrary limit of hashes computed for each non-terminal Merkle path in a batched update. However, it didn't work reliably. `fillup_index` limited to `next_index().trailing_ones()` was resulting in `filled_subtrees` not being updated in case when `fillup_index` was 0. Incrementing it by 1, which we used to do, was a workaround which worked for the most cases in a sense of producing correct roots, but the non-terminal Merkle paths were incorrect - they contained one node too many. Here we are introducing a different approach where we always iterate in `0..self.height` range, but we break the loop as soon as we encounter a node on the left. Before breaking the loop, we update `filled_subtrees`. Therefore, both `filled_subtrees` and all Merkle paths are correct.
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ananas-block
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Jul 2, 2024
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fillup_indexused to be our limit of hashes computed for each non-terminal Merkle path in a batched update. However, it didn't work reliably.fillup_indexlimited tonext_index().trailing_ones()was resulting infilled_subtreesnot being updated in case whenfillup_indexwas 0.Incrementing it by 1, which we used to do, was a workaround which worked for the most cases in a sense of producing correct roots, but the non-terminal Merkle paths were incorrect - they contained one node too many.
Here we are introducing a different approach where we always iterate in
0..self.heightrange, but we break the loop as soon as we encounter a node on the left. Before breaking the loop, we updatefilled_subtrees. Therefore, bothfilled_subtreesand all Merkle paths are correct.